physiology of balance

Question 1

what parts of the ear are responsible for balance

Answer 1

semi circular canals and otolith organs

Question 2

what are the otolith organs and what direction do they detect movement in

Answer 2

1. The utricle - horizontal movement
2. the saccule - vertical movement

Question 3

how do the otolith organs work to detect directional movement

Answer 3

1. The macula (a gelatinous layer embedded with calcium carbonate crystals aka otoliths) lies on the horizontal plane in the utricle and the vertical plane in the saccule
2. The otholiths have a greater specific gravity (resulting in inertia) than the surrounding tissue and movement causes them to bend the cilia in the direction of gravitational pull - this results in AP firing in a similar manner as the IHCs (tip link opening to due mechanical pressure)
3. The otoliths also contain hair cells, with the ‘hairs’ of the hair cells contained within the gelatinous membrane.

Question 4

what nerve inervates the otoliths/semicircular canals

Answer 4

the vestibular nerve - originates from scarpa’s vestibular ganglion

Question 5

how do semicircular canals work

Answer 5

1. the semicircular canals are filled with endolymph and have an enlargement on one end called the ampulla
2. the ampulla has a small crest (cirsta ampullaris) on top of which sits a gelatinous layer (cupula)
3. as the canals are embedded in the skin, they move relative to the rotation of the head
4. the fluid inside the canals moved slower than the movement of the head (transfer of momentum) thus bending the HCs
5. AP fired due to HC tip link opening

Question 6

how is direction of head movement detected (e.g. tilt to the right)

Answer 6

axon tonic firing rate - the HCs have a base firing rate when the head is in a neutral potion -> an increase in firing rate is seen if the head is spun in one direction thus drepressing more tip links and increasing firing from the organs assigned to that direction) –> if spun the other way then this firing rate decreases thus signalling to the brain that the movement has been in the direction opposite to where that organ is assigned to

Question 7

what 3 planes do the semicircular canals detect movement in

Answer 7

1. rotation (head tilt)
2. up/down (nodding yes)
3. left/right (shaking head)

Question 8

where do axons from the otolith organs go to (2)

Answer 8

1. lateral vestibular nucleus
2. vestibulospinal tract

Question 9

where do axons from the semicircular canals go to (3)

Answer 9

1. medial vestibular nucleus
2. medial longitudinal fasiculus
3. motor nerves of trunk and neck muscles

Question 10

what is the vestibular-ocular reflex

Answer 10

stabilisation of the eyes whilst the head moves -> reflex is transmitted throught the vestibular nuclues to the occulomotor nucleus

Question 11

what is one of the fastest reflexes in the body (ent)

Answer 11

vestibular-ocular reflex

Question 12

what is nystagmus

Answer 12

a rhythmical, repetitive and involuntary movement of the eyes

Question 13

why is nystagmus sometimes seen in vertigo (dix-hallpike)

Answer 13

disruption of the vestibular-ocular reflex -> The eye reflex (VOR) is stimulated as if the head has moved quickly to a certain position. The eyes mistakenly move to that position, but quickly register that they have been fooled and almost immediately move back to their correct position of alignment with the head

Question 14

what are the different types of nystagmus and what do they indicate

Answer 14

unilateral horiztonal - vestibular cause
Bilateral horizontal or vertical - brain cause

Question 15

how do stereocilia work to produce an AP

Answer 15

tip links are present between each stereocilia -> stretching these tip links opens cation channels -> cell depol and AP generation
if pushed in the wrong direction the cells become hyperpolarised (e.g. in semicircular canals when moving the head on the opposite direction)