THE AUDITORY AND VESTIBULAR SYSTEMS
THE AUDITORY AND VESTIBULAR SYSTEMS
THE NATURE OF SOUNDSound is an audible variations in air pressure, defined by:1) frequency: Number of cycles (distance between successive compressed patches)per second expressed in units called Hertz (Hz). Human Range is btw 20 Hz to 20,000 Hz2) Intensity: Difference in pressure between compressed and rarefied patches of air. It determines the loudness of the sound.
Sounds propagate at a constant speed: 343 m/sec
THE AUDITORY SYSTEM
THE MIDDLE EARSound Force (pressure) is amplified by the Ossicles, producing greater pressure at oval window (smaller surface) than tympanic membrane, in order to move more efficiently the fluid inside the cochela
The Attenuation Reflex: response where onset of loud sound causes tensor tympani and stapedius muscle contraction. It’s used to adapt ear to loud sounds, or understand speech better in noisy environment (more attenuation of low sounds)
THE INNER EAR
Perilymph: Fluid in scala vestibuli and scala tympaniEndolymph: Fluid in scala mediaEndolymph has an electric potential 80 mV more positive than perilymph (Endocochlear potential)
THE INNER EAR
Basilar Membrane is wider at apex, stiffness decreases from base to apex
THE INNER EARPressure at oval window, pushes perilymph into scala vestibuli, round window membrane bulges out. Endolymph movement bends basilar membrane near base, wave moves towards apex
THE INNER EAR
The Organ of Corti and Associated Structures. Here the mechanical energy of the sound is transformed in electrical signal by the auditory receptor cells (hair cells).Each hair cells has around 100 stereocilia.Rods of corti provide structural support. Hair cells form synapses with bipolar neurons that have their body in the spiral ganglion. Their axons form the auditory nerve
THE INNER EARTransduction by Hair CellsWhen sound arrives, basilar membrane moves. According to the movement, stereociliabends on one or the other direction: i.e. Basilar membrane upward, reticular lamina up and stereocilia bends outward
THE AUDITORY PATHWAY
Auditory nerve
Superiorolive
MGN
Auditorycortex A1
MGN
INFORMATION ABOUT THE SOUNDInformation About Sound Intensity is encoded in 2 ways:
Firing rates of neurons and number of active neuronsStimulus Frequency
Frequency sensitivity: in Basilar membrane is Highest at base, lowest at cochlea apex. This coding is kept separate along the auditory pathways (tonotopy)
Phase Locking is another way to code for frequencyConsistent firing of cell at same sound wave phase. Only for frequency below 4kHz
SOUND LOCALIZATION: HORIZONTAL PLANEInteraural time delay: Time taken for sound to reach from ear to ear
Duplex theory of sound localization:Interaural time delay: 20-2000 Hz
Interaural intensity difference: 2000-20000 Hz
Interaural intensity difference: Sound at high frequency from one side of ear
Sound waves
Sound waves
Sound waves
Sound waves
Sound shadow
Sound shadow
Sound shadow
SOUND LOCALIZATION: VERTICAL PLANE
pinnaPath 2, direct sound
Path 2, reflected sound
Path 2, direct sound
Path 2, reflected sound
Path 3, direct sound
Path 3, reflected sound
Based on reflections from the pinna
THE AUDITORY CORTEX: BA 41
Primary auditory cortex
Secondary auditory cortex
Axons leaving MGN project to auditory cortex via internal capsule in an array called Acoustic Radiation
THE VESTIBULAR SYSTEM
Importance of Vestibular SystemBalance, equilibrium, posture, head position, eye movement
The Vestibular Labyrinth
THE VESTIBULAR SYSTEM
The Otolith Organs (saccule and utricle). Detect force of gravity (linear acceleration) and tilts (change of angle) of the head.Saccule is vertically oriented and utricle horizontally oriented
Crystals of calcium carbonate
Bending of the hairstoward kinocilium: depolarization
THE VESTIBULAR SYSTEM
The Semicircular Canals. Detect rotation of the head and angular acceleration
Crista: Sheet of cells where hair cells of semicircular canals clusteredAmpulla: Bulge along canal, contains cristaCilia: Project into gelatinous cupulaKinocili oriented in same direction so all excited or inhibited together
Filled with endolymph
endolymph
Three semicircular canals on one side helps sense all possible head-rotation anglesEach Canal paired with another on opposite side of headRotation causes excitation on one side, inhibition on the other
CENTRAL VESTIBULAR PATHWAY
S1/M1 Face area
VESTIBULO-OCULAR REFLEX (VOR)
Function: Line of sight fixed on visual target
Mechanism: Senses rotations of head, commands compensatory movement of eyes in opposite direction.
Connections from semicircular canals, to vestibular nucleus, to cranial nerve nuclei excite extraocular muscles
Motion of the head
Motion of the eyes