Vestibular System

Question 1

What is the vestibular system?

Answer 1

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

Question 2

How does this sensory information get back to the brain?

Answer 2

the vestibular nerve which is part of the vestibulocochlear nerve

Question 3

What is the vestibular apparatus?

Answer 3

3 semi-circular canals, the urticle which the semi-circular canals connect and the saccule

Question 4

Describe the semi-circular canals in more detail.

Answer 4

they are at right angles to each other (3D) and their swellings at their bases are called ampulla which contain sensory hair cells

Question 5

What are the otolith organs?

Answer 5

urticle and saccule

Question 6

What does the saccule do?

Answer 6

vertical acceleration

Question 7

what does the utricle do?

Answer 7

back/front head tilt and horizontal acceleration

Question 8

What do semi-circular canals detect?

Answer 8

rotational acceleration

Question 9

where are and how do the sensory receptors in semi-circular work?

Answer 9

embedded in swellings at the base of the bony canals called ampulla-inside are hair cells which act as sensory receptors and are 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 - distortion triggers action potentials which are transmitted along sensory neurone of the vestibular nerve as it synapses directly with the gelatinous cupula

Question 10

What happens when the skull is rotated from left to right from rest?

Answer 10

the endolymph at 1st does not move because of its inertia but the ampulla moves instantly as 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

Question 11

What then happens if you rotate at constant velocity?

Answer 11

the endolymph catches up and rotates at the same speed, removing the shearing forces but this takes several seconds

Question 12

What happens when you stop suddenly?

Answer 12

the endolymph to continue to move due to momentum creating a continuing sense of movement and dizziness

Question 13

What are the 2 types of cilia of hair cells?

Answer 13

single very large kinocilium and a set of progressively smaller stereocilia

Question 14

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

Answer 14

depolarisation and increased discharge of APs in the vestibular nerve

Question 15

what does distortion away from the kinocilium cause?

Answer 15

hyperpolarisation and decreased discharge of APs in the vestibular nerve

Question 16

What does stimulation of the vestibulocochlear nerve cause?

Answer 16

this allows the brain to determine movement in time and space- most integration takes place in the cerebellum

Question 17

what does the orientation of the cupula allow?

Answer 17

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

Question 18

what are the sensory receptors of the otolith organs?

Answer 18

the sensory apparatus of utricle and saccule are called the maculae - macula in utricle are on the horizontal plane orientation and in saccule are on the vertical plane orientation - maculae have cilia (one kinocilium and series of stereocilium) just like the ampulla with protrude into gelatinous mass called the otolith membrane - embedded in the otolith membrane are CaCO3 crystals called otoliths

Question 19

What does tilting the head cause?

Answer 19

the tilt is detected by the macula in the utricle (horizontal plane) and otoliths have a greater density than endolymph and thus are affected more by gravity- tilting the head moves the otoliths and the otolith membrane in which they are embedded which distorts the jelly and moved the cilia

Question 20

what does backwards head tilt cause?

Answer 20

moves the otolith in the direction of the kinocilium causing depolarisation and increased discharge of APs

Question 21

what does forward tilt of the head cause?

Answer 21

moves the otolith away from the kinocilium causing hyperpolarisation and decreased discharge of APs

Question 22

what do vertically orientated macula in the saccule do?

Answer 22

respond to vertical forces eg elevation in a lift and provide orientation of the head when lying down

Question 23

what does the information from the vestibular nerve do?

Answer 23

it keeps the brain informed of body’s position in space and direction of movement

Question 24

Where else does the vestibular nerve connect with?

Answer 24

the cerebellar centres which co-ordinate the postural muscles required to maintain balance without synapsing in the medulla

Question 25

what is vestibular nystagmus?

Answer 25

predictable eye movement in response to stimulation from the vestibular system = 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 the head rotating

Question 26

Describe right nystagmus.

Answer 26

the direction of nystagmus is the direction of the rapid flick back - right rotation causes right nystagmus- the extent of eye movement is restricted by the bony orbit so when the eyeball comes to the end of its range of movement it rapidly flicks back forward

Question 27

How is vestibular function tested?

Answer 27

Nystagmus via caloric stimulation where the outer ear is washed out with either warm or cold fluid and the temperature difference from the core 37*c sets up convection currents through thin bone which affects the endolymph - warm fluid causes nystagmus towards the affected side and cold away (COWS- cold opposite and warm same)

Question 28

What is kinetosis?

Answer 28

motion sickness which is most likely to occur if visual and vestibular system inputs to the cerebellum are in conflict eg if the vestibular system indicates rotation but the visual system does not - the cerebellum generates a sickness signal to the hypothalamus to bring about to bring about the ANS changes= nausea, vomiting, decreased BP, dizziness, sweating and pallor

Question 29

What is labyrinthitis?

Answer 29

acute interference with normal vestibular function due to infection= all ANS symptoms and vertigo

Question 30

What is Ménière’s disease?

Answer 30

vertigo, nausea, nystagmus and tinnitus (subjective noise) - associated with overproduction of endolymph causing increased pressure- cause is unknown

Question 31

What is vertigo?

Answer 31

perception of movement in absence of movement, may also be nystagmus and it is gross impairment of posture and balance which is very disabling

Question 32

does the visual system compensate for vestibular impairment?

Answer 32

chronic vestibular impairment can be compensated quite well by the visual but problems occur if subjects cannot use visual information

Question 33

What do lesions of the brain stem cause?

Answer 33

nystagmus at rest