27- Auditory and Vestibular System

Question 1

what do hair cells detect in mammals as motion-detecting mechanoreceptors?

Answer 1

sound waves of different frequencies

lateral and rotational head movements

direction of gravity

Question 2

describe the structure of a generic hair cell receptor

Answer 2

small nerve cell but with no axon - makes many glutamatergic synapses with afferent dendrites

hair cell embedded within tissue of an apical membrane

stereocilia bundles as thin, rigid processes made of actin - stick out in ascending height order. have mechanically gated ion channels

tip links - few protein molecules linking the tip of a shorter stereocilium to the side of a taller adjacent one. mechanically gated ICs at the base of the tip links

stereocilia are associated with a gelatinous structure - ensures movement of fluid in surrounding compartment maximally affects stereocilia

Question 3

describe how hair cells signal/ how it converts stimulation of the stereocilia into a receptor potential

Answer 3

fluid movement above the apical membrane causes displacement of the stereocilia

at resting potential of -40mV, hair cells have a resting AP firing rate

if fluid movement displaces stereocilia towards the taller stereocilium - causes tension at tip links, stretches and pulls open mechanically gated ICs = cell depolarisation = increased glutamate release and AP firing rate

if fluid movement displaces stereocilia towards shorter stereocilium - tip links slack, mechanically gated ICs close = hyperpolarisation = decrease glutamate release and AP firing

Question 4

describe how the structure of the ear channels sound waves of different frequencies to activate specific populations of auditory receptor cells (hair cells)

Answer 4

width and stiffness of the cochlear duct with its hair cells varies

base of cochlear duct and basilar membrane = narrow and stiff hair cells and fibres - picks up/ stimulated by higher frequency sounds (20kHz)

apex of cochlear duct = broad and floppy - picks up lower frequency sounds (20Hz)

higher freq sounds stimulate base - also stimulates posterior neurons of primary auditory cortex

Question 5

name the two types of extracellular fluid within the inner ear

Answer 5

perilymph
endolymph

Question 6

describe the perilymph

Answer 6

extracellular fluid, fills the bony labyrinth of the inner ear (primary chamber carved out from temporal bone)
high Na+, low K+

Question 7

describe the endolymph

Answer 7

extracellular fluid, fills the membranous labyrinth
high K+, low Na+
excessive positive charge - creates a potential difference between the inside of the membranous labyrinth and the rest of the body

Question 8

name the two compartments of the inner ear

Answer 8

bony labyrinth
membranous labyrinth

Question 9

describe the bony labyrinth

Answer 9

series of channels carved within temporal bone

filled with perilymph - protects membranous labyrinth

contains the vestibule, semi-circular canals and cochlea

Question 10

describe the membranous labyrinth

Answer 10

system of membranes and fluid-filled tubes suspended within the bony labyrinth, perilymph fluid in between

membranous labyrinth is filled with endolymph

contains utricle and saccule, semicircular and cochlear ducts

Question 11

what is unique about endolymph?

Answer 11

high K+, low Na+ = excessive positive charge in fluid for creating a potential difference between the membranous labyrinth and rest of the body

Question 12

describe the cochlea

Answer 12

coiled structure with cochlear duct in membranous compartment

contains three chambers: scala tympani, vestibuli and media

receptor hair cells from base to apex of spiral - fluid movement within cochlear duct tilts stereocilia that extend into endolymph

Question 13

describe the organisation of the semicircular canals

Answer 13

3 semicircular canals, each at right angles from one another

ampulla at the base of the ducts connect each duct to the utricle

Question 14

components of the vestibular system

Answer 14

bony = semicircular canals, cochlea, vestibule

membranous = utricle and saccule, semicircular and cochlear ducts

Question 15

what are the locations for the 5 vestibular receptors?

Answer 15

three = one in each each of the ampulla connecting semicircular ducts to utricle

one in utricle

one in saccule

Question 16

name the two vestibular systems involved in maintaining posture and equilibrium, and controlling eye movements

Answer 16

otolith system (utricle and saccule)

ampulla of each of the semicircular ducts

Question 17

what is the otolith system?

Answer 17

contains utricle and saccule

picks up head movements and direction of gravity

Question 18

what is the role of the ampulla of the semicircular ducts?

Answer 18

picks up any combination of rotational head movements

Question 19

what is conductive hearing loss?

Answer 19

any blockage preventing transmission of sound waves to the oval window, to hair cells and neurological bits of the system

e.g. blockage of ear canal, eardrum rupture

Question 20

what is otitis media?

Answer 20

middle ear infection behind the eardrum, causing the air-filled space of the middle ear to fill with pus-like fluid

restricts movement of membranes and ossicles

Question 21

transmission of soundwaves from outer to inner ear - pathway

Answer 21

sound waves captured and channelled by pinna into ear canal

ear drum vibrates - vibrations passed onto the three bones of the middle ear (hammer, anvil and stirrup). three bones give a certain leverages that allows amplification

stirrup vibrates and is pressed against the oval window - causes vibrations of fluid in inner ear

Question 22

transmission of soundwaves in inner ear/cochlea

Answer 22

stirrup is pressed against oval window and vibrates - vibrations spread in and along perilymph of scala vestibuli of cochlea

basilar membrane bordering cochlear duct is flexible and vibrates = associated with complex structures, passes on vibrations

hair cells with stereocilia extending into endolymph stimulated - tip links stretched, mechanically gated ICs opened, depolarisation and increased AP firing

afferent neurons transmit APs along CN8 (vestibulo-cochlear nerve) to the auditory cortex

Question 23

describe the transduction of soundwaves through the spiral organ/ organ of corti

Answer 23

basilar membrane of the cochlear duct contains two hair cell types = outer and inner

outer = several rows that stretch up through basilar membrane, stereocilia in tectorial membrane (the gelatinous structure that controls tilting)/ stereocilia exposed to endolymph.

inner = one single row

vibrations pass from the scala vestibuli to tympani, through the flexible basilar membrane - cells above vibrate up and down, which causes hair cell stereocilia to tilt

outer and inner hair cells tilt in the same direction towards the tallest stereocilia (outer due to vibrations, inner due to rush of fluid past it). opening mechanically gated ICs = K+ influx = depolarisation = AP firing

Question 24

what are the four different qualities of sound?

Answer 24

frequency

loudness

timing

origin

Question 25

how does the auditory system encode frequency as a sound quality?

Answer 25

frequency encoded by the width and stiffness of the basilar membrane, where different regions have sensitivities to different sound frequencies

the base of the basilar membrane is narrow and stiff = sensitive to 20kHz/ high-freq. sounds

the apex of the basilar membrane is floppy and thick = sensitive to 20Hz/ low freq sounds

signals from different regions of basilar membrane passed to cerebral cortex for interpretation

Question 26

describe the primary auditory pathway

Answer 26

afferent nerves (glutamate)

CN8 (vestibulocochlear nerve)

brainstem cochlear nuclei

crosses over at inferior colliculus

medial geniculate nucleus

primary auditory cortex and higher order structures beyond the cerebral cortex

Question 27

describe the tonotopically organised of the auditory pathway

Answer 27

basilar membrane:
- base = narrow and stiff, sensitive to high. freq sounds (20kHz)
- apex = floppy and thick, sensitive to low-freq sounds (20Hz)

auditory cortex:
- posterior neurons = high-freq sounds
- anterior neurons = low-freq sounds

Question 28

what is Wernicke’s area?

Answer 28

an area in the left hemisphere of the superior temporal lobe

important in language comprehension, damage can cause Wernicke’s aphasia - impaired comprehension, nonsensical but fluent speech

Question 29

how does the auditory system encode loudness as a sound quality?

Answer 29

louder sound means increased AP firing rate

Question 30

how does the auditory system encode timing as a sound quality?

Answer 30

fast axons and powerful synapses help preserve timely info when a hearing event occurs

Question 31

how does the auditory system encode origin as a sound quality?

Answer 31

origin encoded by outputs from superior olivary nuclei, sound localisation comparing laterality and loudness between ears

medial SO cells sensitive to smallest differences in arrival times of soundwaves between two ears – limited to low freq. sounds. higher freq sounds have pressure waves too close together to be identified clearly.

lateral SO compares loudness of sounds in two ears – smallest differences in loudness of a specific freq. indicates laterality of sound based on loudness.

medial SO for lateralisation of sound. lateral SO for loudness of sounds in two ears. both involved in localisation.

signal from cochlear nuclei = MSO & LSO nuclei = primary auditory cortex

Question 32

explain the distinction between inner and outer hair cells and their respective roles in hearing

Answer 32

inner hair cells - associated with 95% of afferent peripheral fibres to the spinal ganglion –> CN8

primarily respond to sound waves - convert sound waves into electrical signals

outer hair cells - associated with only 5% of afferent peripheral fibres despite there being 3-5 rows

modulate sound, act as amplifiers by tugging on the tectorial membrane their stereocilia are associated with and enhance vibrations

Question 33

vulnerabilities in the system that can lead to conductive and sensorineuronal deafness

Answer 33

3500 inner hair cells that aren’t replaced despite damage and have to last a lifetime

vulnerable to louder sounds - louder sounds means greater vibrations/ amplitudes = larger receptor potentials, glutamate release and more APs fired

more likely to cause damage to stereocilia as tugging on the tectorial membrane its associated with is greater - can lead to distortion and death of hair cells. dying hair cells release lots of glutamate = excitotoxicity

larger vibrations destroy hair cells, excess glutamate destroys afferents

vulnerable to genetic conditions - losing hair cells earlier in life, especially those that pick up high freq sounds (near base). can restore hearing with a cochlear implant. apparatus picks up sound frequencies and send electric signals.

otitis media (middle ear infection resulting in pus-like fluid filling middle ear)

conductive hearing loss through blockage or perforated eardrum

Question 34

describe how the structure of the otolith system responds to different types of head movement and changes in the direction of gravity, such as to generate responses in different populations of vestibular hair cells

Answer 34

structure of otolith system: consists of utricle and saccule with macula - patches of hair cells. otolith membrane as gelatinous structure associated with stereocilia. otoconia crystals on top pf otolith membrane give it mass and thus inertia - the tendency for an object to continue its current state of motion.

has all directions of linear head movement and gravity covered considering its macula and structure.

utricle and saccule are orientated at 90 degrees from one another - one picks up vertical, the other horizonal movements

different populations of macula/ hair cells orientated in different directions. different head movements will activate/depolarise one hair cell, hyperpolarise another. information to cerebral cortical structures depends on the hair cells/ macula stimulated

linear head movements - acceleration, decelleeration, constant motion

with acceleration - head moves. otolith initially remains stationary due to inertia, but is then dragged into movement by stereocilia. otolith membrane lags behind. hair cells within a macula are at different orientations - one hair cell is depolarised as stereocilia tilt towards the tallest. another hair cell is hyperpolarised. depolarised hair cell sends increased AP firing signals via glutamate release through afferent fibres.

constant motion - otolith catches up with stereocilia movement. stereocilia return to neutral position with resting AP firing rate.

deceleration - otolith continues moving for a while due to inertia. deceleration causes a burst of APs in the hair cell that initially signalled the start of head movement. burst inhibits that hair cell during deceleration.

change in direction of gravity: head tilts, otolith with otoconia crystals has mass and tilts in the direction of gravity with stereocilia - causes depolarisation and hyperpolarisation of different hair cells within a macula.

Question 35

describe the neural pathways for vestibulo-ocular reflexes

Answer 35

vestibulo-ocular reflex = maintains stability of visual fixation during head movements/ changes in head movement

when the head turns to the left = increases activity from left horizontal canal – initially endolymph is stationary due to inertia, eventually endolymph pushes against cupula gelatinous structure, causing the stereocilia to tilt towards the tallest stereocilium/ towards utricle, depolarisation from K+ influx, generates increased AP firing in afferents

afferents from the left horizontal SC hair cells project to the vestibular nuclei (brainstem), then to the oculomotor and abducens nuclei which are the origins for the motor neurons that control extraocular eye muscles. brain interprets vestibular information, sends signals to contract the lateral recuts and medial rectus eye muscles, adjusting visual fixation for stability depending on the head’s position.

axons connect the vestibular nuclei and abducens nucleus through the medial longitudinal fasciculus - fast, heavily myelinated axons.

Question 36

describe the functions of the semicircular canal system, otolith system and cochlear duct

Answer 36

semicircular canals = detects rotational head movements

otolith system = detects changes in direction of gravity and linear head movements (dec/acc/constant motion)

cochlear duct = transmission of sound waves

Question 37

describe how the structure of the semicircular system responds to different types of rotational head movement, such as to generate responses in different populations of vestibular hair cells

Answer 37

semicircular ampulla contain:
- crest0supproting cells with embedded hair cells connected to an afferent fibre
- stereocilia that extend into endolymph (high K+) associated with cupula/ gelatinous membrane; tallest stereocilium towards the utricle

semicircular canals respond to any combination of rotational head movements

angular acceleration/ head turning - e.g. in the left horizontal canal when head turns
- endolymph in ampulla lags due to inertia before pulling back on cupula = forces stereocilia to tilt towards tallest
- activates hair cells = depolarisation = signal don afferent nerves

angular deceleration/ head turning stops:
- head decelerated, endolymph continues moving for a while due to inertia before stopping
- stops moving = decreases rate of AP firing from afferents

semicircular canals are orientated in pairs - if head turns to left, increased activity in left horizontal canal means decreased activity in right horizontal canal

Question 38

role of the cerebellum in the vestibulo-ocular reflex?

Answer 38

flocculo-nodular lobe in cerebellum ensures accuracy in eye movements

when the eye sends error signals to the brainstem about deviation from central vision, the cerebellum adjusts the strength of vestibular output = reduces errors

restores accuracy in eye movements through long-term recalibration = e.g. with loss of hair cell receptors, cerebellum adjusts for the long-term