Auditory & Vestibular

Question 1

Describe the structure of the auditory system.

Answer 1

Outer ear
* External ear up to tympanic membrane
* auditory canal

Middle ear
* Tympanic membrane to oval window
* Ossicles (Malleus, Incus, Stapes)
* Eustachian tube

Inner ear
* Oval window up to auditory-vestibular nerve
* cochlea

Question 2

Describe how the attenuation reflex protects the inner ear.

Answer 2

Attenuation reflex protects inner ear by adapting ear to loud sounds. It dampens amplification when it could be dangerous. It responds to contraction of tensor tympani and stapedius muscles

Question 3

Describe how the structural properties of the basilar membrane allow discrimination of frequencies.

Answer 3

It has stiff base and wide floppy apex. Pressure wave hits the bottom and travels along it until the energy can be absorbed. Where the pressure gets absorbed corresponds with the frequency.

Question 4

Give a detailed account of the process by which pressure waves lead to transduction of hair cell receptor potentials.

Answer 4

• Sound in
• basilar membrane moves
• pushes stereocilia into the tectorial membrane & they bend (happens in the organ of corti)
• bending of stereocilia opens K+ channels
• endolymph has higher [K+] to K+ flows into cell
• K+ influx triggers VGCCs, influx of Ca++
• Ca++ triggers NT release from hair cells onto spiral ganglion neurons
• AP propagates along auditory nerve (CNVIII)

Question 5

Describe how outer hair cells function as a cochlear amplifier.

Answer 5

Outer hair cells are shortened or lengthened using motor proteins.
This can shorten the gap between basilar & tectorial membranes for INNER hair cells resulting in amplification

Question 6

Explain how neurons within the auditory pathway encode for sound intensity and frequency.

Answer 6

Sound intensity is encoded by the firing rate & # neurons activated
(more neurons, more firing = louder noise)

Sound frequency is encoded by the tonotopic map of characterised neurons on the basilar membrane - location & type of hair cells activated gives frequency

Question 7

Describe the mechanism by which the auditory system localises sound.

Answer 7

Horizontal localisation - using interaural differences
* time delay between ears for low freq
* intensity differences between ears for high freq

Vertical localisation - based on shape of external ear
* using reflections of sound from pinnae

Also through sumation at superior olive
* When AP from each ear reach superior olive at same time, they sum & generate AP
* where in the superior olive they sum, tells us the offset

Question 8

Explain the function and mechanism of the vestibular system

Answer 8

• Uses hair cells to detect changes in balance & equilibrium
• semicircular canals detect head rotation. 3 canals - 1 in each plane. as head rotates, fluid pushes the hairs over
• Otolith organs detet gravity & tilt. head tilts, otoconia (rocks) move, kinocilium bend.

Question 9

Explain the function and mechanism of the vestibulo-ocular reflex.

Answer 9

Reflex acts to fixate line of sight during head movement. Does this by sensing rotation of the head & commanding compensatory eye movements by exciting or inhibiting extraocular motor neurons.

Pathway: vestibular nerve -> medial vestibular nucleus -> extraocular motor neurons.

Question 10

Describe the auditory pathway

Answer 10

• Cochlear
• Brainsteam (ventral cochlear nucleus, dorsal cochlear nucleus)
• superior olive (both sides)
• inferior colliculi
• thalamus (medial geniculate nucleus)
• primary auditory cortex in temporal lobe