Hearing and Balance

SUMBER: http://www.neurophys.wisc.edu/h&b/textbook/textindex.html

Sound TransmissionGeneral Structure and Development of The Ear

External Ear: Functions and Conductive Hearing LossMiddle Ear: Functions and Conductive Hearing LossInner Ear: General Properties of Hair Cell ReceptorsInner Ear: Cochlear FunctionInner Ear: Vestibular FunctionInner Ear: Sensorineural Hearing Loss and Vestibular DisordersLanguage Skills: Acquisition and DisordersHearing Loss and Hearing TestingPractice Questions

I. MECHANICS OF SOUND TRANSMISSION

Objectives

In order to understand how the ear functions we need to know something about what a sound is and how it behaves. This section of the module introduces some basic properties of sound waves and provides a necessary background for understanding normal hearing and hearing impairment. At the end of this section you should be able to:

1. State what constitutes a sound wave

2. State the parameters of a sinusoidal sound wave, defining what is meant by frequency, sound pressure, period and wavelength

3. Define the decibel notation

http://www.neurophys.wisc.edu/h&b/textbook/sound_transmission.html#Obj3




http://www.neurophys.wisc.edu/h&b/textbook/practicemain.html

http://www.neurophys.wisc.edu/h&b/textbook/hearing_test.html

http://www.neurophys.wisc.edu/h&b/textbook/language.html

http://www.neurophys.wisc.edu/h&b/textbook/chap-8.html




http://www.neurophys.wisc.edu/h&b/textbook/mid_ear.html

http://www.neurophys.wisc.edu/h&b/textbook/external_ear.html

http://www.neurophys.wisc.edu/h&b/textbook/general_structure.html

http://www.neurophys.wisc.edu/h&b/textbook/sound_transmission.html

4. Describe the sensitivity of the ear and how that sensitivity is measured

5. Draw the axes of an audiogram and describe how the audiogram is used to evaluate hearing

MECHANICS OF SOUND TRANSMISSION

Objective 1: What is sound?

The vibration of an object, a sound source, causes waves to be transmitted through air (usually) to our ears. It is this undulatory motion of air particles that triggers a cascade of mechanical and electrical events leading, ultimately, to the sensation of hearing. While we usually consider sound waves in air, they can propagate through any elastic medium, as we will describe when we take up the action of the inner ear. In order to understand how the ears and brain process sound information, we need to know something about what a sound is and how it is produced.

Air particles are in constant random motion, exerting very small pressure variations around the steady-state atmospheric pressure. Each particle is subject to both an inertial force (due to its mass and acceleration) and a force which tends to restore the particle too its resting position (due to the elasticity of the medium). When an object (such as a loudspeaker cone) is set into vibration, each air particle moves to and fro about its average position along an axis parallel to the direction in which the wave propagates. Figure 1 is intended to show the spatial distribution of pressure increases (condensations) and pressure decreases (rarefactions) of the particles throughout the medium at a given instant in time created by the vibrating object. For sinusoidal vibration, the distance between succesive peaks is called the wavelength.

Air particles themselves do not move very far, they simply transfer pressure changes by what is referred to as sound propagation. This constitutes what we call a 'sound wave' which moves away from the sound source at a velocity determined by the medium. The velocity of propagation of a sound wave in air is about 344 meters per second while in water it is 1437 m/s. Above the dot pattern shown in Figure I-1 is a curve which plots the instantaneous differential pressure throughout the medium at one instant in time for a sinusoidal vibration. Such a vibratory pattern is heard as a pure tone.




Fig. I-1) Distribution of air particles when a sound source is vibrated as afunction of distance in the direction of sound propagation.

Sound waves move out spherically from a point source of sound (Fig. I-2). As they do so they become less intense. This is because with a source emitting constant power the area of the surface of the sphere increases, thus sound intensity at any point on the sphere must decrease. Sound pressure is inversely proportional to distance from the source as long as the sound does not encounter obstacles, like the head and external ears for example.

Obstacles, which create a change in the medium, impede or resist the propagation of sound. When a sound waved encounters a change in medium, and thus in impedance, a portion of the sound wave is reflected from the surface. That portion not reflected is absorbed and continues to be propagated through the new medium. This is important to remember, for it is at the heart of our understanding of the action of the middle ear, whose purpose is to overcome the impedance mismatch of air and fluid (of the inner ear).

Reflected sound may encounter the original sound wave and, depending on the relative timing of the two, they may either reinforce or cancel one another. This is important with repect to the way in which the head and external ears alter incoming sound waves.

Sound waves may also be diffracted, which means that, depending on the frequency of the sound, they are able to wrap around small or medium-size objects (like the head for example). This can create a 'sound shadow', and is very important when we consider binaural (two-ear) hearing.

Objective 2: How do we describe a sound? What are its parameters?

Sound is described as loud or soft, high pitched or low, rough or smooth, etc. The fundamental physical descriptors of a sound are its frequency and its intensity. These translate into the psychological attributes of pitch and loudness, respectively.

We could use any of several physical quantities to describe the strength of a sound wave, but it is most convenient to use sound pressure, which is the extremely small alternating deviation above and below atmospheric pressure due to the propagated wave of compression and rarefaction. A useful stimulus for testing auditory function is a pure tone for which the sound pressure is a sine wave when plotted against time, as shown in Figure I-3.

The frequency of a pure tone is the number of cycles or complete oscillations of condensation and rarefaction in one second. The unit of measurement is Hertz (Hz). Thus, a pure tone that goes through 1000 cycles per second has a frequency of 1000 Hz, or 1 kHz (kiloHertz).

The period of a pure tone is the time required for one complete cycle, or the time that elapses between two successive condensation or rarefaction peaks. The period is thus arithmetically equal to the reciprocal of the frequency.

period (t) = 1/ frequency (f)

When plotted on a scale of distance (Fig. 1) the wavelength is the distance between two successive peaks on the wave; it may be calculated by dividing the velocity of wave propagation by the frequency.

wavelength (g) = velocity (v)/ frequency (f)

Objective 3: How do we measure the strength of a sound. The Decibel notation

Because the ear operates over a range of sound pressures (its dynamic range) which is greater than a million to one, stimulus strength is usually expressed in logarithmic units known as decibels (dB). The decibel notation always expresses the ratio between two intensities. When stimulus strength is expressed in terms of sound pressure, the following relationship is used, where P1 and P2 are two sound pressures. For studies of hearing, P2 is taken as the sound pressure at the threshold hearing of a normal listener.

dB = 20 log10 (P1/P2)

If, for example, the sound pressure from one source (P1) is is ten times greater than that from a second (P2), the difference is 20 dB.

dB = 20 log10 (10/1) = 20 x 1 = 20

The sound pressure of a very loudsound, such as a jet plane, may be one million times (106) the pressure of the weakest sound that can be detected by someone with normal hearing; these two sounds differ by 120 dB:

20 log10 (P1/P2) = 20 log10 106 = 20 x 6 = 120 dB

The chart below shows on a dB scale the sound pressure level of sounds common to our everyday environment.

Objective 4: Sensitivity of hearing

Figure I-4 illustrates the amazing sensitivity of the ear. The curve shows the threshold of hearing (left axis) and the amplitude of vibration of the eardrum (right axis) at the threshold of hearing at various frequencies. There are several important things to notice about this curve. First, the threshold of hearing varies with frequency of the sound; the ear is most sensitive around 4 kHz. Second, at the most sensitive frequency around 4 kHz the amplitude of motion of the eardrum is about 10-9 cm, which is only about 1/10 the diameter of a hydrogen

http://www.neurophys.wisc.edu/h&b/textbook/fig1-4.gif

atom. Thus, the ear is extraordinarily sensitive, operating in therange of atomic motion.

This almost unbelievable degree of sensitivity is even more impressive when account is taken of the fact that the receptor mechanisms in the inner ear operate in a fluid environment, i.e., the inner ear is really an "underwater" sound receiver. When sound in air strikes a fluid boundary (a boundary between media with different acoustic impendances) there is a theoretical loss of 99.9% of the energy (due to reflection). This 99.9% loss is equivalent to 30 dB; a reduction in stimulus intensity of this amount is quite noticeable to a listener. We shall consider some of the mechanisms that act to overcome this potential loss and increase auditory sensitivity.

We also note on the graph the frequencies and sound pressure levels associated with normal conversational speech. This region is situated to allow for considerable range of modulation of speech intensity.

Objective 5: The Audiogram - a basic clinical test of hearing sensitivity

For convenience in describing the hearing sensitivity of patients in the audiologic clinic, the number of dB by which the threshold sound pressure of an individual exceeds the normal threshold is referred to as dB of "hearing loss."

A graph of "hearing loss" vs. frequency is known as an audiogram. The audiogram (Figure I-5) illustrates a gradually sloping hearing loss that would


become quite noticeable to the listener at frequencies above 500 Hz. We will return to audiograms and how they help us understand the mechanisms of hearing loss later in this section.

Fig. I-5

[NEXT]

II. GENERAL STRUCTURE AND DEVELOPMENT OF THE EXTERNAL EAR, MIDDLE EAR AND INNER EAR

Objectives:

Before delving into the details of auditory and vestibular transduction mechanisms, it is worthwhile to have an overview of the peripheral auditory and vestibular systems. Once the general plan of organization of the external, middle and inner ears is understood, then we can take up the various mechanisms that are involved in normal and abnormal hearing and balance.

Together, the external, middle and inner ears are derived from all three germ layers. Thus, knowing the embryonic origin of different parts of the ear is

http://www.neurophys.wisc.edu/h&b/textbook/general_structure.html

essential in understanding the sensory deficits that result from auditory or vestibular maldevelopment. In this section we seek a basic knowledge of the germ layers that give rise to specific structures of the external, middle and inner ears and how maldevelopment of these structures leads to sensory deficits.

At the end of this unit, each student should be able to:

1. Describe the basic structures of the external ear (including the pinna, external auditory, meatus, tympanic membrane)

2. Describe the basic structures of the middle ear (including the ossicles, muscles, Eustachian tube)

3. Describe the basic structures of the inner ear (including the osseous labyrinth and the membranous labyrinth)

4. Describe the structure of the membranous labyrinth (including the cochlea, ductus reuniens, utricle, saccule, semicircular canals).

5. Describe the fluid composition of the inner ear

6. Describe the blood supply to the inner ear

7. Describe the development of the external, middle and inner ear and state the contributions of each of the germ layers to the development of the ear.

GENERAL STRUCTURE OF THE EXTERNAL EAR, MIDDLE EAR AND INNER EAR

The mammalian statoacoustic organ may be divided into three parts: the external, middle and inner ears. Their structural relationships, highly schematized, are shown in Figure II-1.

Objective 1: The External (Outer) Ear

The external ear consists of the auricle (or pinna) and external acoustic meatus. The external acoustic meatus, lined with skin, leads inward from the bottom of the


http://www.neurophys.wisc.edu/h&b/textbook/general_structure.html#Obj7













concha of the auricle to the tympanic membrane. The stratified epithelium of the skin in the canal is supplied with specialized ceruminous (wax) glands. The first part is supported by the cartilage of the pinna, while the medial 1.5 cm is supported by the temporal bone. Disorders of the external ear include inflammatory, traumatic, and neoplastic lesions. Congenital malformations are not uncommon.

Objective 2: The Middle Ear

The middle ear, or tympanic cavity, is a narrow, air-filled chamber lined with mucous membrane and is situated between the external acoustic meatus and the labyrinth. It communicates with the mastoid air cells and with the nasal pharynx via the Eustachian (auditory) tube. The auditory ossicles, forming a chain of three small bones, connect the tympanic membrane with the inner ear (Figure II-2). The manubrium (handle) of the malleus is attached to the tympanic membrane. The tensor tympani muscle, acting on the malleus, regulates the tension on the tympanic membrane, resulting in two identifiable regions of the tympanic membrane: pars tensor and pars flaccida. The incus is attached to the malleus and to the third ossicle in the chain, the stapes, which in turn is attached via its footplate to the oval window of the cochlea. The stapedius muscle regulates the range of motion of the stapes; the two muscles (tensor tympani and stapedius) thus regulate to some extent the amplitude sensitivity of the ear.

As a result of its development, the Eustachian tube connects the tympanic cavity with the pharynx, and thus provides an important mechanism for equalizing external and internal pressures acting on the tympanic membrane. It also provides a convenient pathway for infections of the pharynx to invade the middle ear.



The middle ear is susceptible to inflammatory disease, trauma, and neoplastic disease. It is also the site of the degenerative disease, otosclerosis. Congenital malformations of the middle ear frequently accompany those of the external ear.

Objective 3: The Inner Ear

The inner ear is called the labyrinth because of the complexity of its shape (Figure II-3). It contains six mechanoreceptive structures: three semicircular canals, utricle, and saccule, which serve the sense of equilibrium, and the cochlea, which is specialized for detection of sound waves. The inner ear consists of two parts: the osseous (or bony) labyrinth, a series of cavities within the petrous portion of the temporal bone, and a membranous labyrinth, which is a series of communicating sacs and ducts within the bony labyrinth.

The inner ear is easily damaged by intense sound, head injury, and ototoxic drugs. It can be affected by microorganisms and is susceptible to degenerative and metabolic disease. It may also suffer abnormal development.

Osseous labyrinth

The temporal bone shell of the inner ear is one of the hardest bones in the body. It is lined with periosteum and is filled with perilymph, a fluid closely resembling cerebral spinal fluid in its chemical composition. Midway between the semicircular canals and the cochlea is the vestibule. It is just medial to the tympanic cavity. The oval window, into which fits the footplate of the stapes, is the lateral wall of the vestibule. Note that motion of the stapes that results from sound waves striking the drum meets considerable resistance at this air (middle ear) - fluid (inner ear) boundary. Mechanisms by which this impedance (resistance) mismatch is overcome are covered in the later section on middle ear function.

The three semicircular canals open into the vestibule. The bony cochlea lies horizontally in front of it. The shape of the cochlea resembles that of a snail shell with two and one-half turns (in humans) and hence its name. The central



conical core of the cochlea is called the modiolus. The outer wall of the modiolus forms the inner wall of a canal which spirals the full two and one-half turns around the central core. A thin shelf of bone, called the osseous spiral lamina projects from the modiolus and partially divides this canal into two parts along its entire length. From the free border of the osseous spiral lamina, a partition reaches across to the outer wall of the bony cochlea and, thus, separates the canal into two passages except for a small communicating opening between them at the apex. This opening is called the helicotrema. Thus, the cochlea can be seen as a long, coiled, fluid-filled tube (about 33 mm in humans) that is divided along its entire length by a partition. This is shown schematically in Figure II-4 below:

The cochlear partition is a complex structure of the membranous labyrinth that is described in a later section. At the basal end of the cochlea (that end nearest the vestibule) there are two openings to the tympanic cavity, one on each side of the cochlear partition, that are covered by membranes. One is called the oval window and, as we already mentioned, is in contact with the stapes foot plate. The other, called the round window, is just below the oval window and in contact with no structure. As we shall see in a later section this membrane yields under pressure developed at the oval window by stapes motion. The two channels formed by the cochlear partition are called the scala vestibuli and scala tympani, respectively. Again, they are filled with perilymph. A tiny canal, called the cochlear aqueduct (or perilymphatic duct), leads from the lowest turn of the cochlea through the temporal bone to the CSF-containing subarachnoid space at the base of the brain.

Objective 4: Membranous labyrinth

The membranous labyrinth lies within the bony labyrinth and, hence, takes on its general form. It is separated from the bony labyrinth by perilymph. The membranous labyrinth is filled with its own fluid, called endolymph. Endolymph is a fluid of somewhat higher specific gravity and different chemical composition than perilymph. That portion of the membranous labyrinth within the bony cochlea is called the cochlear duct or scala media. The receptor organ of hearing, the organ of Corti, is within the scala media. The scala media joins the vestibular organs of the vestibule, the saccule and the utricle through a small tube, the ductus reuniens. The membranous labyrinth continues as the semicircular canals, each of which has at its base a swelling,



called the ampulla within which are the sensory epithelial cells. The membranous labyrinth connects to the endolymphatic sac within the cranium.

The six sensory structures (three canals, utricle, saccule and cochlea) are innervated by the distal process of bipolar neurons of the vestibular or cochlear divisions of the eighth cranial nerve. It should be clear that, because the vestibular and auditory receptor organs share the same continuous fluid environment, diseases of the inner ear often affect both systems.

Objective 5: Fluids of the inner ear

Perilymph and endolymph are of very different chemical composition. Under normal conditions they occupy separate compartments and, hence, do not mix. The distribution of these fluids with respect to the receptor cells may play an important role in inner ear transduction and thus may be a major factor in governing the great sensitivity of the mechanoreceptors of the inner ear. Studies of fluid composition and dynamics in the inner ear are technically challenging, and because of this definitive answers regarding the origin and flow of cochlear fluids remains somewhat controversial.

Endolymph

Endolymph is unlike any other extracellular fluid found in the body. Its predominant cation is potassium; sodium is very low. Like perilymph, it is generally believed that endolymph is not homogeneous in its ionic composition throughout the inner ear. The source of endolymph and its flow dynamics are still controversial. The source of K+ appears to involve active transport by the stria vascularis, although the precise cellular mechanisms by which this is accomplished are not understood. Evidence now points to ionic transport as a mechanism for maintaining constant the chemical composition of endolymph.

Perilymph

Perilymph resembles in its chemical composition other extracellular fluids that are characterized by high Na+ concentration. Osmolarity of perilymph is similar to that of plasma, hence it is in osmotic equilibrium with blood. The origin of perilymph is equally controversial. Two possibilities have been considered: 1) perilymph is derived from CSF, which enters the cochlea via the cochlea aqueduct 2) perilymph is produced locally in the cochlea by ionic or ultrafiltration mechanisms.

Inner ear disorders associated with disturbances of cochlear fluids

Membranes that separate the fluid compartments of the inner ear are permeable to ions. Ions may move passively between compartments along their electrochemical gradients. Thus, the maintenance of the high K+ composition of endolymph depends on active transport mechanisms, and these are believed to be operating in the stria vascularis of the cochlea. If the cochlea becomes anoxic or is treated with a Na/K transport inhibitor, endolymph begins to equilibrate with perilymph, intracochlear potentials fall, and hearing loss occurs.

Meniere's disease, which is characterized by tinnitus, fluctuating hearing loss and veritgo, is generally assumed to be the result of interuption in normal mechanisms of volume regulation of endolymph. The histological sign is expansion of the endolymphatic space, and endolymphatic hydrops.

Perilymphatic fistualae may occur between the perilymphatic scalae and the middle ear or between the perilymphatic and endolymphatic compartments. The origins of such fistuae are varied, and include mechanical trauma, congenital defects and bone erosion associated with cholesteotoma. When they occur perilymph escapes and is replaced by CSF. Clinical symptoms are similar to those seen in Meniere's disease.

Objective 6: Blood supply to the inner ear

The internal auditory artery, a branch of the basilar artery, supplies the entire membranous labyrinth. The artery passes through the internal auditory meatus and then divides into three branches. The first of these three branches, the vestibular artery, supplies the vestibular nerve and parts of the saccule and utricle, and semicircular canals. The second branch, the vestibulocochlear artery, supplies the basal turn of the cochlea, the saccule, utricle, and parts of the semicircular canals. The cochlear artery supplies the entire cochlea via the spiral arteries.

Infarction or acute ischemia of the cochlea and/or vestibular end organs can occur in humans. It has been proposed that symptoms of acute vestibular failure, e.g., vertigo, vomiting, unsteadiness, and nystagmus, may result from occlusion of the vestibular artery. Certain kinds of sudden deafness and tinnitus are common clinical problems and a number of etiological mechanisms have been suggested including cochlear ischemia.

Objective 7: Development of the external, middle and inner ear

The ear is a complex sensory organ of multiple embryonic origin. Different structures are derived from different germ layers. Understanding the origin of each structure is helpful in understanding and diagnosing the functional impairments associated with congenital malformations of external, middle and inner ears, and in knowing when and how to intervene and treat these disorders.

External Ear Development

The external ear is a modification of the surface ectoderm by which the skin is brought into functional relationship with the ossicles at the tympanic membrane. The pinna develops around the first branchial groove (Fig. II-5).

Six hillocks appear on the first (mandibular) and second (hyoid) branchial arches.; three on the facing border of each of the arches fuse to form the elevations, fossae and sulci of the adult pinna. You need not learn the names of these structural features of the auricle but you should know that they form resonance chambers that can profoundly alter the incoming sound waves.

The external acoustic meatus is a derivative of the first ectodermal groove between the mandibular and hyoid arches. The epithelium in the bottom of the first ectodermal groove is in contact with the endoderm of the first pharyngeal pouch. Connective tissue, derived from mesoderm is situated between the epithelial layers. It becomes the fibrous layer of the trilaminar tympanic membrane. Connective tissue around the margin of the membrane begins to ossify at about the third month. This tissue forms the tympanic ring which serves as circumferential support of the tympanic membrane.

Middle Ear Development

The middle ear is developmentally an air sinus that develops along with the Eustachian tube as an outgrowth of the first pharyngeal pouch and thus is lined with endoderm. The ossicular chain is developed from the upper ends of the first (mandibular) and second



http://www.neurophys.wisc.edu/h&b/textbook/fig2-6a.gif

(hyoid) cartilages. Thus, the ossicles, formed from three condensations in the mesenchyme, come to be covered also with endodermal lining of the tympanic cavity. The tympanic membrane or ear drum is formed by the approximation of the ectodermal meatal plug and the endoderm of the tympanic cavity with intervening mesoderm. Thus, this thin membrane is derived from all three germ layers.

The nerve supply of the drum reflects its origin. The ectodermal (outer) surface of the tympanic membrane is supplied by the auriculotemporal branch of the trigeminal nerve and by the auricular branch of the vagus (Arnold's) nerve. The nerves that supply the drum also supply the external auditory meatus. Irritation of the auricular branch of the vagus nerve may cause reflex coughing or vomiting. Foreign bodies in the external ear may, therefore, simulate a thoracic condition. Pain that emanates from the eardrum when it is stretched or torn presumably results from activation of the trigeminal branch.

It should be clear how maldevelopment of the first and second branchial arches leads to developmental abnormalities involving both the external and middle ears. This, in turn, may result in a conductive hearing loss and a lifelong communication handicap.

Inner Ear Development

The membranous labyrinth is the fundamental part of the inner ear. Early in the life of the embryo, even before any other part of the inner ear develops, the peripheral processes of the acoustic nerve reach its membranous wall. In these areas the epithelium becomes modified into neuroepithelium for the end organs of hearing and equilibrium.

The primordium of the membranous labyrinth appears in human embryos of three weeks as a plaque-like thickening of the ectoderm on either slide of the head dorsal to the first branchial groove in the region of the hindbrain. This thickened plate of epithelium, the otic (auditory) placode, soon invaginates into the mesenchymal tissue to form the otic (auditory) pit. The invaginated portion then enlarges, and the mouth of the pit narrows by the growing together of the lips. When these meet and fuse, the otic pit is converted into a closed sac, the otocyst, or otic vesicle. In the 5-week embryo, the future membranous labyrinthine system is represented by an otocyst in which the parts have but

http://www.neurophys.wisc.edu/h&b/textbook/fig2-6b.gif

http://www.neurophys.wisc.edu/h&b/textbook/fig2-6c.gif

lately become definitive. The developmental course whose progress is predicted in the 5-week stage is virtually completed in the ensuing 5-month period. During this period the receptor organs of the vestibular labyrinth and the cochlea are formed. It is now generally accepted that the vestibular (Scarpa) and cochlear (Spiral) ganglia arise from the otic placode.

The inner ear is very sensitive to viral and bacterial infection during the first tri-mester of uterine life. Drugs, including aminoglycoside antibiotics, salicilates, quinine, and diuretics, when administered during pregnancy (and afterward as well) can damage the receptor organs of the inner ear.

[NEXT]

http://www.neurophys.wisc.edu/h&b/textbook/external_ear.html


III. FUNCTIONS AND PATHOPHYSIOLOGY OF THE

EXTERNAL EARObjectives:

The external ear is the entry way to the auditory receptors of the inner ear. Thus, it plays an important role in normal hearing. Because it is a structure that is frequently examined in the clinic, understanding of its functions is important.

At the end of this section you should be able to:

1. Describe the two functions of the external ear, including the effects of the pinnae and ear canals on incoming sound waves.

2. State the type of hearing loss involving the external ear and the ways in which these come about.

THE EXTERNAL EAR

Objective 1: The external ear, which includes the pinna and ear canal, carries out two physiological functions: acoustic and non-acoustic.

The auditory function allows efficient sound transmission from the environment to the tympanic membrane. The nonacoustic functions of the ear canal include protection of the tympanic membrane and the maintenance of a clear passage for sound.

Non-acoustic functions of the external ear.

The protective function of the ear canal is related, in part, to its anatomical structure. The depth of the canal and its tortuous shape and rigid walls provides protections of the tympanic membrane (and the middle ear beyond) from direct injury.

The canal has a self-cleaning function, which keeps the sound pathway clear of debris. The technical problem of self cleansing faced by the ear canal is that it is a blind alley lined with skin. The usual surface contact that removes desquamated keratinocytes from skin on other parts of the body does not operate here. The solution is a mechanism of epithelial migration; the surface

http://www.neurophys.wisc.edu/h&b/textbook/external_ear.html#Obj2




of the skin moves laterally from the tympanic membrane toward the ear canal opening.

The external ear canal is also protected by the arrangement of hairs and the production of wax in the outer (cartilaginous) part. These not only help to prevent objects from entering the ear canal, but they aid in the desquamation and skin migration out of the canal.

Acoustic Functions of the external ear

Although Charles Darwin had concluded in 1907 that the human pinna is vestigial and of no functional significance, several earlier investigations had (correctly) determined that this structure does play an important role in hearing. Specifically, it alters the amplitude of the incoming sound wave and, in doing so, provides a mechanism for amplifying differentially sounds within the range of frequencies that make up human speech. It also plays a role in the 'spatial' hearing of sounds. The curve that relates the amplitude of the sound in free space to that at the ear drum is referred to as a transfer function. Transfer functions are shown in FigureIII-1.

For humans, there is an amplification from the free-field to the eardrum of from 5 to 20 dB over the frequency range from about 1.5 to 7 kHz. While some of this increase is due to reflection of sound from the torso, together with diffraction of sound by the head, most of the increase arises from two mechanisms: a) resonance of the concha (the scooped-out area of the extended ear leading to the ear canal opening) around 5 kHz; and b) resonance of the external canal (closed tube, like an organ pipe) around 2.5 kHz.

At higher frequencies, above about 6 kHz, the shape of the transfer function shifts systematically as the location of a sound sources is changed, both vertically and horizontally, especially for frequencies above about 6 kHz. The external ear acts as a directional amplifier of sound. Thus, the shifts in the structure of the transfer function with changes in sound direction provide important cues for sound localization. Far from being vestigial, the complex structures of the pinna and external ear canal are now recognized as a significant components in the mechanisms that underlie the capacity of a listener to recognize and localize sounds in space.



Objective 2: Pathophysiology of the external ear

Diseases of, trauma to, and maldevelopment of the pinna and external ear canal may include skin, cartilage or bone. Because of the relationships of the external ear to the middle ear, mastoid region, scalp, skull, neck, parotid gland, and temporo-mandibular joint regions, a number of therapeutic approaches are often called for. Secondary involvement of the cochlea and vestibular systems of the inner ear is also occasionally found.

From the standpoint of hearing and hearing disorders, abnormalities of the pinna and external ear canal that result from any condition usually can only cause a blockade to sound conductance. They create a conductive hearing loss. Naturally, hearing loss may be only one of the considerations for intervention in an external ear disorder. There are a number of conditions that may present themselves, some of which are unrelated to hearing disorders. The presence of foreign bodies in the ear canal and infection of the skin of the external auditory canal are conditions most commonly associated with conductive hearing loss in the external ear.

Foreign bodies

Almost anything can become lodged in the external auditory canal. Even naturally occurring cerumen may become the impacted material which results in a noticeable conductive hearing loss. Probing the ear canal with an instrument (e.g., a Q-tip) is dangerous for it can force the impacted material further into the canal and can perforate the tympanic membrane resulting in damage to the middle ear structures

Congenital malformations

Congenital malformations of the pinna and external ear canal are related to developmental defects of the first and second branchial arches and the branchial groove which joins the first pharyngeal pouch to form the external ear canal. Malformation of the external ear canal results in an atresia, which is a conductive blockade of connective tissue or bone. Maldevelopment of the first pharyngeal pouch, leads to abnormalities in Eustachian tube, middle ear, and mastoid differentiation. These malformation may occur singly or in combination.

Recall that the ectodermal otocyst, which eventually differentiates into the inner ear cochleovestibular system, develops independently from external and middle ear primordial tissue, which is of branchial arch origin. Because of this

separate development of the inner ear from that of the external and middle ears, congenital combined lesions of the external, middle, and inner ears are not common, but they do occur. When this happens, there results a sensorineural hearing loss as well as a conductive hearing loss; this combined sensorineural and conductive hearing loss is referred to as a mixed hearing loss.

Other conditions of the external ear

Other conditions that may result in a hearing loss include trauma and neoplasia (benign or malignant). Inflammatory conditions of the external ear (external otitis) may result in canal occlusion, and can have an effect on the non-acoustic protective and cleansing action of the ear canal.

IV. FUNCTIONS AND PATHPHYSIOLOGY OF THE MIDDLE

EARThe middle ear is a crucial component in the transmission of sound from the external world to the inner ear. Disorders of the middle ear are common in the clinic. Understanding the way in which the middle ear functions is crucial to understanding the hearing loss that results when it malfunctions.Moreover, middle ear disease may herald more serious medical problems, and an understanding of middle ear function is necessary to understand to onset and progress of these disease states. We first take up the normal function of the middle ear in some detail, and then go on to describe the hearing loss that results from a wide variety of conditions.

Objectives: At the end of this section you should be able to:

1. Describe the properties of the middle ear which facilitate transfer of sound energy from air to fluid of the inner ear, including impedance matching.

2. State the function of the round window.

http://www.neurophys.wisc.edu/h&b/textbook/mid_ear.html#Obj2



3. Describe the effect upon sound conduction of static pressure difference in the middle ear cavity and the consequent importance of normal Eustachian tube function.

4. Describe the basic attributes of a vibrating mechanical system, including mass, stiffness, friction, resonance, frequency response , and how they are associated with middle ear function.

5. State was is meant by a conductive hearing loss, how this can be differs from a sensorineural hearing loss and how it is measured.

6. Describe common middle ear disorders and how they interfere with hearing.

THE MIDDLE EAR

Objective 1: The middle ear functions to transmit sound efficiently from air to fluid

Although we have seen that the acoustic properties of the outer ear and external canal substantially increase sound pressure at the eardrum above that in a free sound field for the middle frequency range, the middle ear constitutes another important mechanism to increase auditory sensitivity. Recall that the auditory receptors of the inner ear operate in a fluid environment, and that the inner ear is really an "underwater" sound receiver. When sound in air strikes a fluid boundary (a boundary between media with different acoustic impendances) there is a theoretical loss of 99.9% of the energy in a sound wave in air (due to reflection). This 99.9% loss is equivalent to 30 dB; a reduction in stimulus intensity of this amount is quite noticeable to a listener. In order to overcome this mismatch in the impedance of air and fluid, the middle ear is interposed between the tympanic membrane and the oval window. The process is referred to as impedance matching. Impairment in the transmission of sound through the middle ear creates a conductive hearing loss.

Impedance matching

The middle ear acts as a kind of hydraulic press in which the effective area of the eardrum is about 21 times that of the stapes footplate. Thus, the force caused by a given sound pressure in the air acting on the area of the











eardrum is concentrated through the ossicles onto the small area of the footplate, resulting in a pressure increase proportional to the ratio of the areas of the two structures which, in humans, is about 21:1. It also happens that the lever arm formed by the malleus in rotating about its pivot is somewhat longer than that of the incus, giving another factor of about 1.3 in pressure increase. FigureIV-1 shows these relationships.

The 21x of the drum/footplate area ratio, multiplied by the 1.3x lever arm factor, yields about a 27.3x increase in pressure, which is 29 dB, thus just about overcoming the theoretical 30 dB loss due to the air/liquid interface. We say that the middle ear matches the acoustic impedance between the air and the fluid, thus maximizing the flow of energy from the air to the fluid of the inner ear.

Objective 2: The round window allows for fluid displacement in the cochlea

The tympanic membrane moves in and out under the influence of the alternating sound pressure. Motion of the eardrum causes the malleus and incus to rotate as a unit about a pivot point; rotation of the long process of the incus about this pivot causes the stapes to rock back and forth in the oval window, setting up a wave of sound pressure in the fluid of the inner ear. Because fluid of the inner ear in noncompressible, inward movement of the stapes footplate is allowed because of the yielding of the thin membrane which covers the round window. This is essential to the transmission process, since it provides elastic relief for the fluid of the inner ear, thus permitting movement of the stapes and the structures of the inner ear.

Objective 3: The Eustachian tube functions to equalize pressure

The middle ear operates normally when it is filled with air at atmospheric pressure. The Eustachian tube, which connects the middle ear cavity to the nasopharynx, normally opens and closes periodically thereby insuring that the static pressure in the middle ear will remain the same as atmospheric pressure. When the Eustachian tube fails to open, a negative pressure immediately begins to build in the middle ear cavity due to absorption of the trapped air by the middle ear mucosa. There is an increased stiffness in the middle ear mechanical transmission system. The transmission loss is greater for low frequencies and has been observed to be of the order of 20 dB for frequencies below 1000 Hz. A pressure difference may also be experienced when the Eustachian tube fails to open during ascent or descent in an airplane.

Reflex contractions of the middle ear muscles.

The tensor tympani and stapedius tensor muscles in the middle ear contract reflexly in response to loud sounds. Both muscles increase the stiffness of the ossicular chain when they contract and thus reduce sound transmission by up to 15 dB, depending on frequency. In humans the stapedius tensor is much more effective than the tensor tympani. The reflexes are generally thought to be primarily a protective mechanism to shield the inner ear from damage due to intense sound but, because the latency of contraction is at least 10 milliseconds, they cannot protect against impulsive sounds such as a pistol shot. Since the reflexes primarily reduce the transmission of low frequencies, they also act to improve the discrimination of speech sounds in the presence of loud, low frequency background noise.

Objective 4: Attributes of a vibrating system - mechanical resonace in the middle ear

In order to understand the physiological implications of several kinds of pathological change which can occur in sound conduction in the middle ear it is helpful to consider some of the basic properties of a vibrating mechanical system, which the middle ear ossicles are one example. In general, a vibrating system includes three elements: mass, stiffness and friction. The corresponding three types of forces which act on the system are: 1) an inertia force given by the product of the mass and its acceleration; 2) a stiffness force proportional to the deflection of the spring from its resting position; 3) a frictional or damping force which dissipates energy in the form of heat when movement occurs. When such a system is subjected to a sinusoidal driving force of constant magnitude, but which varies in frequency, the resulting amplitude of vibration is maximum at a frequency known as the resonant frequency.

A plot of vibration amplitude vs. driving frequency, referred to as a resonance curve, gives the frequency response of the system as shown in Figure IV-2A. At resonance the inertia and stiffness forces are equal in magnitude but out of phase in time and thus cancel, leaving only the frictional force to limit the amplitude of vibration.

If the stiffness of the system is increased with the mass and friction remaining unchanged, the resulting amplitude is reduced for frequencies below the resonant frequency and the resonant frequency is increased (Figure IV-2B). Conversely, if the stiffness of the original system is not changed but the mass is increased, the response amplitude is little changed for frequencies below the resonant frequency but is reduced for frequencies above resonance and the resonant frequency is lowered (Figure IV-2C).

Objective 5: Conductive hearing loss

We have seen how the normal outer and middle ears participate in sound conduction. When one or more parts of these systems do not function properly a conductive hearing loss results. A conductive hearing loss is the result of a physical blockade of sound transmission to the inner ear. Conductive hearing loss is a problem in the outer ear, the middle ear, or both. A sensorineural hearing loss occurs when there is damage to the cochlear receptor organ, to the fibers of the auditory nerve or to both. A sensorineural hearing loss may also be associated with lesions of the central auditory pathways.

The principles of acoustics outlined above can be applied to the vibrating system composed of the eardrum, ossicular chain and supporting ligaments. Since the actual middle ear is a complex mechanical system in which the mass, stiffness and friction are distributed throughout several structures and the values of these parameters poorly known, we can only do some qualitative reasoning rather than to obtain quantitative results. Nevertheless, the general principles can be used to predict the kinds of hearing impairment that would be expected on the basis of specified changes in the structures of the middle ear.

A conductive hearing loss may be detected and its severity measured with an audiogram. Because in a conductive hearing loss sound will be attenuated by malfunction of the sound-transmission system, an audiogram will show a loss in sensitivity at some or all test frequencies. Later, you will see how the


audiogram can be used to differentiate between a conductive hearing loss and a sensorineural hearing loss.

Objective 6: Pathophysiology of the middle ear

Disorders of the middle ear and mastoid arise from maldevelopment, inflammatory and degenerative processes, trauma, or neoplastic disease. From the point of view of hearing, these disorders may result directly in a conductive hearing loss. Some of them (e.g., inflammation and neoplastic disease) can become serious medical problems if not treated, with involvement of the inner ear and systems beyond.

Congenital malformations of the middle ear

Congenital malformations are often related to malformations of the pinna and external ear canal. This is not surprising since both the external ear and a portion of the middle ear are derived from tissue of the first and second branchial arches. The conditions may be inherited or they may be acquired. In the latter category, for instance, thalidomide is known to be a potent teratogen that

results in total or partial agenesis of the pinna, external canal, and middle ear.

Problems with the ossicles may include agenesis, atresia, fixation or discontinuity. Malleal and incudal fixation are most common. In some cases, a bony atresia plate, a malformation which may completely block the external ear canal, may fuse with an abnormal malleus which, in turn, may be associated with a fused incudomalleal joint (Figure IV-3). This malformation blocks most normal environmental sound being transmitted to the cochlea. The result is a conductive hearing loss. The stapes may also be malformed and immobile. Other structures in the area that may be anomalous include the facial nerve, middle ear muscles, blood vessels, bony partitions, and the oval and round windows.

Tympanicmembrane perforations


Fig. IV-4

Perforations of the tympanic membrane is a common serious ear injury that may result from a variety of causes including projectiles or probes (e.g. Q-tips, pencils, paper clips, etc.), concussion from an explosion or a blow to the ear, rapid pressure change (barotrauma), temporal bone fractures, and other miscellaneous causes. Perforations may include damage to the ossicles. Figure IV-4 are examples.

Hearing loss accompanying tympanic membrane perforation is conductive in nature. There may be two mechanisms at play that contribute to this hearing disorder. First, the normal structure, and hence action, of the tympanic membrane is altered. Sound pressure on either side of the tympanum is quickly equalized. The degree of conductive hearing loss is directly related to the size of the perforation. Second, sound waves that enter the middle ear space reach both the round and oval windows and do so nearly in phase. Recall that under normal conditions inward motion of the stapes footplate in the oval window results in an outward movement of the round window, and vice versa. A "leaky" tympanic membrane means that the normal "push-pull" action of these two membranes is, to some extent at least, circumvented and as a result the sound energy entering the inner ear is reduced.

Ossicular chain injuries

The various injurious mechanisms associated with the tympanic membrane apply to the ossicles as well, occasionally even without rupture to the membrane itself. Violent, closed-head injuries, especially if associated with a temporal bone fracture, are common causes of ossicular chain disruption. A major conductive hearing loss (30-60dB) may result which does not improve after tympanic membrane repair. The most common traumatic ossicular chain lesion is a incudostapedial joint dislocation with or without a fracture of the long process of the incus. However, just about any imaginable fracture or displacement can be found. Ossicular dislocation interrupts the normal



transmission of sound energy from the tympanic membrane to the fluid of the middle ear. Hence, under these conditions the impedance matching mechanism, which alone overcomes the nearly 30 dB loss of energy when sound waves in air meet a fluid boundary, is lost. Coupled with the mechanisms surround a tear in the tympanic membrane (see above), the hearing loss is substantial and disabling.

Metabolic disease - otosclerosis

Otosclerosis is a common cause of hearing loss (Fig. IV-5). It is found in about 10% of the population, and is clinically significant in 10% of affected individuals. It is an osteodystrophy limited to the temporal bone, affecting primarily the otic capsule of the labyrinth. Otosclerosis usually results in fixation of the stapes in the oval window but may involve the cochlea and other parts of the labyrinth. Hence, the symptoms are confined to the auditory and vestibular systems. A conductive hearing loss is the usual outcome, but with cochlear involvement there may be a sensorineural component to the deafness and vertigo as well. The pathogenesis of the disease is not well understood. Early in the course of the disease there is increased mechanical stiffness of the ossicular chain due to a developing fixation of the stapes footplate in the oval window niche. Recall that a stiffness lesion of the middle ear tends to suppress the transmission of low-frequency sounds. This is reflected in the audiogram taken at this time as a low-frequency air-bone gap. Later, as the bone grows, there is added a mass component to the transmission system of the middle ear and a suppression of high tones. Thus, in advanced otosclerosis there is substantial conductive hearing loss at all frequencies usually tested .

Inflammatory processes in the middle ear - Otitis media


Inflammatory diseases of the middle ear are related and the occurrence of one often leads to the other. The progression of otitis media and the oft-accompanying processes of the mastoid, otomastoiditis is shown in the chart at right.

Otitis media evolves from the common cold, allergies, cigarette smoke exposure, or anything that can cause obstruction of the Eustachian tube. For instance, loss of ciliary action, hyperemic swelling, and increased production of mucus associated with an upper respiratory infection leads to temporary closing of the Eustachian tube and, as a result, negative pressure develops within the middle ear. This has two consequences: One involves pressure and pain as the result of distention of the tympanic membrane innervated by the

trigeminal nerve. The other is a progressive conductive hearing loss due to added stiffness of the middle ear transmission mechanism. Because stiffness shifts the resonance point of this mechanical system toward high frequencies, sounds tend to lose their low frequency quality and take on a sharp "tinny" character not unlike that often experienced in high altitude flying. Otitis of this type usually subsides without further complication.

Negative pressure within the middle ear, if left unrelieved, will lead to fluid accumulation in the normally air-filled middle ear space (Figure IV-6). This condition is referred to as serous (or secretory) otitis media. As noted above, it may have predisposing factors including lymphatic engorgement, cleft palate, hypertrophic adenoids, allergic rhinitis, and neoplasms of the nasopharynx and, thus, may develop in the absence of infection. Again, the patient has a conductive hearing loss with a retracted immobile tympanic membrane. The middle ear is filled with an amber-colored serous transudate. The hearing loss is now further complicated by the presence of a fluid-air boundary at the tympanic membrane and mass-friction loading of the ossicles. The degree of hearing loss will vary depending on the amount and viscosity of the transudate, middle ear fibrosis,


and tympanic membrane edema. It may be as low as 5-10 dB (and not too noticeable) to 40-50 dB (where it is disabling).

The disease can develop rapidly into an acute suppurative otitis media as organisms migrate to the middle ear from the nasopharynx. The fluid changes rapidly from serous to sero-sanguineous to sero-purulent and finally to the purulent stage. Hence, acute otitis media is a potentially serious disease.

Because of the relationships between the middle ear cavity and the mastoid pneumatic channels there is a wide range of possible complications that involve areas outside of the middle ear itself. An important one for our considerations is the labyrinth. Involvement here results in labyrinthitis and sensorineural hearing loss. A second important sequelae is acute mastoiditis. Here there is bony destruction and coalescence of mastoid cells. Pus under pressure results in venous stasis, local acidosis, and dissolution of calcium (halisteresis). The infection may break through the confines of the mastoid and lead to intracranial complications including facial paralysis, meningitis, brain abscesses, lateral sinus thrombophlebitis, and otitis hydrocephalus.

Chronic otitis media/cholesteatoma

Left untreated (or if unresponsive), Eustachian tube dysfunction may lead to chronic otitis media and cholesteatoma. Some lesions of chronic otitis media are reversible whereas others are not. In either case, the entire pneumatic system or any portion of the temporal bone and its constituent structures may be involved including the tympanic membrane, ossicles, labyrinth, facial nerve, mastoid cells, arteries, veins, and the bones of the middle and posterior fossae. Conductive hearing loss almost always accompanies this condition. Chronic negative pressure in the middle ear leads to a retraction of the tympanic membrane. The area of retracted tympanic membrane forms a closed pocket or cyst where epithelial debis from the lining of the tympanic membrane collects. This expanding collection of tympanic membrane debris erodes the ossicles, leading to a conductive hearing loss. It can also erode the bony labyrinth or bone of the cranial fossa.

V. THE INNER EAR: GENERAL PROPERTIES OF RECEPTORS

The inner ear is a complex structure that serves both hearing and the sense of balance. Disorders of the inner ear are common and can involve one or the other, or both sense modalities. Knowing how the inner ear functions normally in crucial to our understanding of the mechanisms involved in its malfunctioning. In this section we take up both the auditory and vestibular labyrinths. We draw parallels between their fundamental operations and point out how they achieve their differential sensitivity.

Objectives:


1. Describe the general structure of the inner ear, including the names and locations of each of the six receptor organs.

2. Describe the four structural features common to all inner ear receptor organs.

3. Describe and draw the structure of the sensory cell common to all inner ear receptors: the hair cell, including the distribution of terminals of eighth-nerve fibers. Know what is meant by structural (or morphological) polarization of a hair cell.

4. State the adequate stimulus and describe the transduction mechanisms operating in all inner ear hair cells. Know what is meant by functional polarization.

Objective 1: General overall structure of the inner ear

http://www.neurophys.wisc.edu/h&b/textbook/chap-5.html#Obj4









The membranous labyrinth consists of endolymph-filled vesicles and canals in communication with each other. These vesicles and canals have thin transparent walls which are composed of a connective membrane of mesenchymal origin and are lined on the inner side with an epithelium of ectodermal origin. Within the membranous labyrinth there are six specialized receptor organs within which are

found modified sensory epithelial cells. One of these specialized area, called the organ of Corti, is specialized to transduce sound; the remaining five, the maculae of the utricle and saccule and cristae of the three semicircular canals, are sensitive to head position or head movement. Figure V-1 illustrates the general layout of the inner ear.

Objective 2: Common characteristics of inner ear structure

While the structure of receptor organs in the cochlea and the vestibular labyrinth exhibit structural specialization that provides selectivity to a particular sensory input, their general characteristics and functional properties are very similar. For each of the six receptor organs of the inner ear we can make the following statements:

1. The receptor cells are modified epithelial cells. Such a cell is cylindrical or flask-shaped and is equipped at its apical end with a bundle of sensory hairs called stereocilia. Often found located adjacent the stereocilia is a single kinocilium, or true cilium. The presence of sensory hairs gives these cells their name: hair cells.

2. The hair cells are held in position by a system of supporting cells.

3. Each receptor organ is equipped with an auxiliary structure with which the stereocilia of the hair cells come into contact. Movement of the auxiliary structure relative to the hairs displaces the cilia, and this displacement is the adequate stimulus for activating the hair cell.

4. Each hair cell is innervated at its base by afferent endings of sensory nerve fibers and by one or several endings of efferent centrifugal nerve fibers. Synaptic contact between hair cell and nerve fiber is chemical in nature, although the identity of the neurotransmitter(s) involved has not be made. In

the organ of Corti the sensory cells are innervated by the fibers of the cochlear nerve; in the maculae of the saccule and utricle and the cristae ampullae they are innervated by the fibers of the vestibular nerve.

Objective 3: Hair cells are the receptor cells of the inner ear

Hair cells of the inner ear are epithelial cells which, in the vertebrate embryo, originate from the surface ectoderm and not from the neural tube that forms the central nervous system. Hair cells all

morphologically similar. Figure V-2 illustrates schematically the major structural features of a hair cell.

The cell is cylindrical or flask-shaped and is equipped at its apical end with a bundle of sensory hairs called stereocilia. Hair bundles occur in a variety of sizes and shapes and, for any given hair cell, may vary in number. As a rule, stereocilia uniformly lie in a hexagonal array and increase in length from one edge of the hair bundle to the other (Figure V-3). A single kinocilium is present, which differs in structure from stereocilia and which is asymmetrically placed at one edge of the stereocilial bundle, usually adjacent to the tallest of these (in the cochlea the kinocilium is present in early development but is absent in the adult). The kinocilium is associated with an intracellular organelle, the basal body. This orientation of hair cells with respect to the kinocilium is referred to as morphological polarization. Morphological polarization is related to the directional sensitivity of the hair cell.

In the receptor organ, hair cells rest on a basal lamina and are joined to one another by tight junctions. As epithelial cells, they lack axons and dendrites. Instead, hair cells make synapses onto afferent nerve fibers of the eighth cranial nerve and also receive efferent synaptic contacts from axons originating in the brainstem. Thus, the neural mechanisms that underlie excitation and sensory processing in the cochlea and vestibular labyrinth are unlike those of most other mechanoreceptor nerve preparations (e.g., Pacinian corpuscle) in that the receptor is NOT part of the sensory neuron but is a specialized epithelial cell which excites the sensory neuron by synaptic transmission.

Stereocilia consist of actin filaments within a tubular membrane. Because the filaments are crossbridged, each stereocilium behaves like a stiff rod which

pivots at its base. The adequate stimulus for the hair cell is displacement of the hair bundle. The hair cell is a mechanoreceptor, producing an electrical signal, or receptor potential, in response to

mechanical stimulation of its hair bundle. It is the relative motion between the hair cell and the auxiliary structure specific to the receptor organ that provides the displacement.

Hair cells are extraordinarily sensitive to cilia displacement. If the height of one cilium is scaled to the height of Chicago's Sear's Tower, the movement of the tip of the cilium at the threshold of hearing is equivalent to a two-inch displacement of the top of the Tower (Figure V-4)! At threshold detection levels in humans, the auditory or the vestibular systems are operating at the same order of magnitude of displacement as that of thermal motion.

Objective 4: Mechano-electrical transduction by hair cells - Functional polarization

Although it has been possible to learn a considerable amount about the operation of hair cells in the mammalian inner ear, a more nearly complete picture of the transduction process has emerged from in vitro studies of hair cells from lower vertebrates. Under these conditions it is possible to insert a glass microelectrode into a hair cell and thereby record the intracellular electrical events when the hair bundle is deflected. In a quiescent cell, the resting membrane potential is around 60 mv.

As in the case in neurons, the electrical signals in the hair cell originate from the flow of ionic currents across the membrane through specific pores, or ion channels. The identity of ion channels and their kinetics is studied electrophysiologically using 'patch clamp' techniques. Movement of the hair bundle in the depolarizing direction leads to increases in membrane conductance, meaning that the membrane becomes more permeable to positively charged ions. It is now believed that the ion channels admitting positively charge ions during positive deflection, the transduction channels, are located at the tips of the stereocilia. These are opened by a mechanical linkage that exists between adjacent hairs of the hair bundle. Movement in the opposite direction closes channels thereby stemming the flow of ionic current. Additional ion channels with different specificity and function are located on the cell body and in the synaptic region. It is known that amino glycosides, such as occur in certain ototoxic antibiotics, interfere with channel operation. Also, very loud sound may physically disrupt the bundle structure. Thus, interfering with ion-channel operations or changing the physical arrangement

of the stereocilia is sufficient to alter mechanoelectrical transduction in the inner ear.

The change in membrane potential associated with movement of the hair bundles results in changes in the discharges of eighth-nerve afferent axons connected at the hair cell base. Depolarization of the hair cell leads to increased firing of the fiber, while hyperpolarization results in cessation of firing. This relationship between the electrical properties of hair cells and their eighth-nerve discharge patterns is referred to as functional polarization. Figure V-5 illustrates functional polarization of hair cells.

VI. THE INNER EAR: THE COCHLEAObjectives:


1. Describe the structure of the Organ of Corti, including the following: Reissner's membrane, stria vascularis, tectorial membrane, hair cells, supporting cells, tunnel of Corti, VIII nerve fibers, spiral ganglion, basilar membrane, osseous spiral lamina.




2. Describe the traveling wave pattern of vibration on the basilar membrane. Know the physical properties of the cochlear partition that are responsible for this form of mechanical displacement.

3. Describe the excitation process: how basilar membrane vibration leads to depolarization of the hair cells and stimulation of auditory nerve fibers. Know the structural arrangements in the organ of Corti that make this possible.

4. Describe the innervation pattern within the cochlea.

5. Describe the four coding mechanisms used in the auditory nerve to transmit information from the ear to the brain. Know what is meant by: threshold tuning curve, characteristic frequency, firing rate, place theory of hearing, phase locking, volley theory of hearing.

6. State what is meant by otoacoustic emissions.

Objective 1: Structure of the cochlea

Figure VI-1 illustrates increasingly expanding cross sections of the cochlea. In Figures VI-1B and C are seen the scala vestibuli and scala tympani separated by the cochlear partition, except in the apical turn where the two scalae are in continuity via the helicotrema. Within the modiolus is seen the spiral ganglion. The central processes of spiral ganglion neurons form the cochlear nerve and exit the temporal bone in the internal acoustic meatus. The cochlear partition, which includes the endolymph-filled scala media, is bounded by Reissner's membrane and the basilar membrane, on which sits the auditory receptor organ, the organ of Corti.

Expanded views of a cross section of one turn of the cochlea are shown in Figure VI-1D. At this magnifications it is possible to identify clearly the tectorial membrane, inner and outer hair cells, supporting cells and distal terminals of auditory nerve fibers.

The sensory cells of the organ of Corti are held in place by specialized supporting cells. Some of these are relatively rigid structures containing stiff protein filaments. The hair cells are tightly gripped at their apical ends by processes of one of the filamentous supporting cells, the phalangeal cells.












These phalangeal processes together form the reticular lamina.

The lateral wall of the cochlear duct is formed by the stria vascularis. Histologically, the stria is a very highly specialized stratified epithelium, with capillaries that invade the territory of the epithelium. The lower cells of the epithelium are richly supplied with mitochondria and basal infoldings which are characteristics of cells involved in the control of water and electrolytes. The stria plays an important role in cochlear function by producing endolymph, whose ionic composition more closely resembles intracellular fluid than it does CSF. In the process of producing endolymph (the fluid of the scala media), the stria is also responsible for maintenance of the +90 mv endocochlear potential within the cochlear duct. Endolymph is resorbed through the wall of the endolymphatic sac into the vessels of the dura matter.

Reissner's membrane is the third side of the cochlear duct and separates perilymphatic space (scala vestibuli) from endolymphatic space (cochlear duct). It is two layers thick being composed of simple squamous epithelium (toward endolymph) and a connective membrane.

As can be seen in the cross section of the cochlear spiral (Figure VI-1B), the bony cochlea becomes smaller and smaller in cross-sectional area as the apex is approached. At the same time the basilar membrane becomes progressively wider toward the apex. This is because the osseous spiral lamina is broadest at the cochlear base where the basilar membrane is only about 0.16 mm wide

(in humans); at the apex the basilar membrane has broadened to about 0.52 mm. This variation in width of the basilar membrane correlates with its variation in stiffness which, in turn, underlies the kind of mechanical motion it undergoes in response to sound

waves.

There are two kinds of hair cells in the organ of Corti: inner hair cells (IHC) and outer hair cells (OHC) (Figure VI-2). Usually there is but a single row of IHCs and three to four rows of OHCs. In humans there are approximately 3,500 IHCs and 12,000 OHCs. The hairs of both species are related to the auxiliary structure of the organ of Corti, the tectorial membrane. Unlike vestibular hair cells, these have no kinocilium in adult life (although a remnant, the basal body, remains in the cell body). They differ from one another in their shape and in the pattern of the stereocilia. They are also innervated differently. Nearly 95% of the afferent fibers of the cochlea division of the eighth nerve originate at the base of inner ear cells. Most of the efferent input to the cochlea from the central nervous system reaches the bases of the OHCs. Motion of the basilar membrane, under the influence of sound, results in a shearing motion between the stereocilia and tectorial membrane resulting in activation of the hair cell and the afferent auditory nerve fibers connected to it.

Objective 2: Cochlear mechanics - the Traveling Wave

Our understanding of the operations of the inner ear began in the late 19th century. Hermann von Helmholtz, a brilliant German physicist at the time viewed the basilar membrane in the inner ear as a series of mechanical resonators arranged like the strings of a harp, varying in tuning from high frequency at the base of the cochlea to low frequency at the apex. According to this view, the basilar membrane vibrates maximally at the spot whose resonance frequency matches the frequency of the stimulus, giving rise to excitation of only those nerve fibers which innervate that spot. This is a form of labeled-line, or "place," theory of pitch perception in which the locus of maximal vibration along the cochlear spiral determines the perceived pitch by activating a specific small group of nerve fibers. Many years passed before Helmholtz's theory was tested by direct observation of the mechanical vibration patterns of the inner ear by the Hungarian physicist, Georg von Bekesy, a feat which earned him the Nobel Prize.

Over the past 50 years considerable progress has been made using modern physical recording methods in further elucidating the mechanical properties of the organ of Corti. We can now state the mechanical events that take place in the cochlea when a sound wave

enters the inner ear:

1. Sound waves normally enter the inner ear via the oval window and are transmitted rapidly through the cochlear fluid. Movement of the oval window by the stapes footplate is met with equal and opposite action of the round window (Figure VI-3).

2. The basilar membrane is a resonant structure. The basilar membrane is deflected in response to sound waves in the inner ear. Each location along the basilar membrane responds best to a small range of sound frequencies. The basilar membrane is NOT under tension, however. Therefore, it does NOT operate as a series of independent tuned resonators, like the strings of a harp. Figure VI-4 illustrates resonance curves derived from different regions along the basilar membrane.

3. The deformation of the basilar membrane is a traveling wave. When motion of the stapes establishes a sound wave in the fluid of the inner ear, each small region of the basilar membrane deflects in response to this pressure with a time

delay that depends upon its own mechanical properties. The wave then diminishes rapidly in both amplitude and velocity as it continues to move toward the apex. Figure VI-5A illustrates a traveling wave at four successive instants in time, together with

the envelope of the peaks of the displacement.

The part near the base, with its high resonance frequency and correspondingly short mechanical "time constant," moves first, followed by successively more apical segments. The displacement thus constitutes a traveling wave of deformation which progresses from base toward apex. Figure VI-5B is a three-dimensional rendition of the traveling wave envelope.

4. Different regions of the basilar membrane respond maximally to different sound frequencies based on the local physical properties. There is a systematic shift in the locus of maximal vibration from the apex toward the base as the frequency of a pure tone stimulus is raised. The curves in Figure VI-6 show in cartoon form the travelling wave for low, mid, and high frequency sounds. Thus, the basilar membrane is said to be tonotopically organized.

5. The primary factor which changes the tuning along the cochlear partition is the variation in width of the basilar membrane (Figure VI-7). This results in a systematic change in effective stiffness from base to apex.

6. Hair cells are 'active' participants in the mechanoelectric transduction process. By mechanisms not yet fully understood, outer hair cells change shape under the influence of sound. Shape change is driven by a 'molecular motor,' which has not yet been characterized structurally or chemically. This mechanism, nonetheless, renders the operation of the organ of Corti highly non-linear, which is essential to account for the ears phenomenal sensitivity and dynamic range of frequency and intensity.

Objective 3: Excitation of auditory nerve fibers

When sound energy is introduced into the inner ear, the resultant up-and-down motion of the basilar membrane produces shearing motion between the stereocilia projecting from the apical surfaces of hair cells and the tectorial membrane. Shearing occurs because of the relative positions of the hinge points for the basilar membrane and the tectorial membrane (Figure VI-8). This shearing action displaces the stereocilia which, in turn, results in cellular depolarization or hyperpolarization through the transduction mechanisms described earlier.

The transduction process results in a non-propagated receptor potential, which is associated with the release of neurotransmitter at the base of the hair cell and the excitation of primary afferent neurons. The propagated, all-or-none impulses that arise in the auditory nerve fibers then carry to the CNS the coded information concerning the auditory stimulus. All information about our acoustic environment is, thus, carried in trains of nerve impulses in bundles of auditory nerve fibers of the left and right ears.

What is the innervation pattern in the organ of Corti and what are the codes used in carrying acoustic information from the ear to the brain?

Objective 4: Innervation of the organ of Corti

There are two types of fibers that innervate hair cells in the organ of Corti: afferent and efferent. Afferent innervation comes via peripheral processes of bipolar neurons in the spiral ganglion; the central processes of the spiral ganglion neurons project to cells in the brainstem. This accounts for about 95% of the axons in the auditory nerve. The remaining efferent innervation arises from neurons in the brainstem and carries information from the brain to the cochlea.

Afferent innervation: The greatest number of afferent fibers make contact with inner hair cells (Figure VI-9). There is little divergence of afferent fibers in the cochlea: each auditory nerve fiber contacts but one inner hair cell. Each inner hair cell contacts as many as 20 auditory nerve fibers. A relatively small number (about 5%) of afferents innervate outer hair cells.

Efferent innervation: Axons arising from neurons in the superior olivary complex of the brainstem reach the cochlea (in the olivocochlear bundle) where they synapse mainly at the base of outer hair cells. The bundle arises from olivary neurons on both sides of the midline, forming two olivocochlear systems.

Objective 5: Coding of information in the auditory nerve

Like in all other sensory systems, information about the outside world is carried to the brain in trains of all-or-none action potentials in ensembles of peripheral

afferent nerve fibers. As applied to the auditory system, the term "code" is simply a way of describing the manner in which information about sound is represented in such neural activity.

The Place Principle of Hearing - a labeled-line code

Johannes Müller suggested more than a century and a half ago that different nerve fibers elicit different sensations by virtue of their

"specific nerve energies." In modern terms this theory states that: different sets of auditory nerve fibers, when active, elicit different auditory sensations by virtue of their central connections. It is most often applied to the perception of pitch and the quality of tones. This is because auditory nerve fibers exhibit a selectivity for the frequency of a sound. Figure VI-10A illustrates this with a curve that relates the threshold of response of a single auditory nerve fiber to

sound frequency. Such a curve is called the threshold tuning curve, and it mirrors the mechanical tuning of the spot on the basilar membrane innervated by the fiber. The frequency to which a fiber is

most sensitive is referred to as the characteristic frequency (CF). Different fibers will have a CF dependent on the location of the hair cell to which they are attached (Figure VI-10B). Because these auditory nerve fibers each innervate a single inner hair cell, and because the basilar membrane is itself tonotopically organized in a mechanical sense, the characteristic frequency of a nerve fiber is directly related to a location (or a 'place') along the basilar membrane.

Figure VI-11 shows the relationship between cochlear place and the regions of greatest frequency sensitivity. Hair cells that are traumatized, either mechanically, by disease or by ototoxic drugs, exhibit very poor frequency selectivity. Destruction of hair cells results in loss of hearing sensitivity in the frequency region represented by those cells. This is a major cause of a sensorineural hearing loss.

Rate code

Acoustic information may be carried by the rate or frequency of the discharge of a neuron. The peripheral encoding of sound intensity is associated with this type of code. Figure VI-12 illustrates the change in discharge rate as a function of sound pressure level for a single fiber of the auditory nerve. Over a range of some 40-70 dB a very small change in intensity results in a relatively large change in discharge rate. Because a single fiber can not respond over the full listening range of 120 dB, intensity must be coded in a population of fibers with different thresholds, as illustrated in Figure VI-12B.

Temporal code - the Volley Theory of Hearing

It has long been known that the auditory system preserves temporal information. Listeners use temporal cues, along with others, in encoding the low frequency information in our language -- the vowel sounds -- and in preserving interaural time differences used by listeners in localizing the source of a sound

in space. Temporal coding comes about because the hair cells in the cochlea are functionally polarized, which means that deflection of the stereocilia in one direction is excitatory for the hair cells and movement in the opposite direction is inhibitory. Thus, when the basilar membrane vibrates in response to low-frequency signals, below about 4 kHz, the hair cells in the region of vibration exhibit an alternating excitation-inhibition at the frequency of vibration. This, in turn, generates action potentials in auditory nerve fibers attached to those hair cells. The action potentials in the nerve reflect the time-pattern of excitation and inhibition in the hair cell. The result is a train of nerve impulses time locked to the individual cycles of the acoustic stimulus. For a simple sine wave, the impulses are generated around a particular point on the sine-wave cycle, a process that is referred to as phase locking. Because of its refractory period, an auditory nerve fiber can not respond to every successive cycle of a stimulus. When it responds, however, it does so around a constant phase angle of the stimulus. Consequently, the impulses occur around integral multiples of the period of the sine-wave stimulus (Figure VI-13).

A population of auditory nerve fibers, all phase-locking to the same stimulus, represent in their combined discharge pattern the complete temporal representation of the stimulus. The combined time sequence of events in called 'volleying,' and the theory that describes it as a way of carrying information is called the 'Volley Theory of hearing.' Figure VI-14 illustrates this phase-locking of an ensemble of auditory nerve fibers to a low frequency pure tone. Each fiber is incapable of responding to every cycle of the stimulus, but collectively they can do so.

Ensemble code

It is unlikely that any natural stimulus engages but a single inner hair cell (IHC) and thereby excites but one or even that small number of auditory nerve fibers associated with a single receptor cell. Indeed, a single auditory nerve fiber is probably not capable of encoding unambiguously the frequency or intensity of a sound, as described above. Rather, when some finite number of receptor cells is brought to threshold level of activation in a temporal sequence that is governed by the velocity and slope of the traveling wave envelope on the cochlear partition, the information about that displacement pattern is coded in

terms of the profile of the rate and timing of activity across all or part of the eighth-nerve array.

Knowledge of coding mechanisms is used in diagnosis and treatment of hearing disorders

Increasing clinical use is being made of our growing knowledge of the neural coding of auditory information. Recording of the compound action potential of the auditory nerve by electrodes placed near the round window is used in certain situations for testing the integrity of the cochlea and auditory nerve. Hearing aids are becoming highly sophisticated, with their circuitry tuned to take advantage of the function of surviving receptors. Surgeons are now implanting electrode arrays inside the cochleas of patients who have suffered loss of hair cells but who have at least partially intact auditory nerves. Such a prosthetic device permits direct electrical stimulation and partial restoration of hearing. Success in these efforts requires that the spatial and temporal patterns of neural excitation which occur in the normal ear be duplicated as closely as possible.

Efferent effects

Efferent axons, arising in the brainstem in the vicinity of the superior olivary complex, contact mainly outer hair cells. Stimulation of efferents has a profound effect on the afferent input to the brain. Such effects are mediated via active processes in outer hair cells. Outer hairs cells change shape in response to efferent activation, which in turn alters the micromechanical action of inner hair cells, and hence the flow of information from the ear to the brain.

Objective 6: The cochlea produces otoacoustic emissions

The cochlea, once thought to be a 'passive' transducer of sound energy into electrical nerve impulses, is now knows to contain 'active' elements. This means that the cochlea not only responds to energy imposed on it, but it also generates energy. One way in which this is now believed to occur is by movement of the outer hair cells. A change in configuration of an outer hair cell may have a substantial influence on the mechanical response of inner hair cells, the main transducers and signallers of acoustic information entering the inner ear.

Some years ago it was discovered that in response to a brief sound there appeared in the ear canal a second brief, time-delayed sound - an echo of the first. This acoustic echo is now known to be generated by 'active processes' in

the cochlea. Sounds appear in the ear canal under other conditions as well. They are called otoacoustic emissions, and may appear spontaneously as well as being evoked by an external sound. Emissions are generally believed to arise from outer hair cells under control of efferent input from the central nervous system. Studies of this phenomenon have added greatly to our understanding of the non-linear properties of the normal cochlea and results are being applied to developing tests for sensorineural hearing loss.

VII. THE INNER EAR: THE VESTIBULAR APPARATUS

Objectives:


1. State the three major functions of the vestibular system.

2. Describe the structure of the vestibular receptors, including the cristae, maculae, cupula, otolithic membrane, hair cells, vestibular (Scarpa) ganglion, vestibular nerve. Know the adequate stimulus for each receptor organ.

3. Describe and diagram the spatial arrangements of hair cells of the maculae and cristae and state how these spatial patterns relate to the directional sensitivity of each receptor organ.

4. Describe the coding properties of vestibular nerve fibers under conditions of rest, angular acceleration and tilting the head.

Objective 1: The vestibular apparatus in humans serves three major functions:

1. It is the primary organ of equilibrium and thus plays a major role in the subjective sensation of motion and spatial orientation.










2. Vestibular input to areas of the nervous system involved in motor control elicits adjustments of muscle activity and body position to allow for upright posture.

3. Vestibular input to regions of the nervous system controlling eye movements helps stabilize the eyes in space during head movements. This reduces the movement of the image of a fixed object on the retina.

The block diagram below illustrates the role of the vestibular system in control of posture, eye movements and perception of orientation.

Objective 2: The vestibular labyrinth contains five receptor organs

Sense organs of the vestibular system are mechanoreceptors. The vestibular apparatus or vestibular labyrinth contains the three semicircular canals, the utricle and the saccule. The semicircular canals are so arranged that they lie in planes orthogonal to one another (Figure VII-1).

Semicircular canals sense angular acceleration

Sense cells within each organ are hair cells, which are specialized epithelial cells, hair cells, having ciliary tufts protruding from their apical surface. The three semicircular canals have swellings, called ampullae and within each ampulla is the sense organ, called the crista. In the cristae the hairs of the hair cells are embedded in a gelatinous mass,

called the cupula, which extends across the ampulla (Figure VII-2).

Fluid inertia during angular acceleration results in displacement of the cupula and bending of the sensory hairs. This is the

adequate stimulus for exciting the hair cell. Figure VII-3 illustrates movement of the cupula and its embedded hairs during rotation first in one direction and then in the opposite direction.

Canal functions can be tested clinically. This may be done by rotating a patient in a special chair (creating angular acceleration) or by irrigating the ear with cold or warm water (caloric test). In order to test the function of each canal receptor organ, it is necessary to place the canal in its most

effective position. For example, the plane of the horizontal or lateral canal is 30 degrees off the horizontal during normal upright posture. Thus, if one wishes to test the lateral semicircular canal, the head of the seated patient is tilted to bring the canal to a horizontal position (Figure VII-4).

The utricle and saccule sense linear acceleration or head tilt (gravity)

The sense organs within the saccule and utricle are called maculae (Figure VII-5). Both the saccular macula and utricular macula are covered by a gelatinous mass called the otolithic membrane containing concretions of calcium carbonate called otoconia or otoliths.

This loading of the cilia by inertial mass makes the organs sensitive to linear acceleration and changes of position of the head in the gravitational field. Figure VII-6 is a schematic representation of the macula and its relationship to the otolithic membrane.

Objective 3: Vestibular hair cells are organized differently in different receptor organs

In the vestibular organs of birds and mammals, two types of hair cells are usually distinguished although they may in fact represent the extremes of a spectrum of morphological types. Along with the most common type, called type II, are found flask-shaped cells, called type I enclosed up to their neck by a large nerve chalice which may enclose more than one cell. Between the hair

cell and membrane of the chalice is a complicated pattern of contacts where the synaptic space is reduced to less than 100A. Synaptic terminals densely packed with vesicles are regularly observed in contact with the base of the chalice. These are interpreted as being presynaptic terminals. Type II hair cells are innervated by several thin nerve branches forming synaptic contact with the bottom of the cell. Efferent endings are presynaptic to the hair cell and are filled with vesicles.

Afferent endings are formed by the distal branches of bipolar cells of the vestibular (Scarpa's) ganglion. Terminals may be transitional between chalice and bouton-type endings. Information is transmitted between hair cell and eighth nerve terminal by normal chemical transmission.

Adequate Stimulus of the Hair Cell

The vestibular hair cells, like those in the cochlea, are directionally sensitive displacement detector. During head tilt or head rotation, lateral force is transmitted to the sensory hair bundle via the overlying auxiliary structure (cupula or otolithic membrane). As in all hair cells, regardless of their differences in morphology, the resultant displacement of the stereocilia opens ion channels resulting in inward ionic current. This leads to a receptor potential, release of neurotransmitter and the generation of action potentials in the distal processes of afferent nerve fibers.

The output from the crista ampullaris is proportional to angular displacement of the cupula. The output of the maculae of the utricle and saccule are excited by very small linear movement of the otolithic membrane. Thus, while the adequate stimulus for the different sense organs may differ, the adequate stimulus for the sensory cell appears to be the same, shearing displacement of the sensory hairs.

Directional Sensitivity of the cristae - Functional polarization of the receptor organ

Recall that hair cells are functionally polarized: displacement of the sensory hair bundle in the direction in of the kinocilium is excitatory, resulting in

depolarization of the hair cell and increased firing of vestibular nerve fibers; displacement in the opposite direction is inhibitory and results in hyperpolarization of the hair cell and reduced firing in the vestibular nerve.

The cristae of the semicircular canals, as receptor organs, are also functionally polarized. In each crista ampullaris all cells are oriented with their kinocilia pointing in the same direction. Angular acceleration causes deflection of the cupula in only one direction and thus affects simultaneously all the hair cells oriented in that direction (Figure VII-8). Thus, all afferent fibers innervating each cristae fire together depending on the

direction of angular motion.

The orientation of the hair cells is such that the receptors of the horizontal canal, for example, are excited by deflection of the cupula towards the utricle (utriculo-petal endolymphatic flow) whereas the two vertical canals are excited by deflection of the cupula away from the utricle (utriculo-fugal endolymphatic flow), as illustrated in Figure VII-9.

Otolithic organs are omnidirectional

The utricle and saccule serve in the maintenance of body posture by responding to linear acceleration and changes in head position. The adequate stimulus is displacement of the otolithic membrane which is free to move in any direction determined by the direction of acceleration. Again, as in other inner ear receptors, the hair cells are functionally polarized. The spatial arrangement of hair cells in the utricle is very elaborate, however, and hair cells are not oriented in the same direction. Instead, the hair cells are oriented in a "fanning" fashion on either side of a line called the strioli of Werner, dividing the utricle into medial and lateral portions. Thus, for essentially any direction of otolithic motion a pair of orthogonal hair cells can be found to supply information about stimulus direction. Figure VII-10 is a schematic drawing of directions of polarization of sensory cells in the maculae of the utricle. A similar situation obtains for the saccule although the orientation of hair cells there is not identical to that of the utricle.

Objective 4: Vestibular nerve fibers encode information about head position and motion

A microelectrode inserted into the ampullar nerves (or cells of Scarpa ganglion) innervating the canals records a resting discharge from individual nerve fibers. The average resting discharge with the head motionless is about 90 impulses per second in canal fibers and about 60 per second in fibers innervating the utricular macula. If we assume that there are about 20,000 fibers in the vestibular nerve then the central nervous system (and the vestibular nuclei in particular) with the head motionless receives a tonic input amounting to 1.5 x 106 impulses per second. This high resting discharge is necessary for the expression of functional polarization of the hair cells; when the cupula moves towards the utricle in the horizontal canal there is an increase in discharge rate; movement in the opposite direction results in a decrease in discharge rate. In the vertical canals excitation occurs when the cupula moves away from the utricle and inhibition occurs when the cupula moves toward the utricle.

The high resting discharge also makes the hair cell very sensitive since very small movements of the cupula in either direction affect the discharge of the fiber. This may account for the low human perceptual threshold to angular acceleration (0.1 deg/sec2).

Differential sensitivity to sound, head position and head movement

How can a hair cell respond differentially to sound, to head position and to head movement? The answer lies not in the function of the hair cell per se, but in its relationship to other elements of the receptor organ. A hair cell responds only to the deformation of its cilia. Thus, the various receptor organs of the inner ear, each of which is equipped with hair cells, supporting cells and an auxiliary structure, are specialized for the kind of mechanical distortion they can detect. The organ of Corti, but not the vestibular organs, is set in motion by small pressure waves set up in the cochlear fluid; this movement results in a shearing motion between the auxiliary structure (tectorial membrane) and the sensory hairs on hair cells. Neither angular or linear acceleration of the head, nor head tilt can create such a shearing motion. Angular acceleration, on the other hand, is a most effective way to create in the ampullae of the semicircular canals a shearing motion between the hair cell cilia of the cristae and the overlying auxiliary structure, the cupula. While refractory to angular acceleration and to sound, the otolithic organs in the saccule and utricle are especially sensitive to linear acceleration and to head tilt (gravity). Again, the action is between the auxiliary structure (the otolithic membrane) and the cilia of the hair cells in the maculae.

Projections of primary vestibular nerve fibers

The maculae and cristae are innervated by bipolar neurons of the vestibular ganglion. The central processes of these cells form the vestibular nerve which enters the brain stem at the cerebellopontine angle medial to the cochlear nerve. The vestibular nerve bifurcates into short ascending and long descending branches which are distributed to the vestibular nuclei. Some vestibular nerve fibers continue without interruption to the ipsilateral cerebellar cortex and one of the deep cerebellar nuclei. Most primary vestibular fibers terminate differentially in the four main vestibular nuclei in the floor of the fourth ventricle. The vestibular nuclei give rise to secondary vestibular fibers which project to specific portions of the cerebellum, certain motor cranial nerve nuclei and to all levels of the spinal cord.

[NEXT]

VIII. THE INNER EAR: DISORDERS OF THE INNER EAR

Objectives:


1. State what is meant by sensorineural hearing loss, and distinguish it from conductive hearing loss.

2. Name the two major categories of inner ear disorders, and the major disorders associated with each of them.

3. Describe the sites of lesions of each of the disorders.

Objective 1: Impairment in the cochlear transduction mechanisms, in auditory nerve transmission, or in both, results in a sensorineural hearing loss







Because both the auditory and vestibular structures of the inner ear are derived similarly embryologically and because they share the same fluid environment, a disorder of one frequently includes a disorder of the other, resulting in a complex of symptoms.

The inner ear is vulnerable to damage or destruction from a variety of sources. Malformations of the labyrinth may be inherited or acquired. Inflammatory and metabolic processes may disrupt permanently normal vestibular and auditory function at the level of the end organ. Drugs and other substances have teratogenic effects on the inner ears of the fetus and destructive effects on the cochlea and vestibular organs in young and adult individuals. Trauma, either physical or acoustic, can cause hearing loss and vestibular damage. Viral infections may destroy the receptor organs, especially in utero.

Objective 2: Disorders of hearing fall into two broad categories: congenital and acquired

Congenital disorders

Recall that the auditory placodes in humans are formed at three weeks and the membranous labyrinth differentiates between the sixth and seventh weeks. Before and during this period opportunity exists for the maldevelopment of the inner ear.

Dysplasias - there are several morphologic congenital abnormalities of the inner ear. These include maldevelopment of the bony labyrinth, the membranous labyrinth, or both. Within the membranous labyrinth there may be maldevelopment of any or all of the receptor organs and their supporting cells. Severe sensory neural hearing loss results.

Hereditary syndromes - include labyrinthine disorders that are associated with no other abnormalities, and those that are associated with external ear malformations, integument disease, ophthalmic lesions, CNS lesions, skeletal malformations, renal disease, and miscellaneous defects. One example is Usher's Syndrome, which is a diplasia that accounts for about 10% of all hereditary deafness; there is no vestibular involvement and it is associated with retinosis pigmentosa. Another cochlear deformation is associated with Waardenburg's Syndrome. This accounts for 2-3% of all cases of congenital deafness in the U.S. It is associated with a white forelock and widened intercanthal distance.

Commonly acquired viremic diseases

Maternal rubella can cause profound congenital sensorineural hearing loss. It is exceptionally virulent to fetal organs, especially the cochlea, when it occurs in the first trimester of pregnancy. The severity of the hearing loss varies considerably from patient to patient and to a lesser degree from one ear to the other in the same individual. The pathogenesis of this disorder is not clear.

Cytomegalovirus (CMV) - is a common congenital viral infection which is associated with a number of neurological disorders including hearing loss. In temporal bones of infants who died from CMV there are reported changes in the stria vascularis with cochlear duct and saccular hydrops.

Disorders associated with chromosomal abnormalities - Trisomy syndromes

Trisomy refers to the condition in which the nucleus of the cell contains an extra chromosome. Thus, there are under these circumstances 47 chromosomes rather than the normal 22 pairs of autosomes and 2 pair of sex chromosomes. Depending on the chromosomes, there may be multiple physical anomalies including malformed ossicles and underdeveloped otic capsule and organ of Corti.

Neonatal hyperbilirubinemia - bilirubin encephalopathy

Bilirubin, a yellow pigment, is the major end product of hemoglobin metabolism. It has long been known that, in human neonates, there is a close association between elevated blood bilirubin levels and disorders of the central nervous system. The most extreme neurological consequence of hyperbilirubinemia is referred to as "kernicterus" - a condition that may include hearing impairment, choreoathetosis, spasticity, oculomotor problems, cognitive dysfunction, and mild forms of mental retardation. Classical kernicterus in term infants, resulting from Rh incompatibility, has been in many places nearly eliminated by prophylaxis and the use of early exchange transfusion. With the decrease in the incidence of classical kernicterus induced by Rh incompatibility, attention has shifted to the occurrence of this disorder in premature and gravely ill infants. The hearing loss that accompanies hyperbilirubinemia is of the sensorineural type. In studies of temporal bones of humans and animals with this condition there has been no clear-cut evidence of damage to the inner ear structures. Rather, the damage appears to occur in the auditory nuclei of the brainstem; neurons in the cochlear nuclei, in particular

are severely damaged or destroyed.

Acquired disorders

Lesions acquired at birth

Trauma - Hearing loss may result from trauma that occurs prenatally or during delivery.

Hypoxia and anoxia - Fetal hypoxia around the time of delivery is believed to play a role in some cases of otherwise unexplained hearing loss. Hypoxia or anoxia has been shown in experimental animals to produce lesions of the auditory pathways of the central nervous system and may also produce cochlear damage. Premature infants are at risk for sensorineural hearing loss. Histological signs associated with neonatal hypoxia or anoxia include atrophy of the organ of Corti, labyrinthine hemorrhage and degeneration of neurons in the central auditory pathways of the brainstem.

Traumatic Lesions

Noise induced hearing loss - Excessive noise can cause permanent damage to the cochlea. It may occur as the result of a sudden blast (e.g. gun shot) or it may come because of lengthy exposure to high intensity sound (e.g. factory noise). Even relatively brief exposure to a high-noise environment is potentially hazardous to the health of the organ of Corti as evidenced by the studies done at a 4-hour Bruce Springsteen concert in St. Louis and during the 1987 Twins-Cardinals World Series games in domed stadiums (see following journal abstracts).

Temporary threshold shifts from attendance at a rock concert. W. W. Clark and B. A. Bohne, Central Institute for the Deaf and Dept. of Otolaryngology, Washington University School of Medicine, St. Louis, MO 63110). From J. Acoust. Soc. Am., 79:548, 1986.

The relation between exposure level and hearing loss in rock concert attendees was studied. Six volunteer subjects, ages 16-44, participated. All except the 44 year old had normal hearing sensitivity as revealed by audiometric evaluations made immediately before the concert. They attended a Bruce Springsteen concert at the St. Louis Arena and returned to CID for another hearing test within 30 min. following the concert. Noise exposure was assessed by having two subjects seated at different locations in the arena, wear calibrated

dosimeters during the event. Sixteen hours after the concert all subjects returned for a final audiometric evaluation. Results indicated the average exposure level was 100-100.6 dBA during the 4 1/2 hr concert. Five of the six attendees had significant threshold shifts (<50 dB) predominately in the 4-Khz region. Measures made 16h after the concert and thereafter indicated that hearing returned to normal in all subjects. Although no PTS was observed, comparison of these data with studies of hearing loss and cochlear damage in animal models suggests that these subjects may have sustained some sensory cell loss from this exposure. (Work supported by NIOSH and NINCDS.)

NOISE EXPOSURE DURING THE 1987 WORLD SERIES: CARDS SOUNDLY BEATEN BY TWINS. W.W. Clark, Central Institute for the Deaf, St. Louis, MO 63110. From paper presented at the 1987 meeting of the Association for Research in Otolaryngology.

The 1987 World Series, matching the St. Louis Cardinals and the Minnesota Twins and played at the Hubert H. Humphrey Metrodome in Minneapolis and Busch Memorial Stadium in St. Louis was the first World Series in history in which the home team won every game. Among the factors most commonly cited by players, fans and the press as contributing to the difference in performance in the two stadiums was the extraordinarily high levels of noise caused by cheering fans inside the enclosed Metrodome. To assess the difference in noise exposure to fans and indirectly to players, measures of noise exposure were made during game 4 of the series held in St. Louis and game 6 of the series, held in the Metrodome. Two subjects wore logging dosimeters (Quest M-27, 90 dR criterion, 80 dB threshold, 5 dB trading ratio, A weighting network) while they attended the game. Seats for both games were in approximately the same location: behind 3rd base in the upper deck.

During the 3 hr 11 min. game in St. Louis, spectators were exposed to an average noise level of 90.6 dBA (49.3% of allowable OSHA dose). The maximum level was 117 dBA, and levels above 95 dBA occurred for 13% of the total time. In contrast, spectators at game 6 in the Metrodome were exposed to an average noise level of 94.4 dBA (90.4% of allowable OSHA dose) during the 3 hr 22 minutes game. The maximum level was 114 dBA and levels above 95 dBA occurred for 28.2% of the time. Inspection of the records for 1 minute averages from each game suggested a different pattern of exposure. In St. Louis, the noise level was fairly constant, peaking only in the middle innings when the Cardinals scored. In game 6 in the Metrodome, however, levels declined to a relatively quiet 77.90 dBA when the Cardinals held the lead, but increased to 95-109 dBA in the later innings when the Twins assumed the lead.

These results show clearly that noise levels recorded in the Metrodome significantly exceeded those measured in St. Louis under similar conditions. They suggest that the excessively high levels of noise in the Metrodome may have differentially affected player communication, concentration, and performance of the St. Louis Cardinals, who were not acclimated to playing in such a noisy environment. Finally, because the levels are sufficient to produce significant temporary threshold shift in most individuals, spectators and players at such events should be strongly advised to wear ear protection.

"Warning: Walkmen May Be Dangerous to Your Hearing Health"

There is mounting evidence that miniature Walkman type radios and tape players are a threat to hearing. Powered by as little as a single AA battery, these players can pump out as much as 115 dB at full volume, much of it reaching the ear through foam rubber plugs or earphones that cup the ear. The New York City Health Department took a sound level meter into Manhattan's streets and subways and found among 35-40 headset wearers' levels ranging from 95 to 112 dB. If you walk past someone wearing a headset and you can hear any of what they are listening to, that sound could be at a dangerous level.

The loss is of cochlear origin and is most pronounced in the vicinity of 4 kHz. This frequency corresponds to the frequency region of enhanced sensitivity due to the resonance properties of the external ear. There is a considerable amount of information now available on the pathophysiology of noise-induced hearing loss. Because of this and because this kind of hearing loss is very widespread in today's noise-filled environment, a separate section on the mechanisms of this disorder is presented. The consequence of exposure to intense sound is a temporary or a permanent hearing loss. Whether one or the other condition prevails depends on a number of variables including the intensity, frequency, and duration of the sound exposure. It is believed that the structural damage to the inner ear that accompanies a permanent hearing loss arises from the interplay of mechanical and metabolic processes.

Temporary Threshold Shift

Hearing loss after exposure to long and intense sound often is transient in nature and over time normal hearing returns. This is referred to as a temporary threshold shift (TTS). Many of us have experienced this after, for example, listening to a rock concert. In animals exposed to sounds that create a TTS (as tested in behavioral experiments), there is no evidence for structural changes in

the cochlea despite the fact that the hearing loss may be as great as 50 dB. The mechanisms of TTS are, therefore, unclear. It may be that the structural damage is subtle and has so far escaped detection. Perhaps there is no structural alteration at all and it is all in an altered biochemistry which leads to malfunctioning of the transducer mechanisms or synaptic transmission. It is important to know, however, that different individuals exhibit different susceptibilities to intense sounds and that there is probably a fine line between a temporary threshold shift with little residual damage to the cochlea and a permanent hearing loss.

Permanent Threshold Shift

A permanent threshold shift is accompanied by irreversible cochlear damage. An obvious question to ask is whether there is an orderly relation between exposure conditions, level of permanent hearing loss, and the degree of cochlear injury. While obtaining an answer to this question seems straightforward, it is not, and so far no satisfactory one has been put forward.

Objective 3: There are several mechanisms that underlie peripheral hearing disorders

Our understanding of the cellular/molecular mechanisms that cause anatomical changes in the organ of Corti after intense sound exposure is incomplete. Two processes, mechanical and metabolic, have been suggested as the principal mechanisms responsible for hair cell damage associated with acoustic overstimulation. In general, mechanically induced injuries have a rapid onset whereas those produced by "metabolic exhaustion" have a more gradual onset. Mechanical factors related to excessive movements of the cochlear partition would include disruption of the internal structure of the cell. They might result in tears in the basilar membrane or Reissner's membrane, which would result in the cytotoxic mixture of endolymph and perilymph. Excess motion might also separate the organ of Corti from the basilar membrane or from the tectorial membrane. The synaptic junction between the hair cell and eighth nerve fiber could be widened or the OHC stereocilia may lose their connections with the tectorial membrane.

The long-term changes in hearing consequent to acoustic overstimulation are believed to involve the hair cell metabolic machinery. Structural changes in the endoplasmic reticular system and mitochondria suggest deficits in fuel utilization, protein synthesis, and energy production. There may be disruption in cellular enzyme systems critical to energy metabolism, protein synthesis and

ion transport. It is also suggested that acoustic injury may involve regional ischemia, although there is little direct support for this hypothesis. A third possibility has been suggested that relates both to mechanical and metabolic hypotheses, namely that damage results from changes in the cochlear vascular system.

Inner vs. Outer Hair Cells

Outer hair cells (OHCs) are more susceptible than inner hair cells (IHCs) to acoustic over-stimulation. One reason may be that OHCs, because of their greater distance from the fulcrum of the basilar membrane, undergo greater velocity of motion and hence are at greater risk of mechanical damage. Second, the direct mechanical linkage of OHC stereocilia with the tectorial membrane may enhance this cell's susceptibility. Thirdly, the difference may be metabolic, reflecting the differences in internal organelle structure of the IHCs and OHCs. It is noted that OHCs are also more susceptible to ototoxins. Also, the first row of OHCs seems to be at greatest risk.

Damage to the Stereocilia

Recall that stereocilia act as rigid rods and that this rigidity is imparted on them by compact bundles of microfilaments composed of actin strands cross-bridged with another protein, possibly fibrin. The application of scanning electron microscopy has revealed another dimension to acoustic trauma to the organ of Corti: the disarray, collapse, fusing, elongation, breaking, and/or elimination of sensory hairs. These observations are accompanied by those showing that the crystalline fine-structure of the stereocilia are altered by intense sound, a condition that could reduce the rigidity of these hairs several-thousand-fold. Floppy stereocilia occur when the crystalline lattice along the length of the sensory hair is damaged. The disarray occurs after damage to the base of the hair or its rootlet. The consequences of damage to these cytoskeletal elements would be reduction in efficiency of coupling vibrational energy to the hair and subsequent impairment of hair cell function. It would appear that some forms of stereociliary damage are permanent. Remembering that the tips of the hairs may be location of the ionic current source of the receptor potential, damage to this membrane could have profound effects on the transduction process itself. Fusion and elongation of cilia are not specific to sound exposure and similar observations have been made following ototoxic treatment with antibiotics.

Injury to the Auditory Nerve

Dendritic nerve endings beneath damaged IHCs and OHCs, including their mitochondria, appear swollen after high-intensity noise exposure. Under these sound conditions there may be signs of degeneration of spiral ganglion neurons as well. The question of whether there can be nerve damage or loss without hair cell destruction has not been answered. The mechanisms of nerve damage may include degeneration secondary to hair cell loss, direct mechanical injury, and/or metabolic dysfunction.

Changes in the central auditory system as a consequence of cochlear damage

Injury to the receptor cells and auditory nerve can result in degenerative changes in the cochlear nuclei of the brainstem and transneuronal atrophy in the superior olivary complex and inferior colliculus. The available evidence also supports the conclusion that the developing ear and brain is more susceptible to acoustic injury than those of the adult. This has been observed in a wide variety of mammalian species and thus there is good reason to believe that it would

hold for humans as well.

Temporal bone fractures

Fractures may occur along the longitudinal axis of the temporal bone or transversely to this long axis. Transverse fractures are usually more serious because they extend into the labyrinth resulting in a total loss of hearing and destruction of the vestibular function. Both kinds of fractures may injure the facial nerve resulting in a facial nerve paralysis.

Cochlear concussion

A cochlear concussion results from a blow to the head not severe enough to cause a fracture. It may however, produce a mild-to-total hearing loss. This is because of tears of the membranous labyrinth which may involve the vestibular structures as well as the cochlea. In such cases, an individual may also experience vertigo.

Presbycusis

The term "presbycusis" has been traditionally applied to the hearing loss that normally accompanies aging. Although it commonly refers to hearing loss resulting from degenerative changes in the cochlea alone, it is now clear that

the aging process affects the whole auditory system and that hearing loss of old age probably involves changes in the middle ear, inner ear and central auditory pathways. Although there is no clear relationship between age-related changes in the middle ear and the audiographic findings, there are documented cases of ossicular fixation and arthritic changes in ossicular joints with fibrous and calcific changes. The correlation between the types and patterns of cochlear lesions and the patterns of hearing loss has long been recognized. While there may be wide variations in these patterns, in general, the common finding is a high frequency sensorineural hearing loss associated with degeneration of the organ of Corti in the base of the cochlea. Hearing loss may progress over time to lower frequencies. Figure VIII-1 shows audiograms taken at decade intervals. Note here the gradual and progressive loss of sensitivity at high frequencies. Presbycustic individuals may also have central nervous system involvement in their hearing disorder. Within the cochlear nuclei, for example, the injury may range from little or no alteration in cellular structure to complete destruction of cells. Whether this occurs independently of a cochlear lesion is currently not known.

Ototoxicity

It has long been known that certain drugs and chemicals can have strong effects on the auditory and vestibular receptors of the inner ear. The clinical signs of ototoxicity are variable but include one or more of the following symptoms: sensorineural hearing loss, tinnitus, "dizziness" of one description or another, and depressed vestibular function with or without nystagmus. Over the past 40 years, there has been a steady accumulation of data, from both the clinic and the laboratory, on the mechanisms of action of various ototoxins. With current understanding of the normal cellular-molecular mechanisms of receptor cell action, we are on the threshold of understanding the mechanisms of many of the disorders that affect hair cells. A brief description is given a few of some of the more common agents and their actions.

Aminoglycoside antibiotics

Most of the antibiotics recognized as having ototoxic properties belong to the family of aminoglycosides. The primary ones that require respect are streptomycin, dihydrostreptomycin, neomycin, kanamycin, gentamicin and tobramycin. Figure VIII-2 shows the audiograms from the left and right ears of an individual treated with kanamycin for severe renal failure. Below the audiogram are plots of hair cell and spiral ganglion cell loss in each of the ears taken after postmortem histological preparation of the temporal bones. Note the correspondence between hair cell loss in the basal half of the cochlea and

the high frequency hearing loss which is typical of aminoglycoside ototoxicity.

Clinical and experimental evidence collected from human and animal studies over many years has given a picture of the pathophysiological mechanisms which underlie the damage inflicted by these agents. First, the toxic substances must reach the labyrinthine fluid either via the blood stream or, when applied topically to the middle ear, by direct penetration of the oval and/or round windows. Second, primary damage is to the hair cell; auditory nerve fibers may degenerate secondary to sensory cell degeneration. Both kanamycin and neomycin affect first the outer hair cells of the cochlea base; over time the lesion progresses to the cochlear apex. Inner hair cells seem less vulnerable to these agents. In cases where permanent damage is to outer hair cell regions alone, the physiology of auditory nerve fiber innervating the inner hair cells in the region of the lesion is clearly abnormal. A primary effect is to greatly alter the frequency selectivity of an auditory nerve fiber. Third, at the cellular-molecular level, the action of aminoglycosides seems to alter plasma membrane permeability for there is microscopic evidence for the swelling of sensory hairs with the deformation of the cell surface. This may involve several processes upon which cellular integrity depends. Two of them are the cellular metabolic and protein synthesizing machinery, for there is also evidence that mitochondria and ribosomes are damaged. Another is that the ionic channels which are responsible for mechano-electric transduction to occur may be blocked or otherwise affected.

A number of other antibiotics with differing molecular structure are known to have ototoxic properties. These include viomycin (polypeptide), polymyxins (polypeptides), vancomycin (contains both sugars and amino acids) and minocycline (a tetracycline).

Diuretics - Two potent diuretics in general use have well-recognized ototoxic effects. These are ethacrynic acid and furosemide. Rapid intravenous infusion of either of these agents may produce a sensorineural hear loss which is usually immediate in onset and transient, lasting a few hours to several days. There may be vertigo. Severe and permanent hearing loss is reported.

Animal studies have shown that intravenous injection of ethacrynic acid or furosemide produces within seconds depression of the cochlear microphonic potential (hair cell receptor potential) and auditory nerve action potentials and a decrease in endocochlear potential which is necessary for normal transduction and transmission in the inner ear receptor organs. Anatomical changes include outer hair cell degeneration in basal and middle turns of the cochlea. In those cells that survive there may be distortion of the stereociliary bundle. Moreover, there are marked changes in the stria vascularis, with intra- and extracellular edema and destruction of the intermediate cell layer. Thus, it would appear that diuretic ototoxicity involves changes in the transduction and transmission properties of the hair cells and a breakdown in the intra-labyrinthine secretory mechanisms of the stria vascularis.

Quinine derivatives - It has long been known that quinine derivatives produce irreversible sensorineural hearing loss with tinnitus as the major symptom. Administration to women in the first trimester of pregnancy has resulted in severe abnormalities of the inner ear of the fetus. There may be a complete absence of hair cells and supporting cells throughout much of the organ of Corti.

Salicylates - High doses of salicylates predictably produce a bilaterally symmetric, flat hearing loss up to about 40 dB with some reduction in speech discrimination. The magnitude of the hearing loss is directly related to the serum levels of the substance. The hearing loss and accompanying tinnitus are completely reversible within 24-72 hours after the drug is discontinued. There is no consistent morphological change observed in the inner ears of humans or animals subjected to high doses of salicylates. While biochemical changes of the perilymph and endolymph have been noted along with consistently reduced electrical activity of the cochlea and auditory nerve, the precise mechanisms of this form of ototoxicity are not known.

Meniere's disease (idiopathic endolymphic hydrops)

The prototypic intra-labyrinthine auditory-vestibular disorder is Meniere's disease. The classic description of the symptom complex was given more than a century ago: (1) tinnitus, (2) spontaneous, episodic vertigo and (3) hearing loss. Endolymphic hydrops (i.e. distention of scala media with Reissner's membrane bulging into scala vestibuli) was clearly described as the basic histopathologic lesion associated with the disease (hence the clinical synonym) although a cause-and-effect relationship between the hydrops and the clinical findings remains obscure. Vertigo of Meniere's disease occurs in sudden attacks and may be accompanied by spontaneous positional nystagmus. Hearing loss is characteristically unilateral, fluctuating, and sensorineural in nature. Bilateral hearing loss occurs in 10-20% of the cases although a figure as high as 40% has been reported. Subjectively, the hearing loss is frequently accompanied by fullness and feelings of pressure in the affected ear. Tinnitus may be roaring, buzzing, whistling or mixed in nature. Histopathologic studies of temporal bones taken postmortem from Meniere's patients usually show hydrops of the scala media and, less frequently, hydrops of the utricle and/or one or more semicircular canal. In severe cases, secondary complications such as rupture of membranes, herniation of the vestibular organs, and degeneration of the organ of Corti may occur. Occasionally, there are noted changes other than hydrops involving the endolymphic sac and its surrounding tissue and the vestibular aqueduct and endolymphic duct. Traditionally, clinical experience indicates that there may be a number of secondary factors that contribute to the syndrome. These include metabolic, infectious, allergic neurogenic and psychosomatic factors.

Labyrinthitis - Invasion of the labyrinth by inflammatory reaction is manifested by vertigo, nystagmus, sensorineural hearing impairment, nausea and vomiting. Serous (non-suppurative) labyrinthitis represents a serous inflammation of the labyrinth secondary to an acute or chronic infection of the surrounding bony labyrinth. There is not active invasion of the labyrinth by the infecting organism. Normal vestibular and auditory function returns. Suppurative labyrinthitis occurs secondary to otitis media and mastoiditis and represents a direct extension of the infection into the labyrinth. The symptoms include roaring tinnitus, progressive sensorineural hearing loss to total deafness, and violent vertigo with associated nausea and vomiting. Complications of this disease are meningitis or epidural abscess.

Tumor of the VIIIth nerve - Lesions of the eight nerve are characterized by tinnitus, sensorineural hearing loss, mild vertigo, and in some patients, other

cranial nerve signs. The classic lesion is the so-called "acoustic neuroma", a benign tumor that is usually not of auditory nerve origin nor is it a neuroma. The tumor is a schwannoma typically arising from the vestibular nerve within the internal auditory meatus. The growth of the tumor in the vestibular nerve does not typically produce vestibular signs and it is not until the tumor compresses the auditory nerve that it is noticed. The most common first symptom is unilateral tinnitus. This may be followed by a high frequency hearing loss. The mechanism for the hearing disorder probably involves the disruption of normal transmission of action potentials in the fibers of the auditory nerve due to compression by the tumor. Sudden occlusion of the internal auditory artery, which supplies the organ of Corti, may produce a severe or total cochlear hearing loss. Pressure from the growing tumor may eventually involve cranial nerves VII, VI and V.

Tinnitus is a major disorder of the peripheral auditory system

What is tinnitus?

No doubt everyone has experienced sounds that do not originate from a source or sources outside of the body. Instead, they originate 'within the head'. These auditory sensations may take many forms, such as roaring noise, tones and clicks, and they may be intermittent or continuous. They all are referred to as tinnitus. Thus, tinnitus can be defined broadly as a conscious experience of sound that originates within the head of its owner.

Tinnitus is not a disease per se, but is a symptom of a wide range of disorders. Severity of tinnitus ranges widely, from being mildly irritating and hardly noticed to being severely debilitating. The incidence of tinnitus is very high. About 32% of all U.S. adults report having tinnitus at one time or another, and about 6.2% of them report it to be severe or debilitating.

What are the causes of tinnitus?

Tinnitus may have a physical basis. That is, sound energy is produced somewhere in the head, which is then heard by a subject. This could include such things as vascular anomalies, muscular contraction, clicking jaws. Also, sounds may be generated within the inner ear that are loud enough to be heard by a subject (or a nearby listener), the so called otoacoustic emissions. In all of these cases, the inner ear and central auditory pathways are normal in their transmission and processing of the unwanted and often annoying sounds.

Tinnitus may have no physical basis. In these cases it is the result of abnormal physiological activity in the inner ear or in the central auditory pathways, which gives rise to the perception of sound even though there is no physical sound present.

Except for those cases for which there is a clear mechanical cause, the physiological mechanisms that underlie tinnitus are unknown.

Because mechanisms are unknown and because tinnitus may arise from any of a number sites in the ear and brain, it is not surprising that there is no single treatment that works in all cases.

Tinnitus has multiple etiologies. It may be associated with:

a. blows to the head. Tinnitus may be transient or long-lasting.

b. drugs and general anesthetics used during surgery may initiate or exacerbate a tinnitus.

c. tumors of the eighth nerve (schwannoma's), and is an early symptom in a high percentage of such cases. Rarely does it go away after surgery.

d. sensorineural hearing loss. In cases of acoustic trauma and presbycusis tinnitus is reported to be high pitched, matching the frequency in the transition zone between regions of greater and lesser hearing loss.

e. otosclerosis, which may be associated with pregnancy.

f. Menier's disease.

X. HEARING LOSS AND MEASUREMENT OF HEARING

ABILITY

Objectives:

There are a number of ways to test how well a person hears. In the clinic you may try some rather crude screening methods. At other time you may need to refer patients to trained audiologists for special testing. This section is aimed at introducing you to the major ways in which hearing is tested.

At the end of this section, you should be able to:

1. Describe the effects of different degrees of hearing loss on speech and language

2. Describe the various methods used for evaluating hearing loss

3. Describe how the audiogram can be used to differentiate a conductive hearing loss from a sensorineural hearing loss

4. Describe objective procedures that may be used in evaluating the hearing of difficult-to-test subjects.

5. Describe the several approaches to rehabilitation available for hearing impaired individuals.

Objective 1: Hearing loss and its effects on communication

Hearing loss may be categorized by degree. This table below does not take into account some important variables, including age of the individual which, as we will see later impacts critically on language development.

25-40 dB

Misses hearing many consonants, difficulty in auditory learning, mild speech - language problems

40-65 dB

Speech - language retardation, learning disability, hears little or no speech at normal conversational levels

65-90 dB

Voice pathology, aural language seriously compromised, severe learning problems

>90 dB

http://www.neurophys.wisc.edu/h&b/textbook/hearing_test.html#Obj5









Profound hearing loss (deaf), voice-speech characteristic of deaf, severe learning disabilities

Objective 2: Hearing Assessment

Accurate otologic diagnosis often depends on reliable and accurate tests of hearing. The results of such tests help identify the sites of lesions and and point to strategies for intervention and treatment. Patients may be screened in a relatively course way, or may be evaluated using more sophisticated testing procedures.

Screening

Some simple procedures can be carried out to give clues as to the nature of the hearing loss:

In conversation, how does the person respond when you speak to him/her? Does the person have to see your face to understand what you are saying? Does the person tend to favor one or the other ear? Using a tuning fork may help evaluate the nature of a hearing loss short of an audiologic examination.

Audiometric Testing of Adults

The basic battery of audiometric tests normally includes pure-tone testing of threshold by both air and bone conduction, speech reception threshold (SRT), and speech discrimination (SDS). These tests normally require the cooperation of the subject whose response to a sound is indicated by some gesture (e.g. raised hand). Testing is typically carried out in a sound-attenuated room with the subject listening to carefully calibrated sounds.

Threshold sensitivity testing using airborne pure tones.

Testing is done using earphones thereby allowing each ear to be examined independently. Tones are reduced in intensity until they are no longer heard, at which point the examiner alternately raises and lowers the intensity of the sound until a just-detectable threshold is determined. This is repeated at several frequencies within the audible range and the results plotted as an audiogram. The shape of the curve is a measure of the frequency sensitivity of both the middle ear and the inner ear. To differentiate between middle ear (conductive) and inner ear (sensorineural) components to a hearing loss it is necessary to conduct additional tests.

Threshold sensitivity testing using bone-conducted pure tones

Here testing is done with a vibrator placed somewhere on the skull (usually the mastoid). The testing and plotting procedures are the same as with air conduction testing. In this case, however, sound is transmitted directly to the cochlea via bone conduction, thereby by-passing the transmission mechanism of the middle ear. Thus, audiograms obtained using both bone and air conducted sounds provide information about the integrity of both the middle and inner ears. Difficulty in hearing only air conducted sounds results in a separation of the bone and air conduction audiograms - the so-called "air-bone gap".

A B

C DFig. X-1

Objective 3:

Thus, the shape of the bone conduction audiogram is a measure of the sensitivity of the inner ear alone, while the difference between air conduction and bone conduction is a measure of the degree of hearing loss attributed to the middle ear. Figure X-1 illustrates bone- and air-conduction audiograms from normal and hearing impaired subjects.



http://www.neurophys.wisc.edu/h&b/textbook/fig10-1c.gif

http://www.neurophys.wisc.edu/h&b/textbook/fig10-1d.gif

Threshold sensitivity testing using speech

The threshold for hearing easily-recognized two-syllable words (e.g. baseball, iceberg, eardrum) is called the speech reception threshold(SRT). Since speech is made up of pure tones, this threshold should be very close to pure tone thresholds up to about 4 kHz, which is the range offrequencies occupied by most normal speech sounds.

Speech discrimination testing

Speech discrimination, as opposed to speech sensitivity, is the person's ability to not only hear words but to identify them. The procedure includes presentation of 50 selected monosyllabic words at an easily detectable intensity level. The speech discrimination score (SDS) is the percentage of words correctly identified. Pathology of the inner ear, auditory nerve, and/or central auditory pathways can affect this score. The ability of an individual to discriminate speech is not well predicted by the pure-tone audiogram. An individual may hear a sound well enough, but the neural signals may be altered to the extent that the sound is unintelligible.

Individuals suffering only a conductive hearing loss will be able to identify words if the sound is loud enough. For persons with sensorineural hearing loss, there is a marked drop in the score without a proportionate loss of pure-tone or speech sensitivity.

Auditory assessment of children

Infants indulge in vocal play within the first year of life. If they fail to respond to their auditory world in this time, a parent will often ask questions about the child's ability to hear. Perhaps another year passes before the question is raised of why the child fails to speak. This early period in the child's life is critical for the development of normal communication skills which, if not achieved then, will often result in lifelong communication handicaps. Thus, newborns at risk for hearing loss and those whose hearing is suspect can, and should, be accurately evaluated at the earliest time. Accurate hearing tests can be conducted at any time in a person's life. Obviously, testing procedures used successfully on adults may have to be modified for use on infants and small children

Infants respond to sounds in the free field or through earphones with facial responses (grimaces, smiles, brow raising) or with head turning. A 3-month old child sitting in its mother's lap may exhibit a startle response to sound. A 6-

month old child may initiate or cease activity upon hearing a stimulus. These responses are difficult to quantify, but in the hands of an experienced person, can give clues as to the ability of that infant or child to hear sounds. When testing young children it is necessary to remember to apply standards and stimuli appropriate for the child's developmental level.

By 24 months of age other test strategies can be used based on the child's natural curiosity about objects in the environment. Play audiometry is a technique used that makes a game of the test and may be used up to the age of about 6 depending on the abilities of the child. This technique allows for tests of pure-tone detection and speech sensitivity and intelligibility.

Testing is also possible in the more difficult situations in which the child is 1) profoundly deaf, 2) developmentally disabled, 3) aphasic, 4) emotionally disturbed (e.g., autistic), 5) deaf-blind, or some combination of these disorders. Specially designed behavior tests include those using positive reinforcement (candy, toys) to reward appropriate responses to auditory stimuli.

Objective 4: Objective methods for assessing hearing and hearing loss

Testing may also be objective in the sense that it is not required that the subject consciously respond to the sound. In these tests, physiological responses are monitored and, in general, the cooperation of the patient is not needed. There are several special reasons for using such tests. First, they are designed to test the integrity of specific parts of the auditory pathway. Second, they can be used to test patients who are unable or unwilling to cooperate with hearing examiners (e.g., infants, mentally retarded persons, malingerers). Commonly used methods include:

Tympanometry

This method assesses the mobility or compliance of the tympanic membrane and thereby provides important information about the function of the middle ear including the tympanic membrane, ossicles, and Eustachian tube.

Electrocochleography

Cochlear and auditory nerve electrical activity can be recorded from human patients from electrodes advanced through the tympanic membrane and placed on the otic capsule. This method allows assessment of cochlear and auditory nerve function independent of the patient's subjective response. Two electrical events are recorded from the inner ear in response to sound: the cochlear

microphonic (receptor) potential and the compound action potential of the auditory nerve. Distortion of the waveform of either of these potentials is an indication of inner ear disease.

Evoked potential

A silver disc electrodes taped to the skull record the electrical activity of the brain (EEG). When a brief acoustic stimulus (e.g., a click or short tone burst) is presented to the ear there is a synchronized burst of action potentials generated in the auditory nerve which spreads throughout the central auditory pathway. Because of its very low amplitude (in the microvolt range) this wave of activity is generally buried in the EEG and can only be recovered using computerized signal-averaging techniques. When such methods are employed the complex waveform recorded is called the auditory evoked potential and it includes contributions from many sites that are activated sequentially in time along the auditory pathway. An averaged waveform has multiple peaks and valleys stretched out over a period of several hundred milliseconds after the presentation of the acoustic stimulus. By examining only certain epochs of the waveform it is possible to study the integrity of certain portions of the auditory pathway.

The time period most commonly studied covers the first 12 msec after the stimulus is presented to the ear and, hence, represents the electrical activity evoked in neurons in the auditory nerve and brainstem. This is referred to in the experimental and clinical literature as the brainstem evoked response (BER) or averaged brainstem response (ABR). Other time periods reveal later waves which represent activity generated in higher centers of the auditory pathway including the auditory areas of cerebral cortex. This technique is very useful in studying hearing loss of central origin in cooperative patients. It is essential in determining the hearing loss in infants and others who are unwilling or unable to cooperate.

Objective 5: Rehabilitation for Hearing Loss

Traditionally, persons with a hearing loss have been fitted with a hearing aid, a device that simply amplifies the sound. Hearing aid may be helpful at any time in life and, because they are amplifiers only (and therefore make sounds louder but not clearer), they tend to work best in cases of conductive hearing loss.

For treating profound sensorineural hearing loss, a cochlear prosthesis has been developed to aid those individuals with little or no residual hearing.

Alternatively, other means of communication may be substituted (e.g., sign language, lip reading). In some cases, all means of communication are used.

Hearing and Balance

Documents