Vestibular Function Flashcards by Adam Davie

What is the main function of the vestibular system?

To control posture and balance

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What is the name for the fluid filled membranous tubes that make up the vestibular system?

Labyrinths

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Aside from fluid, what do the labyrinths contain?

Areas of sensory cells responsible for detecting tilt of the head with respect to the body as well as angular acceleration

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What are the functions of the areas of sensory cells within the labyrinths?

Detecting head tilt

Detecting angular acceleration

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What type of cells are the sensory cells within the labyrinths?

Hair cells, whose cell bodies are embedded in the bone of the skull

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What is the name of the fluid found in the labyrinths?

Endolymph

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What is the feature of the hair cells and how does the feature differ from normal?

The hair cells have cilia which protrude into the endolymph. Unusually, the cilia have one large kinocilium and sets of progressively smaller stereocilia

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What are stereocilia?

The smaller projects of the vestibular cilia that stem from the kinocilium

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What is the kinocilium?

The large origin of the cilia on the hair cells from which stems the stereocilia

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What is the effect of distortion of the stereocilia in the direction of the kinocilium?

Depolarisation and increased discharge of APs

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What is the effect of distorsion of the stereocilia away from the kinocilium?

Hyperpolaristaion and decreased discharge of APs

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Which organs within the vestibule detect head tilt?

Macula

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Describe the cilia of the macula

They are embedded in a blob of jelly which contains crystals of CaCO3

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What is the name for the crystals in which the macular cilia are embedded?

Otoliths

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Describe the features of otoliths and concequences of the features

They have a greater density than the endolymph and are therefore affected by gravity. When the head tilts the blob is distorted and shearing forces are exerted on the macular cilia

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What is the term form side to side tilting of the head?

Utriculus

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What is the term for forward to back titling of the head?

Sacculus

What features of the semi-circular canals contain the cupula organs?

Swellings called ampullae

Describe the hair cells of the cupula organs

They are contained in gelatinous material like macular hair cells, but there are no otoliths. They detect angular acceleration rather than head tilt

How do the cupula organs detect angular acceleration?

If the skull is initially at rest, the endolymph does not at first move because of its inertia. however the base moves instantly because it is embedded in the skull. This momentum at the base causes shearing movements which bend the cilia. If they rotate at a constant velocity, the endolymph rotates at the same speed, therefore no shearing forces, but this takes several seconds to equilibrate. When the movement stops, the momentum contained in the cilia cause shearing in the opposite direction momentarily after the base stops immediately

What are the names and functions of the three sets of semi-circular canals?

Posterior - detects head over heels rotation
Anterior - side to side rotation
Horizontal - spinning

What is the passage of information form the SSCs?

Information form the SSCs and maculae pass along CNVIII (vestibulocochlear) to the vestibular nuclei and make many connections

What are the three key connections that the information from the SSCs and maculae make from the vestibular ganglia?

Vestibular nuclei on one side project to those on the other side of the head
They receive input form proprioceptors signalling limb and body position, also from the neck and eye muscles
Vestibular nuclei project via the thalamus to the cerebral cortex to provide perception of movement and body position - KINAESTHESIA

Describe the tonic labyrinthine reflexes mediated by the vestibular system

They keep the axis of the head in a constant relationship with the rest of the body using information form the maculae and neck proprioceptors

Describe the dynamic righting reflexes mediated by the vestibular system

Rapid postural adjustments that are made to stop someone falling when they trip unexpectedly. They are long reflexes involving the extension of all limbs

What are the two vestibuloochlear reflexes?

``` Static reflex Dynamic reflex (vestibular nystagmus) ```

Describe the static vestibuloochlear reflex. Wht are the inputs to the reflex?

When the head tilts, the eyes roll up to compensate, so that over a certain range, the image stays the right way up. Macula input plus proprioceptors from the eye muscles to the oculomotor nucleus and then to the oculomotor muscles

Describe the vestibuloochlear dynamic reflex/vestibular nystagmus

This reflex is a series of eye movements that rotate the eye against the direction of rotation of the head and body so that the original direction of gaze is retained. The extent of the movement is restricted, when eye muscles come to the end of their range of movement, they rapidly flick back to the zero position. If the rotation continues the slow phase starts again, then flicks back. By convention, the direction of the nystagmus is the direction of the rapid flick back i.e. right rotation = right flickback = right nystagmus

What are the two tests relating to nystagmus that are used to test vestibular function?

Post-rotatory nystagmus | Caloric stimulation

Describe the test for post-rotatory nystagmus

Only done in research units. Subjects are rotated in a Barany chair, if rotation is to the left the subject should get a left nystagmus. At the end of the rotation, for around 20 seconds, the subject should get a right nystagmus

Describe the caloric stimulation test for vestibular function

Simple test which only applies to the horizontal SSC and associated circuitry. When the outer ear is washed with either cold or warm fluid, the temperature difference from core 27 degrees gets through the thin bone and sets up convection currents which affect the endolymph. Warm fluid (44 egrees) causes nystagmus towards the affected side, cold fluid (30 degrees) causes nystagmus away from the affected side Warm into R -> R nystagmus Cold into R -> L nystagmus

What is important to remember when testing vestibular function?

That the vestibular system works with the visual system to control body position and therefore the visual input must be removed to properly test vestibular function

What symptom can be provoked by sustained stimulation of the vestibular system?

Kinetosis - motion sickness

What is kinetosis?

Motion sickness

What are the features of kinetosis/ motion sickness?

Nausea, vomiting, decreased BP, dizziness, sweating, pallor - all autonomic NS symptoms

What is the most likely cause of kinetosis/motion sickness

If the visual and vestibular systems are in conflict eg if the vestibular system senses rotation but the visual system does not

What happens when the vestibular and visual systems are in conflict to bring about the ANS responses of motion sickness?

The archicerebellum generates a 'sickness signal' to the hypothalamus to bring about the ANS changes

Give some diseases/causes of damage to the vestibular system

``` Labyrinthitis Vertigo Meniere's disease Drug damage e.g. streptomycin Brain lesions ```

Describe labyrinthitis

An acute interference with normal vestibular function as a result of infection. Often occurs post viral infection. Displays all ANS symptoms plus vertigo

Describe vertigo

The perception (hallucination) of movement in the absence of movement. There will also be nystagmus at rest leading to gross impairment of posture and balance - very disabling

Describe Meniere's disease

Unknown cause, associated with over production of endolymph causing increased vestibular pressure. Causes vertigo, nausea, nystagmus and tinnitus

Can vestibular impairment be compensated in any way?

If the impairment is chronic, it can be quite well compensated by the visual system. Learning circuits are set up I the archicerebellum but are ineffective if the patient cannot use visual information