Hearing and Balance

Question 1

What 3 systems need to integrate signals to maintain good posture and balance?

Answer 1

Vestibular, proprioception and vision (and important connections with cerebellum and motor systems)

Question 2

What are the 2 organs of equilibrium?

Answer 2

Semi circular canals and otiliths

Question 3

Explain the bony labyrinth

Answer 3

3 semicircular canals, from top - anterior, lateral and posterior. Ampulla is the dilated region at the end of the canals. 
Fenestra vestbuli (oval window)
Vestibule
Fenestra cocheae (round window)
Cochlea

Question 4

Explain the membranous labyrinth

Answer 4

Utricle, saccule, cochlear duct.
Endolymph - low Na, high K (inside membranous labyrinth)
Perilymph - high Na low K (between bony labyrinth and membranous labyrinth).

Question 5

Explain hair cells

Answer 5

Sensory receptors of the vestibular system
They have no axon but the release their neurotransmitter directly from their cell body onto the axon of an adjacent cell.
Apical surface - kinocilium is the tallest sterocillia (all rest = stereocilia and are 40-70 actin rich).
Stereocilia are connected by tip links - only join up and down and all stereocilia point in the direction of the kinocilium.
Tip links are linked to mechanotransducer channels.

Question 6

What happens when stereocillia are pulled in the direction of the kinocilium?

Answer 6

Causes the channels to open, Ca++ and K+ flow in and depolarise the cell - cause VOCC to open and stimulate NT release to an afferent fibre.

Question 7

What happens if they are pulled away from the kinocillium?

Answer 7

The channels that would have been slightly open at rest close and the cell hyperpolarises. Decrease frequency of impulse. No NT release.

Question 8

What is the ampullary crest?

Answer 8

Inside the ampulla (dilated region at end of canals) and is where the hair cells sit. It is enlarged epithelium.

Question 9

What is the cupula?

Answer 9

It is a gelatinous mass that the hair cells are embedded into.

Question 10

Explain what happens in the semi-circular…

Answer 10

Semi circular canals respond to angular acceleration. All the hair cells point in the direction of the saccule. Therefore each amuplla responds to depolarisation in one direction and hyperpolarisation in the other. The semicircular canals work in pairs. Fluid moves in the opposite direction from movement.
So, looking from top (utricle is at front), if turning left - fluid moves to the right. Hyperpolarises right ampulla, decrease firing and depolarise left, increase firing.

Question 11

What are otiliths and where are they found?

Answer 11

They are crystals of calcium carbonate and are found in the utricle. They are heavy, rest on macula - otiliths sags in direction of tilt and bends hair cells. MOVEMENT is nto required - static head position can be detected.

Question 12

Explain the oreintation of the stereocilia in the utricle and the consequences of this organisation.

Answer 12

They are all point towards a curving landmark - the striola. Therefore tilit or linear acceleration in many directions can be responded to. Complex signals will be sent to the brain to accurately measure head position

Question 13

How is the macula orientated in the saccule? Why?

Answer 13

Vertically when the person is upright. Therefore responds to vertically directed linear force - and directs the position of the head in space.

Question 14

Organisation of the cilia in the saccule?

Answer 14

All point away from the striola.

Question 15

Where does the vestibulocochlear nerve enter the brainstem?

Answer 15

cerebellopontine angle

Question 16

Explain the projections from the 4 vestibular nuclei…

Answer 16

Lateral vestibular nuclei - to cerebellum and limb motor neurones to maintain posture and balance.

Medial vestibular nuclei - to the extraocular muscles of the eye and also the the neck muscles to control the vestibuloocular reflex - head moves - move eyes in opposite direction but same angle.

Question 17

What is Meniere’s disease?

Answer 17

Too much endolymph and distension of the membranous labyrinth

Causes - severe vertigo, nausea, nystagmus, hearing loss and tinnitus

Question 18

What is benign positional vertigo?

Answer 18

When otoconia dislodges from utricle to the semi circular ducts. When head moves, grsavity dependant movement of otoconia causes abnormal fluid displacement in the affected semicircular canals - resultant vertigo.

Question 19

What is acoustic neuroma?

Answer 19

benign tumor of the myelin forming cells of VIII located in the cerebellopontine angle.

Question 20

What is the function of the ear?

Answer 20

To transduce sound waves into neural signals that can be processed by the brain.

Question 21

What are sound waves?

Answer 21

They are alternate compression and rarefraction (to make less dense).

Question 22

Give 4 features of a sound wave and do they correlate to anything?

Answer 22

Waveform - amplitude plotted against time…sine waves.
Phase
Amplitude (dB) = LOUDNESS
Frequency (Hz) = PITCH (cycles/second)

Question 23

What frequency range can human ears detect?

Answer 23

20Hz to 20kHz.

Question 24

Explain the external ear

Answer 24

Pinna, concha and auditory meatus gather sound and focus it on the ear drum (tympanic membrane) - increase the pressure 30-100fold. The pinna and concha provide cues about the eleveation of the sound source.

Question 25

Explain the middle ear

Answer 25

Function; to match the realtively low impedance airborne sounds to the higher impedance fluid of the inner ear…by focussing force from large diameter tympanic membrance to the smaller diameter oval window.

Vibrate tympanic membrane…malleus, incus…stapes…oval window

Question 26

Explain the organisation and function of the inner ear.

Answer 26

Cochlear. Spiral shaped. if unfolded, long…contains the basilar membrane - stiff and narrow at the base - HIGH FREQ, but wide and flexible at the apex for LOW FREQ (tonotopy)

The cochlear has 3 compartments - scala vestibuli, scala media (where organ of corti sits on basilar membrane, with the hair cells covered by the tectorial membrane) and the scala tympani.

THe verticle movement of the basilar membrane causes the tectorial membrane to slide sideways over the hair cells causing them to bend. If towards kinocilium then depolarisation - release glutamate onto afferent auditory fibre. But if bent away from kinocilium - hyperpolarisation.

Question 27

The auditory nerve fibre has an anatomical arrangement that follows that of the basilar membrane. Explain what the tonotopic rate and place code indicate.

Answer 27

Pattern code - rate of firing is porpotional to amplitude.

Place code - where hair cell is determines frequency.

Question 28

Explain the function of the outer hair cells.

Answer 28

3x as many as IHC.
Respond to electrical stimulation by changing their length - depolarisation = contraction and hyperpolarisation = elongation.
Motility contributes to motility of the basilar membrane and therefore amplifies the signals (especially weak ones).
OHC also emit sounds - otoacoustic emission - propagate in reverse through middle ear to tympanic membrane. Infant hearing test.

Question 29

Explain the pathways to the auditory cortex.

Answer 29

Cochlear, spinal ganglion, ventral and dorsal cochlear nuclei, superior olive, inferior colliuculus, MGN, auditory cortex.

Tonotopy is preserved throughout.

Question 30

Explain intraural time and level differences.

Answer 30

Sound from left arrives in left ear first. Arrives in left cochlear nucleus first, first to superior olivary nucleus. Sound from right ear will reach the left sup olive’s 3rd neucles at same time - summation and AP.

Question 31

What is conductive deafness and what causes it?

Answer 31

Can hear better through bone

Earwax, damage to ear drum, otosclerosis of the middle ear, trauma, middle ear infection,

Question 32

What is sensorineural deafness?

Answer 32

cochlear - infection, trauma, noise, ototoxic drugs, genetic defects and tumor,

Question 33

What is central deafness

Answer 33

Vascular accident, trauma, MS, infection, tumor, neonatal distress.

Question 34

What is tinnitus?

Answer 34

Constant sensation of tone. Years. Spontaneous activity of hair cells, spiral ganglion cells or neurones of CNA. hard to treat.