Vestibular systems Flashcards by Sophia Terry

What are the functions of the vestibular system?

Gaze stabilisation.

Maintain spatial orientation- plane illusion or train illusion may impair this.

Self vs. world motion perception.

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What vestibular structures are responsible for detecting angular acceleration?

3 semi-circular canals.

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What vestibular structures are responsible for detecting linear acceleration?

2 otolith organs.

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What connections does the vestibulo-ocular reflex have?

Ocular

Spinal

Autonomic

Cortical

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What sensations are the different vestibular projections responsible for?

Vestibulo-ocular = nystagmus.

Vestibulo-spinal = unsteadiness.

Vestibulo-autonomic = nausea.

Vestibulo-cortical = vertigo.

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What are the 2 components of the vestibular labyrinth?

Bony labyrinth- bounded by petrous temporal bone filled with perilymph.

Membranous labyrinth- filled with endolymph, contains receptor cells, located within the utricle, saccule and semi-circular canals.

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Where are epithelial hair cells located in the vestibular system?

Macula- in utricle and saccule.

Crista- of ampulla (swelling of semi-circular canals).

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Where do the primary afferent neurons of the vestibular system have their cell bodies?

Vestibular ganglion.

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Where do the peripheral processes of the primary afferent neurons of the vestibular system terminate?

Macula and ampulla.

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Where do the central processes of the primary afferent neurons of the vestibular system synapse?

Brainstem vestibular nuclei.

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What are hair cells?

Mechanical transducers detecting static tilt and acceleration.

Embedded in a gelatinous matrix contain calcium carbonate crystals- uloconia.

Stereocilia are arranged in rows of varying heights, and there is a single long kinocilium.

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What are the 2 types of nerve endings on hair cells?

Type I: chalice-like endings form ribbon synapses.

Type II: simple nerve terminals.

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What is the transduction mechanism of vestibular hair cells?

Mechanoelectrical transduction mechanism similar to the cochlea.

Hair cell receptor potential: toward the kinocilium = depolarisation; away from the kinocilium = hyperpolarisation.

Ganglion cell discharge: toward the kinocilium = increased frequency; away from the kinocilium = decreased frequency.

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What are the 4 main targets that the vestibular nuclei project to?

Spinal cord.

Nuclei of the extraocular muscles.

Cerebellum.

Centres for cardiovascular and respiratory control.

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How do the vestibular pathways influence the static labyrinth (otoliths)?

Vestibulospinal tract.

Antigravity muscles that contribute to upright posture and equilibrium.

Blood pressure.

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How do the vestibular pathways influence the kinetic labyrinth (semi-circular canals)?

Medial longitudinal fasciculus (MLF).

Eye movements that ensure that the retinal image is kept stationary when the head moves.

Postural adjustments.

Cardiovascular and respiratory function.

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How do postural reflexes work?

Integrate sensory information from vestibular apparatus, somatic receptors and the eyes to increase extensor muscle tone and maintain upright posture.

What are postural reflexes mediated by?

Brainstem descending tracts: vestibulospinal tracts and reticulospinal tracts.

What is the vestibulo-ocular reflex?

When the head rotates to the left, the eyes rotate in compensation to the right with repositioning saccades to the left.

VOR operates to maintain the gaze on a selected target.

How may the integrity of the vestibular system be tested to assess the condition of the brainstem, e.g. in comatose patients?

A horizontal vestibulo-ocular reflex can be elicited by warming or cooling the endolymph in the semi-circular canals.

A warm caloric test applied to one ear should produce a slow drift of the eyes away from the stimulated side followed by a fast saccade towards the stimulated side.

The direction of the nystagmus is named in accordance to the fast saccadic phase.

What is vertigo?

Illusion of movement.

Usually rotational or ‘true vertigo’.

Perceptual abnormality.

How many people experience balance disorders such as vertigo and dizziness?

25% of people experienced dizziness at some time.
20% of people experienced dizziness severe enough to see a doctor.
50% experience dizziness over 75 years old.
25% of referrals to ENT and neurology clinics.

Give examples of peripheral vestibular disorders.

Labyrinth and CNVIII, e.g. vestibular neuritis, BPPV, Meniere’s disease, BVF, UVF.

Give examples of central vestibular disorders.

CNS (brainstem/cerebellum), e.g. stroke, MS, tumours.

What is the effect of a static head tilt on the static labyrinth of the vestibular system?

Tonic discharge modulated by head tilt and the change in firing is maintained as long as the head remains tilted.

What is the state of the static labyrinth of the vestibular system in an upright position?

Vestibular ganglion neurons that innervate the saccule have tonic discharge due to constant hair displacement imposed by gravity.

What is the effect of linear acceleration on the static labyrinth of the vestibular system?

In the horizontal plane, induces transient hair displacement of some cells of the utricular macula and modulation of spontaneous discharge in ganglion cells.

What is the effect of angular acceleration on the kinetic labyrinth of the vestibular system?

Hair cells in the crista respond to alteration of the velocity of the rotational movement (angular acceleration) in the plane of the semi-circular canals- output signal on CNVIII is velocity. Acceleration towards the side of the receptor increases firing rate, while deceleration decreases it. Rotation of the head in any plane produces change of activity of the receptors in one or more semi-circular canals.

Describe the bilateral semi-circular canal activity during head rotation (kinetic labyrinth of vestibular system).

Head rotation deforms the hair cells of the left and right semi-circular canals in opposite directions. Inertia of movement causes endolymph to move in opposite direction with respect to the canals. Firing rate of vestibular ganglion cells increases on the left side and decreases on the right side. Push-pull arrangement operates for all 3 pairs of canals.

Where do primary afferent fibres in CNVIII terminate?

Vestibular nuclei of the brainstem and in the cerebellum.

List vestibular nuclei.

Superior Lateral Medial Inferior

How are the vestibular nuclei somatotopically organised?

Static labyrinth (otoliths)- lateral and inferior. Kinetic labyrinth (semi-circular canals)- superior and medial.

Describe the path of the lateral vestibulospinal tract.

Descends ipsilaterally in ventral funiculus of spinal cord. Axons terminate in lateral part of ventral horn and influence motor neurons to limb (especially extensor antigravity) muscles.

Describe the path of the medial vestibulospinal tract.

Descends bilaterally in medial longitudinal fasciculus (MLF) to cervical and upper thoracic spinal cord. Axons terminate in medial part of ventral horn and influence motor neurons to neck and back muscles.

Describe the vestibular pathways to the thalamus and cortex.

All vestibular nuclei project to ventral posterior and ventral lateral nuclei of the thalamus. Thalamic nuclei project to 2 cortical areas (2V and 3a)- part of the head region of the primary somatosensory cortex. Projection also to superior parietal cortex- ‘vestibular cortex’ concerned with spatial orientation. Cortical projections may account for feelings of dizziness (vertigo) during certain kinds of vestibular stimulation.

Give examples of acute vestibular disorders.

Vestibular neuritis ('labyrinthitis'). Labyrinthine concussion.

What is vestibular neuritis?

Sudden, unilateral vestibular loss (vertigo, nausea, unsteadiness, nystagmus). Hearing spared. No CNS symptoms or findings. Viral- after URTI; mini-epidemics. Days to weeks.

Give an example of an intermittent vestibular disorder.

Benign paroxysmal positional vertigo (BPPV).

Give examples of recurrent vestibular disorders.

Meniere's disease. Migraine.

What is Meniere's disease?

Rare. Build-up of endolymphatic pressure ('hydrops'). Hearing impaired. Triad: vertigo, tinnitus, deafness.

Give an example of a progressive vestibular disorder.

Acoustic neuroma (CNVIII).

What are some non-vestibular causes of dizziness?

Heart disorders Presyncopal episodes Orthostatic hypotension Anaemia Hypoglycaemia Psychological Gait disorders