Hearing, Sight And Balance

Question 1

to achieve balance and posture, the brain needs to integrate signals from which 3 key inputs?

Answer 1

• Vestibular system
• Proprioception (somatosensory system)
• Vision

(As well as important connections with cerebellum and motor systems)

Question 2

What is the vestibular system and what is it important for?

Answer 2

2 organs of equilibrium: semicircular canals and otolith organs Both found in the inner ear, or labyrinth

Vestibular system is important for balance, posture and eye movements

Question 3

Where do you find endolymph and what is in it?

Answer 3

Membranous labaryinth
- high K
- low Na
(Opposite of plasma)

Question 4

What is peri lymph and where is it found?

Answer 4

Fluid in the bony labaryinth?
High Na (150mM)
Low K (7mM)

Question 5

What do hair cells release at their synapses with the primary afferent fibres?

Answer 5

Glutamate

Question 6

What are the sensory receptors of the vestibular system?

Answer 6

Hair cells

See page 2 of vestibular system lecture

Question 7

What do tip links do?

Answer 7

Stereocilia are connected by tip links

Join up the lower with the higher neighbours but they do not connect horizontally - only along the axis of polarity

(40-70 actin-rich stereocilia)

Question 8

By what mechanism are the membrane potentials of hair cells changed?

Answer 8

Bend hair cells over along axis of polarity - Stretch the tip links - open channels - ions in fluid can flow into the hair cells - pottasium and calcium. Rush into the tip - depolarisation
- slightly depolarised already - 10% of channels open already - bend the other way - close them - hyper polarised

Question 9

What are the effects of changes in hair cell membrane potential on neural firing?

Answer 9

Depolarisation - increased neural firing (impulse frequency)

Hyperpolaristaion - decreased neural firing (impulse frequency)

Question 10

How do hair cells respond to motion and position changes in the Semicircular ducts?

Answer 10

Ampulla - dilation of the semicircular canals -
Ampullary crest - thickened epithelium - Hairs sit on it
- cupula - gelatinous mass in ampulla - sound - pushes it to move the hair cells
(Angular acellaration)

(All hair cells in ampulla are arranged so that the axis of polarity always points in one direction, eg in horizontal semicircular duct they go towards utricle.
Therefore, because hair cells only respond along their axis, each ampulla responds with depolarization in one direction, and hyperpolarization in the other.)
(Pairing between ant. and post. On opposite sides - 3D)

Question 11

How do hair cells respond to motion and position changes in the utricle?

Answer 11

Macula

Otoliths - ear rocks - calcium crystals on otolithic membrane on a gelatinous mass - directly on top of macula

(Muscles of support, muscles of the necks, eye movement)

Question 12

How is the utricle macula able to detect static position etc?
What type of acellaration can it detect?

Answer 12

When upright, macula roughly horizontal, so otoliths rest directly on it. If head is tilted, gravity acts on heavy otolithic mass so it sags in direction of tilt and bends hair cells. So movement is NOT REQUIRED.
This means static head position can be detected.

Macula can also detect linear acceleration (frequent fluctuations lead to motion sickness).

Question 13

Do the cilia of utricle macular hair cells all face in the same direction?
What does this mean?

Answer 13

• No
  …they all point towards a curving landmark – the striola

This means the utricle can respond to tilt or linear acceleration in many directions. Tilt in any direction will depolarize some cells and hyperpolarize others – leads to complex signal to brain with accurate measure of head position

Question 14

How do hair cells respond to motion and position changes in the saccule?

Answer 14

Saccule essentially like utricle: also has a macula, but this is oriented vertically when person in upright position.

Therefore responds to vertically directed linear force and detects position of head in space

In the saccule, cilia also oriented in different directions, but unlike maccule, they point away from striola

Question 15

Where is the sensory ganglion of the vestibular nerve fibres?
What are the vestibular nerve fibres like?

Answer 15

Internal acoustic meatus

Fast, myelinated (efferent a and afferents)

Question 16

Where does the 8th cranial nerves enter the brainstem and have its nuclei?

Answer 16

Enter brain stem at CPA

nuclei near floor of fourth ventricle - near cerrebellar peduncle

Question 17

What is the role of the ascending and descending tracts from vestibular nuclei?

Answer 17

Project to nuclei of extra-oculomotor nerves and to cervical spinal cord to co-ordinate head movements with eye movements -this is the vestibulo-ocular reflex “small rotation of the head is
accompanied by movement of the eyes through the same angle but in opposite direction” – ‘doll’s eyes’

Project to cerebellum and lower spinal levels to co-ordinate extensor and flexor muscles to maintain balance and posture

Question 18

What do you know about Ménière’s Disease?

Answer 18

too much endolymph and distention of membranous labyrinth =>

attacks of severe vertigo, nausea, nystagmus, hearing loss and tinnitus,

plus eventual permanent progressive hearing loss.

Question 19

What do you know about Benign Positional Vertigo?

Answer 19

otoconia dislodged from utricle and migrate into semicircular ducts (often posterior one).

When head moves, gravity-dependent movement of otoconia causes abnormal fluid displacement in the affected semicircular duct and resultant vertigo

Question 20

What is acoustic neuroma?

Answer 20

Benign tumour of the myelin forming cells of the 8th cranial nerve - classically at the CPA
- balance and hearing problems