19 Posture + balance

Question 1

What is the structure of the vestibular apparatus?

Answer 1

Bony labyrinth with inner membranous labyrinth.

Semicircular canals + vestibule (utricle + saccule).

Question 2

How is the vestibule innervated?

Answer 2

Vestibulocochlear nerve.

Vestibular branch: utricular, saccular and ampullary nerve.

Question 3

What does the vestibular apparatus provide information about?

Answer 3

Head orientation, movement and location in space.

Question 4

What are the six degrees of freedom?

Answer 4

Translational: x, y, z.
Rotational: roll, pitch, yaw.

Question 5

What are the structural features of a maculae?

Answer 5

Matrix of supporting cells.

Hair cells possess cilia, penetrate into otolithic membrane (gel containing calcium carbonate).

Question 6

What are the structural features of hair cells?

Functional features?

Answer 6

One kinocillium.
Many cilia, functionally connected by ‘tip links’.
Bending towards kinocilium increases nerve firing.
Bending away decreases firing.

Question 7

How do hair cells produce an action potential?

Answer 7

Distortion of sterocilia opens K+ channel. K+ moves in from endolymph. Activates voltage gated Ca channels, triggering glutamate release.

Question 8

What do macula provide information about?

Answer 8

Static equilibrium (head relative to horizon, 0.5o sensitivity).
Dynamic equilibrium (linear acceleration).

Question 9

How are the maculae orientated?

Answer 9

Horizontal in utricle.

Vertical in saccule.

Question 10

What do the semicircular canals provide information about?

Answer 10

Rotational movements.

Angular acceleration.

Question 11

Where are the sensory organs in the semicircular canals located?

Answer 11

Ampulla.

Question 12

How are the ampulla structured?

Answer 12

Sensory hair cells embedded in cupula (gelatinous mass).

Cupula forms barrier against endolymph movement.

Question 13

How does head rotation activate the ampullary nerve?

Answer 13

Cupula + cilia distorted by head rotation due to inertia of the endolymph. Cupula bends in opposite direction to the endolymph.

Question 14

How are the semicircular canals arranged in pairs?

Why?

Answer 14

Ant + post. Post + ant. Two lateral.

Allows comparison - firing will increase on one side and decrease on the other.

Question 15

Describe the vestibular pathway associated with the cupula:

Answer 15

Afferent to medial vestibular nucleus.

Efferents to neck/body/trunk (vestibulo-ocular reflex).

Question 16

Describe the vestibular pathway associated with the otoliths:

Answer 16

Afferents to lateral vestibular nucleus.

Efferents to cerebellum + limb motor neurons (maintain posture).

Question 17

What is the function of the vestibulo-ocular reflex?

Answer 17

Maintain forward vision during head movements.

Question 18

What is Meniere’s syndrome? Symptoms?

Answer 18

Increased volume of endolymph rupture membranous labyrinth.

Pressure, earache, tinnitus, dizziness.

Question 19

What is Benign Paroxysmal Positional Vertigo? Cause?

Answer 19

Calcium carbonate crystals become dislodged from otoliths.

Illusion of movement, disorientation, dizziness.

Question 20

Which drugs can cause ototoxicity? (8)

Answer 20

Antibiotics.
Anticancers.
Diuretics.
Aspirin.
Quinines.
Metals.
Xylene.
Toluene.

Question 21

What does a medulloblastoma cause?

How?

Answer 21

Postural instability.

Cerebellar tumour leads to inability to utilise vestibular information from utriles + saccules.

Question 22

Describe the vestibulo-ocular reflex.

Answer 22

Rotation to right.
Right horizontal ear canal depolarisation, vestibular afferent increases activity.
Neurons to LVI + RIII fire more.
Neurons to LIII + RVI fire less.
L lateral rectus + R medial rectus contract.
L med rectus + R lat rectus relax.
Eyes rotate leftward.

Question 23

What is nystagmus?

Answer 23

Slow tracking movement in lone direction, then fast in the other.

Question 24

How does nystagmus change over time with right rotation?

Answer 24

Start: rightwards nystagmus.
30s endolymph catches up + nystagmus decreases.
End: fluid momentum causes leftwards nystagmus.

Question 25

What is the basis of electrooculography?

Answer 25

Cornea +vely charged compared to retina.

Eye movement changes voltmeter potential difference.

Question 26

What is caloric testing?

Answer 26

Lie patient down, irrigate ear with water.
Warm causes endolymph to rise - increasing firing.
Cold causes endolymph to fall - decreasing firing.

Question 27

What is caloric testing used for?

Answer 27

Checking brainstem function in unconscious patients.