vestibular function

Question 1

what is thr vestibular system?

Answer 1

A sensory system essential in the control of posture and balance. Found in the inner ear, it is a series of fluid-filled membranous tubes, (labyrinths), which are embedded in the temporal bone

Question 2

what does the vestibular apparatus consist of?

Answer 2

of 3 semi-circular canals, the utricle to which the semi-circular canals all connect and the saccule.

Question 3

what are the otolith organs?

Answer 3

the utricle and saccule

Question 4

function of the utricle

Answer 4

back/front tilt and horizontal acceleration

Question 5

function of the saccule

Answer 5

vertical acceleration

Question 6

function of the semi-circular canals

Answer 6

rotational acceleration.

Question 7

where are the sensory cells of the semi-circular canals?

Answer 7

embedded in swellings at the base of the bony canals called ampulla.

Question 8

what are the function of the hair cells inside the swellings

Answer 8

sensory receptors

Question 9

where are the hair cells embedded in the swellings?

Answer 9

embedded in a flexible gelatinous structure called the cupula that stretches across the entire width of the ampulla and which becomes distorted by movement of the endolymph fluid within the canals.

Question 10

how does the semi circular canals detect rotational acceleration?

Answer 10

If the skull is rotated left or right from rest, the endolymph at first does not move because of its inertia. However the ampulla moves instantly because it is embedded in the skull.

The inertia of the endolymph produces drag which bends the cupula, and consequently the cilia embedded in it, in the opposite direction to movement. If rotate at constant velocity, the endolymph catches up and rotates at the same speed, removing the shearing forces, but this takes several seconds.
Sudden stop will cause endolymph to continue to move due to momentum creating a continuing sense of movement and dizziness.

Question 11

what are the two types of hair cells?

Answer 11

there is a single very large kinocilium and a set of progressively smaller stereocilia

Question 12

what does Distortion of the cilia in the direction of the kinocilium cause?

Answer 12

depolarisation and increased discharge of APs in the vestibular nerve.

Question 13

what does Distortion of the cilia away from the kinocilium lead to

Answer 13

hyperpolarization and decreased discharge of APs in the vestibular nerve.

Question 14

whats the purpose of the orientation of the cupua being slightly different?

Answer 14

so the brain can build a 3D image of body position using the pattern of firing and inhibition received.

Question 15

what is the maculae?

Answer 15

The sensory apparatus of the utricle and saccule

Question 16

what are otoliths

Answer 16

Embedded in the otolith membrane are CaCO3 crystals

Question 17

what is the tilt of the head detected by?

Answer 17

the macula in the utricle (horizontal plane);

Question 18

why are otoliths affected mroe by gravity?

Answer 18

otoliths have a density greater than endolymph and thus are affected more by gravity. Tilting the head moves the otoliths and the otolith membrane in which they are embedded. This distorts the jelly, and moves the cilia.

Question 19

what happens when the head is tilted backwards?

Answer 19

Backward tilt moves the otolith in the direction of the kinocilium causing depolarisation and increased discharge of APs. Opposite for forward tilt.

Question 20

what keeps the brain informed of the body’s position in space and any direction of movement.

Answer 20

The distinct pattern of information being sent from the three ampulla of the semi-circular canals, and the two maculae of the otolith organs, to the brain via the vestibular nerve

Question 21

where do the vestibular nuclei on one side project?

Answer 21

ipsilaterally, bilaterally and contra-laterally to descending motor pathways (and also the extraocular nuclei (extraocular muscles)

Question 22

where do vestibular nuclei receieve input from?

Answer 22

proprioceptors signalling limb and body position, also from neck and eye muscles

Question 23

where does the vestibular nuclei project vie?

Answer 23

via the thalamus to the cerebral cortex —> perception of movement and body position = kinaesthesia

Question 24

what do the vestibular system reflexes involve?

Answer 24

the vestibulocoritcal and vestibulospinal tracts

Question 25

what are the 3 types of vestibular system reflexes?

Answer 25

Tonic labyrinthine reflexes
Dynamic righting reflexes
Vestibulo-ocular reflexes

Question 26

what does the Dynamic righting reflexes do

Answer 26

rapid postural adjustments that are made to stop you falling when you trip. Long reflexes, involving extension of all limbs. Most profound in cats.

Question 27

what does the Vestibulo-ocular reflexes do?

Answer 27

strong association is evident between the vestibular apparatus, the visual apparatus and postural control.

Question 28

what are the types of Vestibulo-ocular reflexes?

Answer 28

Static reflex

Dynamic Vestibular Nystagmus

Question 29

what is the Dynamic Vestibular Nystagmus

Answer 29

A series of saccadic eye movements that rotate the eye against the direction of passive rotation of the head and body so that the original direction of gaze is preserved despite head rotating. Think about being on a slowly rotating roundabout

Question 30

how is Nystagmus is used to test vestibular function

Answer 30

Post-rotatory nystagmus,

Caloric stimulation

Question 31

what is Post-rotatory nystagmus,

Answer 31

Subjects are rotated in a Barany chair. If rotate to the left then during acceleration get a left nystagmus. At the end of rotation, for about 20 seconds, during deceleration get a right nystagmus.
Due to endolymph catching up and now pushing the cupula in the opposite direction.

Question 32

what is Caloric stimulation

Answer 32

Simple test for the horizontal SCC. When the outer ear is washed with either cold or warm fluid, the temperature difference from core 37C, gets through the thin bone and sets up convection currents which affect the endolymph. Warm fluid (44C) causes nystagmus towards affected side, cold (30C) causes nystagmus away from affected side (COWS – Cold Opposite, Warm Same).

Question 33

what gives rise to motion sickness (kinetosis)

Answer 33

Powerful maintained stimulation of the vestibular system can give rise to kinetosis

Question 34

when is motion sickness most likely to occur?

Answer 34

occur if visual and vestibular system inputs to the cerebellum are in conflict e.g. if the vestibular system indicates rotation but the visual system does not.

Question 35

what is Labyrinthitis

Answer 35

Acute interference with normal vestibular function as a result of infection = all ANS symptoms + vertigo

Question 36

what is vertigo?

Answer 36

is the perception (hallucination) of movement in the absence of movement. There may also be nystagmus. Gross impairment of posture and balance, very disabling.

Question 37

what is Ménière’s disease

Answer 37

Vertigo, nausea, nystagmus and tinnitus = subjective noise. Associated with over production of endolymph causing  pressure. The cause is unknown.