Vestibular Function

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

series of fluid filled membraneous tubes (labyrinths)

fluid is called endolymph

Question 2

What makes up the vestibular system?

Answer 2

consists of 3 semi-circular canals which are at a right angle to each other and have swelling at the bottom of them called ampulla which contain sensory hair cells

also made up or an utricle, which all the semi-circular canals connect to, and a sdaccule

Question 3

What are the otolith organs?

Answer 3

utricle

saccule

Question 4

What are the semi-circular canals, utricle and saccule responsible for sensing?

Answer 4

Utricle - back/front tilt and horizontal acceleration

Saccule - vertical acceleration

Semi-circular canals - rotational acceleration

Question 5

Where are the sensory receptors in the semi-circular canals?

Answer 5

The sensory cells of the semi-circular canals are embedded in swellings at the base of the bony canals called ampulla

Question 6

What is found inside the ampulla at the base of the semi-circular canals that are the sensory receptors?

Answer 6

hair cells which act as sensory receptors

The hair cells 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

Question 7

What is the cupula?

Answer 7

the cupula is a gelatinous strucutre that streches the entire width of the ampulla and containts the hair cells

Question 8

Where are the cilia of hair cells?

Answer 8

The cilia of the hair cells embedded within the gelatinous cupula synapse directly with the sensory neurons of the vestibular nerve (CNVIII)

Question 9

How do the sensory organs of the semi-circular canals detect rotational acceleration?

Answer 9

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

Question 10

What is inertia?

Answer 10

The principle of inertia, one of the most fundamental laws of physics, holds that objects resist changes in their state of motion: If an object is at rest, it will stay at rest until something forces it to budge, and if it’s moving, it will keep moving until ground to a halt. That’s why spinning makes you dizzy

Question 11

What does the inertia of the endolymph cause when moving?

Answer 11

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 12

What happens if you rotate at a constant velocity?

Answer 12

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

Question 13

What happens if you suddenly stop rotating?

Answer 13

Sudden stop will cause endolymph to continue to move due to momentum creating a continuing sense of movement and dizziness

Question 14

What are the 2 types of cilia of the hair cells

Answer 14

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

Question 15

What does distorsion of cilia in the direction of the kinocilium cause?

Answer 15

Distortion of the cilia in the direction of the kinocilium causes depolarisation and increased discharge of APs in the vestibular nerve

Question 16

What does distortion of the cilia away from the kinocilium cause?

Answer 16

Distortion of the cilia away from the kinocilium leads to hyperpolarization and decreased discharge of APs in the vestibular nerve

Question 17

Where does a lot of integration of sensory information take place?

Answer 17

cerebellum

Question 18

the sensory apparatus of the utricle and saccule are collectively known as what?

Answer 18

maculae

Question 19

How is the macula orientated in the utricle and saccule?

Answer 19

utricle - horizontal plane

saccule - vertical plane

Question 20

WHat is the structure of the maculae?

Answer 20

The maculae, like the ampulla, have a set of cilia (one kinocilium and a series of stereocilium) which protrude into a gelatinous mass called the otolith membrane

Embedded in the otolith membrane are CaCO3 crystals called otoliths

Question 21

What do verticallty orientated macula in the saccule respond to?

Answer 21

Vertically orientated macula in the saccule respond to vertical forces e.g. elevation in a lift, and provide information on orientation of head when lying down

Question 22

What happens when you tild your head?

Answer 22

Tilt of the head is detected by the macula in the utricle (horizontal plane); 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 23

Does backwards tilt cause an increased or decreased discharge of APs and why?

Answer 23

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

Question 24

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

Answer 24

the vestibular nerve

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

Question 25

Numerous neural pathways connect the vestibular centres of the medulla (where many of the sensory afferents of the vestibular nerves terminate) and the cerebellar centres which co-ordinate the postural muscles required to maintain balance, what are they?

Answer 25

Projections from the vestibular nuclei on one side project ipsilaterally, bilaterally and contra-laterally to descending motor pathways (and also the extraocular nuclei (extraocular muscles))

Vestibular nuclei receive input from proprioceptors signalling limb and body position, also from neck and eye muscles

Vestibular nuclei project via the thalamus to the cerebral cortex - perception of movement and body position = kinaesthesia.

Question 26

What is the tonic labyrinthine reflexes?

This is one of the vestibular system reflexes

Answer 26

Keep the axis of the head in a constant relationship with the rest of the body

Use information from maculae and neck proprioceptors

Question 27

What is the dynamic righting reflexes?

it is one of the vestibular system reflexes

Answer 27

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 28

What are vestibulo-ocular reflexes?

Answer 28

Within the medullary vestibular nuclei, afferents from the vestibular system synapse with afferent fibres travelling to the extraocular nuclei and thus influence eye movement (see static and dynamic reflexes)

where activation of the vestibular system of the inner ear causes eye movement. This reflex functions to stabilize images on the retinas (when gaze is held steady on a location) during head movement by producing eye movements in the direction opposite to head movement, thus preserving the image on the center of the visual field

Question 29

one type of vestibular-ocular reflex is the if the static reflex, what is it?

Answer 29

When you tilt your head, the eyes intort/extort to compensate, so that over a certain range, the image stays the right way up.

Question 30

What is nystagmus?

Answer 30

Nystagmus is a condition of involuntary (or voluntary, in some cases) eye movement, acquired in infancy or later in life, that may result in reduced or limited vision

Question 31

What is dynamic vestibular nystagmus?

(a type of vestibular occular reflex)

Answer 31

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

Question 32

What is the direction of the nystagmus?

Answer 32

direction of nystagmus is the direction of the rapid flick back R rotation - R nystagmus

Question 33

What is nystagmus used to test?

Answer 33

vestibular function

Question 34

One test of vestibular function is post-rotatory nystagmus, how is it carried out?

Answer 34

Subjects are rotated in a 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 35

What is caloric stimulation?

nystagmus test to check vestibular function

Answer 35

Simple test for the horizontal SCC

When outer ear is washed with either cold or warm fluid, the temperature difference from core 37°C, 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).

Warm into R ear - R Nystagmus

Cold into R ear - L nystagmus

Question 36

What is kinetosis?

Answer 36

motion sickness

Question 37

What is the cause of kinetosis?

Answer 37

powerful maintained stimulation of the vestibular system

motion sickness is most likely to occur if visual and vestibular system inputs to the cerebellum are in conflict, e.g. vestibular system indicated rotation but the visual system does not

the cerebellium generates a sickness signal to the hypothalamus to bring about ANS changes - nausea, vomiting, decreased BP, dizziness, sweating and pallor

Question 38

What is Labyrinthitis?

Answer 38

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

Question 39

what is vertigo?

Answer 39

the perception (hallucination) of movement in the absence of movement

There may also be nystagmus

Gross impairment of posture and balance, very disabling

Question 40

What is Ménière’s disease and what does it cause?

Answer 40

Vertigo, nausea, nystagmus and tinnitus = subjective noise

Associated with over production of endolymph causing ­pressure

The cause is unknown

Question 41

What is the inner ear sensitive to attack by?

Answer 41

drugs such as streptomycin

Question 42

if vestibular impairment is chronic, can it be well compensated for by the visual system?

Answer 42

yes

learning circuits are set up in the cerebellum but problems occur if subjects cannot use visual information

Question 43

What can leasions in the brain lead to?

Answer 43

nystagmus at rest