The physiology of the ear

The Physiology of the Ear

Presented by:

Coojacinto, Stephanie JoyceGaleno, Chris Carlo

Ong, Charles AdrianTamayo, Tacyni Monece

STOP AND JUST LISTEN!

Second Sound

First Sound

General Anatomy of the Ear

Three Main Divisions of the Ear

External Ear› Auricle/ Pinna: Flap of elastic cartilage

shaped like flared end of a trumpet and covered by skin

External Ear (cont.)

External Auditory Canal: › Curved tube about 2.5 cm (1 in.) › Lies in the temporal bone and lead from the auricle to

the eardrum › Contains a few hair and ceruminous glands

(specialized sebaceous glands) Tympanic membrane:

› A thin, semitransparent partition between the external auditory canal and middle ear

› Consists of a connective tissue core lined with skin on the outside and mucous membrane on the inside

› Covered by epidermis and lined by simple cuboidal epithelium

EAC and Ear Drum

Middle Ear

Small, air-filled cavity in the temporal bone that is lined by epithelium.

Contains two small membrane-covered openings:› Oval window› Round window

Contains the three smallest bones in the body› Malleus (hammer)› Incus (anvil)› Stapes (stirrup)

Joints: synovial joints

Auditory Ossicles

Malleus: attached to the internal surface of the eardrum

Incus: the middle bone; articulates with the head of the stapes

Stapes: its base fits into the oval window (which is right above the round window)

Auditory Ossicles

Muscles of the Middle Ear

Tensor Tympani muscle: inserts into the handle of the malleus. It reduces the vibrations of malleus that could potentially harm the tympanic membrane (i.e. loud noise)

Stapedius muscle: Reduces the vibrations of stapes

Middle Ear

Eustachian Tube

An opening on the anterior wall of the middle ear

Consists of both bone and hyaline cartilage Connects middle ear with the nasopharynx Functions to equalizes the pressure within

middle ear and the atmospheric pressure (between tympanic cavity and nasopharynx)

A route where pathogens may travel from the nose and throat to the middle ear

Eustachian Tube

Inner Ear (Labyrinth)

Called a labyrinth because of its complicated series of canals

Two main divisions: › Bony Labyrinth: lined with periosteum and

contains perilymph (similar to CSF)› Membranous Labyrinth: surrounded by the

CSF. A series of sacs and tubes inside the bony labyrinth and having the same general form

Inner Ear

Inner Ear (cont.)

› Bony Labyrinth: series of cavities in the temporal bone. Divided into three areas: Semicircular Canals: projects posteriorly and

superiorly from the vestibule. Consists of an anterior, posterior and lateral semicircular canal. Ampulla: swollen enlargement at the end of each

canal Semicircular ducts: allows communication between

the utricle and the vestibule

Inner Ear (cont.)

Vestibule: contains receptors for equilibrium Oval central portion of the bony labyrinth Communicates anteriorly with the cochlea and

posterosuperiorly with the SCC The membranous labyrinth in the vestibule consists

of: Utricle Saccule

Inner Ear (cont.)

Cochlea: contains receptors for hearing› Anterior to the vestibule› A bony spiral canal that resembles a snail

shell and makes almost three turns around a central bony core (modiolus). It is divided into three channels The partitions that separate the channels are

shaped like a letter Y Scala vestibuli: channel above the bony partition and

ends at the oval window Scala Tympani: channel below and ends at the round

window

Inner Ear (cont.)

Cochlea:› Adjoins the wall of the vestibule (where the

scala vestibuli opens)› It has two membranes: basilar membrane

and vestibular membrane (which separates the cochlear duct from the scala vestibuli)

› Spiral Organ of Corti: Rests on the basilar membrane and contains hair cells, which are receptors for hearing

Membranous Labyrinth

Lodged within bony labyrinth Filled with endolymph Surrounded by perilymph

Part I of Ear Physiology: Equilibrium

Vestibular Apparatus

Sense of equilibrium---- provides orientation with respect to gravity

Forms the inner ear along with the cochlea

Consists of two parts:› Otolith Organs: utricle and saccule› Semicircular canals

Vestibular Apparatus (cont.)

Vestibular Apparatus (cont.)

The sensory structures of both the vestibular apparatus and cochlea are located within the membranous labyrinth (which is filled with a fluid called endolymph) which is located within the bony cavity in the skull, bony labyrinth.

Perilymph is the fluid between the membranous labyrinth and the bone

Sensory Hair Cells of the Vestibular Apparatus

Utricle and Saccule: provide information about linear acceleration› Refers to the changes in velocity when

traveling horizontally or vertically (i.e. riding in a car)

Semicircular Canals: provides a sense of rotational and angular motion It helps maintain balance when turning the head, spinning, or tumbling. › Refers to the changes in direction

Hair Cells

Receptors for equilibrium; modified epithelial cells

Named as they are because each cell contains twenty to fifty hairlike extensions› Stereocilia: processes containing filaments

of protein surrounded by part of the cell membrane

› Kinocilium: larger extension that has the structure of a true cilium

Hair cells

Hair Cells

Sensory Process of Equilibrium

1. When the stereocilia are bent in the direction of the kinocilium, the cell membrane is depressed and becomes depolarized.

2. The hair cell releases a synaptic transmitter, thus stimulating the dendrites of sensory neurons that are part of the vestibulocochlear nerve.

3. When the stereocilia are bent in the opposite direction, the membrane of the hair cell becomes hyperpolarized, which causes the release of a less amount of synaptic transmitter.

Sensory Process of Equilibrium

In this way, the frequency of action potentials in the sensory neurons that innervate the hair cells carries information about movements that cause the hair cell processes to bend.

The Otolithic Organs

Utricle and Saccule have a patch of specialized epithelium called a macula that consists of hair cells and supporting cells. › The hair cells project into the membranous

labyrinth, with their hairs embedded in a gelatinous otolithic membrane Contains microscopic crystals of calcium

carbonate, these increase the mass of the membrane, and increase the resistance to change in the movement

Otolithic organs (cont.)

Utricle is more sensitive to horizontal acceleration› Otolithic membrane lags behind the hair

cells› Hair cells are pushed backward

Saccule is more sensitive to vertical acceleration› Causes the hairs of the saccule to be

pushed upward

Utricle and Saccule

Semicircular Canals

Semicircular duct: inner extension of the membranous labyrinth in each canal› Ampulla

Crista ampullaris: elevated area of the ampulla where the sensory hair cells are located. Cupula: gelatinous membrane where the processes

of the hair cells are embedded. It can be pushed in several directions because of the endolymph.

Semicircular Canals (cont.)

Endolymph:› Provides inertia so that the sensory

processes will be bent in a direction opposite to that of the angular acceleration. Through this, it stimulates the hair cells

Semicircular Canals (cont.)

The Semicircular Canals:

Anterior Semicircular canal: hair cells are stimulated when doing a somersault.

Posterior Semicircular canal: stimulated when performing a cartwheel.

Lateral Semicircular canal: stimulated when spinning around the long axis of the body.

Neural Pathways of the Vestibular Apparatus

Neural Pathways

Stimulation of hair cells in the vestibular apparatus activates sensory neurons of Vestibulocochlear nerve (CN VIII)

These fibers transmit impulses to the cerebellum and to the vestibular nuclei of the medulla oblongata

The vestibular nuclei then send fibers to the oculomotor center of the brain stem and to the spinal cord

Neural Pathways

Nystagmus and Vertigo During a spin, the bending of the cupula

produces smooth movements of the eyes in a direction opposite to that of the head movement so that a stable visual fixation point is maintained.

When the spin is abruptly stopped, the eyes continue to smoothly in the previous direction of the spin, and then are jerked rapidly back to the midline position

This produces involuntary oscillations of the eyes called vestibular nystagmus.

Nystagmus

Vertigo

• Loss of equilibrium as a result of spinning• May be caused by anything that alters the

firing rate of one of the CN VIII compared to the other Usually due to a viral infection causing

vestibular neuritis• Severe vertigo is accompanied by

dizziness, pallor, sweating, nausea, and vomiting due to involvement of ANS, which is activated by vestibular input tothe brain stem

Vestibular nystagmus

Involuntary movement of the eye resulting from abnormal stimuli to the inner ear.

One of the symptoms of an inner-ear disease called Ménière's disease› Early symptom: “ringing in the ears” or

tinnitus Vestibular symptoms of vertigo and

nystagmus accompany hearing problems in this disease

Vestibular nystagmus

Types› Central

produce one-way or two-way eye movement› Peripheral

exhibits only one-way eye movement. Treatment

› Botulinum toxin, the substance that causes botulism, is sometimes injected to reduce eye movement

› Surgery is also necessary in some cases

THE EARS and hearing

Sound causes vibrations of the tympanic membrane, and they produce movements of the middle-ear ossicles, which press against a membrance called the oval window in the cochlea.

Movements of the oval window produce pressure waves within the fluid of the cochlea, causing movements of the basilar membrane.› Bending of the sensory hair cells follows› Stimulation of action potentials transmitted to

the brain in sensory fibers and interpreted as sound

Sound waves

Alternating zones of high and low pressure traveling in a medium (air or water)

Are characterized by:› Frequency (Hz)

cycles per second (cps) Pitch

› Intensity (dB) Amplitude of the sound waves

Outer Ear

Sound waves are funneled by the pinna (auricle) into the external auditory meatus, and these 2 form the outer ear.

External auditory meatus channels the sound waves (while increasing the intensity) to the eardrum, or tympanic membrane

Sound waves in the EAM produce extremely small vibrations of the tympanic membrane.

The ear: Outer, Middle and Inner ear

Middle Ear

The cavity between the tympanic membrane on the outer side and the cochlea on the inner side

3 middle-ear ossicles – protection› Malleus (hammer)

attached to the tympanic m. vibrations are transmitted via the malleus and incus to

the stapes› Incus (anvil)› Stapes (stirrup)

attached to the oval window in the cochlea vibrates in response to the vibrations of the tympanic

m.

Middle Ear

Stapedius muscle› Attaches to the neck of the stapes› Increases protective function› Helps prevent nerve damage within the

cochlea in very loud sounds as it contracts and dampens the movements of the stapes against the oval window

Middle Ear

Auditory (eustachian) tube› A passageway leading from the middle ear to the

nasopharynx› Is usually collapsed; to prevent debris and infectious

agents from traveling from the oral cavity to the middle ear.

› Tensor tympani muscle Must contract to open the auditory tube Occurs during swallowing, yawning, sneezing “popping” sensation in swallowing when driving up to a

higher altitude The auditory canal opening allows air to move from the

region of higher pressure (middle ear) to the region of lower pressure (nasopharynx)

INNER EAR

Charles Adrian Ong

COCHLEA

COCHLEA

• cochlea which serves as the body's microphone, converting sound pressure impulses from the outer ear into electrical impulses which are passed on to the brain via the auditory nerve.

• The inner ear structure called the cochlea is a snail-shell like structure.

Sections of the cochlea

Sections of the cochlea

Sections of the cochlea

The pressure changes in the cochleacaused by sound entering the ear travel down the fluid filled tympanic(scala tympani) and vestibular canals(scala vestibuli) which are filled with a fluid called perilymph. This perilymph is almost identical to spinal fluid and differs significantly from the endolymph which fills the cochlear duct(scala media) and surrounds the sensitive organ of Corti.

Spinal organ of corti

Spinal organ of corti

• Receptor organ of hearing• It contains four rows ofhair cells which

protrude from its surface. Above them is the tectoral membrane which can move in response to pressure variations in the fluid- filled tympanic and vestibularcanals. There are some 16,000 -20,000 of the hair cells distributed along the basilar membrane which follows the spiral of the cochlea.

Spinal organ of corti

The place along the basilar membrane where maximum excitation of the hair cells occurs determines the perception of pitch according to the place theory. The perception of loudness is also connected with this organ.

Spinal organ of corti

Tiny relative movements of the layers of the membrane are sufficient to trigger the hair cells. Like other nerve cells, their response to stimulus is to send a tiny voltage pulse called an "action potential" down the associated nerve fiber (axon). These impulses travel to the auditory areas of the brain for processing.

Effects of sounds at diff. frequency on basilar membrane

Neural Pathways for Hearing

Prepared by: Chris Carlo M. Galeno

Neural Pathways for Hearing

Sensory neurons in the vestibulocochlear nerve (VIII) synapse with neurons in the medulla oblongata that projects to the inferior colliculus of the midbrain.

Neurons in this area project to the thalamus thats sends axons to the auditory cortex of temporal lobe.

Neurons in different regions of basilar membrane stimulate neurons in corresponding areas in auditory cortex.

Each area of the auditory cortex thus represents a different part of the basilar membrane and a different pitch.

The cochlea acts like a frequency analyzer, in different frequencies (pitches) of sound stimulate different sensory neurons that project to different places in the auditory cortex

The analysis is based on which hair cells activate the sensory neurons

It is related to the position of the hair cells on the basilar membrane. This is known as the PLACE THEORY OF PITCH.

Since the different sensory neurons project to different places in the auditory cortex, the organization of this cortex is said to be tonotopic.

tone frequencies are transmitted separately along specific parts of the structure.

Analysis of Pitch….. AMAZING!

Able to recognize that a given sound frequency (such as 400 Hz) is the same regardless of whether it is played by violin or piano

In harmonics, can vary, depending on their amplitudes. However, if the fundamental frequency is the same, the pitch is recognized being the same on the different instruments

Hearing Impairments

2 Major Categories of Deafness

Conduction Deafness› Transmission of sound waves through the

middle ear to the oval window is impaired

Sensorineural or Perceptive Deafness› Transmission of nerve impulses anywhere

from the cochlea to the auditory cortex is impaired

Conduction Deafness

Caused by middle–ear damage from otitis media or otosclerosis

Impairs hearing at all sound frequencies

Can be helped by Hearing Aids Device that amplify sounds and conduct the

sound waves through bone to the inner ear.

Hearing Aids

Sensorineural Deafness

Result from a wide variety of pathological processes and from exposure to extremeley loud sounds

Unfortunately, the hair cells in the inner ears cannot regenerate once destroyed.

Impairs the ability to hear some pitches more than others.

This may be due to pathological processes or to changes that occur during aging.

Sensorineural Deafness

Can be corrected by Cochlear Implants

It consists of elctrodes threaded into the cochlea, a receiver implanted in the temporal bone, and an external microphone, processor and transmitter.

Cochlear Implants

Presbycusis Age-related hearing impairment Begins after age 20 when the ability to

hear high frequencies (18000-20000 Hz) diminishes

Men are affected to greater degree than women, but the progression is variable

Deficits may gradually extend to 4000-8000 Hz range

Impairment can be detected by Audiometry

A technique in which threshold intensity of different pitches is determined.

The ability to hear speech is particularly affected by hearing loss in the higher frequencies

END! THANK YOU!