Physiology of Auditory and Vestibular Systems

Question 1

describe hair cells?

Answer 1

auditory receptors with stereocilia located between basilar membrane and reticular lamina - responsible for stimulus responsive cation entry
cell body releases glutamate onto efferent nerve endings (does not create an AP itself, but glutamate causes an AP in the nerve)
afferent nerves regulate the response
specialised structure at the tip which gates or closes the TMC1 channel mouth depending on the tilt of cilia

Question 2

how is AP frequency modulated?

Answer 2

tonic release of glutamate

Question 3

common feature in all 3 sensory organs in the labyrinth?

Answer 3

hair cells

cochlea, semi-circular canals, otolith organs/otocysts

Question 4

what is sound?

Answer 4

repetitive changes in density of air
an object vibrates > produces changes in air pressure > air vibrates and travels in 3 dimensions > we hear a sound
(speech = vocal cord vibrations)

Question 5

give a brief overview of how hearing occurs?

Answer 5

sound > vibration of air > vibrate the tympanic membrane the malleus then incus then stapes > vibration spreads to cochlea > vibration of air converted to movement/vibration of fluids in cochlea > vibration in cochlea is captured by hair cells > transduction (physical vibration is transduced to neural energy) > perceived in auditory cortex

Question 6

what are the 3 mechanisms for impedance matching?

Answer 6

1. area ratio of the eardrum to the stapes footplate (vibration converges onto smaller area)
2. lever action of the ossicles
3. buckling of the eardrum (increases pressure)

Question 7

where are hair cells found in the cochlea?

Answer 7

sit on basilar membrane in the organ of corti within the scala media of the cochlea

Question 8

describe transduction through the cochlea?

Answer 8

organ of corti hair cells > basillar membrane > tectorial membrane

Question 9

what separates the 3 layers of the cochlea?

Answer 9

reissner’s membrane separates scala vestibuli and tympani

basilar membrane separates scala media and tympani

Question 10

anatomy of cochlea?

Answer 10

less rigid at apex, so requires less force to get movement of basilar membrane and therefore fluid and therefore hair cells (low frequency sounds)
more rigid at base so needs lot of force to move basilar membrane (high frequency?)
scala media closed off at apex
scala tympani and vestibuli connect at helicotrema (at apex)
scala tympani meets oval window
scala vestibuli meets round window
pressure at oval results in complimentary motion at round

Question 11

endolymph movement in cochlea causes what?

Answer 11

movement of stereocilia > displacement of organ of corti > change in tilt of stereocilia > change in entry level (increased/decreased?) > release of glutamate onto neurons

Question 12

describe hair cells?

Answer 12

basis for tontopy - hair cells are basically tuned to different frequencies (different frequency sounds stimulate hair cells at different locations)

Question 13

describe current in hair cells?

Answer 13

K+ current is inwards rather then outwards
fluids in inner ear are therefore different to other cells
- endolymph has high K+ conc, low sodium (opposite to perilymph in Scala vest/tmp) creating inwards conc gradient
hair cell is slightly depolarized at rest (due to potassium gradient?)
potassium enters the hair cell and is recycled via channels, transporters and gap junctions

Question 14

inner hair cells?

Answer 14

main source of afferent signal in CN VIII, main part of sound perception
multiple hair cells innervated by a single afferent

Question 15

outer hair cells?

Answer 15

primarily get efferent inputs

control stiffness and amplify membrane vibration

Question 16

how do outer hair cells act as cochlear amplifiers?

Answer 16

contains motor protein “prestin” in membrane which can change the length of the cell (changes ratio of B membrane and T membrane)
respond to sound with both a receptor potential and a change in length
length change of OHC > increased movement of BM > increased bending of IHC > increased transduction

Question 17

what are the 2 mechanisms for frequency coding in CN VIII?

Answer 17

place code

temporal code

Question 18

how is the auditory system different to the visual system>

Answer 18

signals from right/left are all mingled

as you need to localise where sound is coming from

Question 19

how is tonotopy preserved in the auditory system?

Answer 19

VIII nerve branches into 3 cochlear nuclei
- dorsal cochlear nucleus (DCN)
- posteroventral cochlear nucleus (PVCN)
- anteroventral cochlear nucleus (AVCN)
tonotopy ensures that each neuron innervates several different areas and different neuron types

Question 20

how and where does sound location mainly occur in the pathway?

Answer 20

determined by time taken (interaural time difference) and intensity of sound at arrival at the ears
lateral and medial superior olives involved
- medial = computes sound arrival at the two ears (e.g ipsilateral ear earlier than the contralateral, this generates an “interaural time difference” using a “delay line”

Question 21

functions of vestibular system?

Answer 21

provide information on gravity, rotation and acceleration
reference for somatosensory and visual systems
integration of arousal, conscious awareness of body via connections with vestibular cortex, thalamus and reticular formation
allows for gaze and postural stability, orientation and detection of linear and angular acceleration

Question 22

what does the vestibular organ sense?

Answer 22

head angular acceleration (semi-circular canals) - head rotation
head linear acceleration (saccule and utricle) - translational motion, gravity and extension by head tilt

Question 23

peripheral sensory apparatus of vestibular system?

Answer 23

detects and relays information to vestibular nucleus about head angular and linear velocity to central processing system

Question 24

central processing system of vestibular system?

Answer 24

processes information in conjunction with other sensory inputs for position and movement of head in space

Question 25

motor output system of vestibular system?

Answer 25

generates compensatory eye movements and compensatory body movements during head and postural adjustments

Question 26

how does movement cause stimulus?

Answer 26

different head movements stimulates different hair cells as different ones are displaced by fluid depending on direction of movement

Question 27

function of semi circular canals?

Answer 27

detect head movements via rotatory acceleration

Question 28

how are semi-circular canals orientated?

Answer 28

lie in 3 orthogonal planes on both sides of the head
posterior canal shares plane with contralateral anterior canal
horizontal canals on both sides share a plane
this forms a push-pull pairing between canals with their contralateral counterparts (increases movement - hence firing- in one and reduces in the other giving direction of movement)

Question 29

what are the otolith organs?

Answer 29

utricle and saccule
sense linear acceleration and gravity
utricle = horizontal movement
saccule = vertical movement

Question 30

organisation of otolith organs?

Answer 30

hair cells lie in macula
striola divides hair cells into 2 groups with opposing polarities
hair cell cilia orientated in all directions
crystals of calcium carbonate called otoconia are on the otolithic membrane

Question 31

what is the striola of macula?

Answer 31

serves as a structural landmark
arranges otoconia into narrow trenches dividing each otolith
orientation of hair cells change over course of the macula
allows otolith organs to have multidirectional sensitivity

Question 32

what are the 3 main vestibular reflexes?

Answer 32

vestibulo-ocular reflex (keep eyes still while head moves)
vestibulo-colic reflex (keeps head still or level while you walk)
vestibular-spinal reflex (adjusts posture for rapid changes in position - balance)

Question 33

describe the structure of semi-circular canals

Answer 33

3 endolymph filled canals at right angles to each other
crista = sheet of cells where hair cells cluster
ampulla = bulge along canal containing cristae
cilia = project into gelatinous cupula
kinocilia = orientated in same direction so all excited/inhibited together