Flashcards in Vestibular System Deck (38):
Physiology of Balance
What are semicircular canals?
Organs of balance filled with inert endolymph similar to scala media.
What are the semicircular canals of the vestibule?
Horizontal. Anterior. Posterior.
Where do the canals meet?
What do semicircular canals end in?
What is the ampulla?
Compartment containing the cupula, hair cells and endolymph.
How is the ampulla innervated?
Afferent neurones going to the vestibular nerve.
What is the function of the endolymph?
Pushes against cupula to cause depolarisations/hyperpolarisations.
What activates the hair cells of the ampulla?
How do hair cells detect horizontal movement?
As well as the endolymph moving against the cupula, the otolithic membrane containing otoconia adds a gravitational pull on the hairs as the head rotates to detect linear acceleration.
Describe the orientation of hair cells in semicircular canals.
All hairs in the same direction in each ampulla.
Central Processing in the Vestibular System
What may cause vertigo and hearing loss?
Local blood supply deficit in the internal auditory and common cochlear arteries.
What causes only vertigo?
Local blood supply deficit in the anterior vestibular artery.
What is nystagmus?
Involuntary eye movement/jerking in one direction.
What are the types of nystagmus?
Peripheral. Central. Spontaneous. Positional. Gaze-induced. Post-rotational.
What causes peripheral nystagmus?
Normal or diseased functional states of the vestibular system.
What causes central nystagmus?
Normal or abnormal processes not related to the vestibular system. Lesions of midbrain/cerebellum.
What causes spontaneous nystagmus?
Random. Regardless of position of head.
What causes positional nystagmus?
Specific position of head.
What causes gaze-induced nystagmus?
Changing of gaze.
What causes post-rotational nystagmus?
Rapid rotation/shaking of head.
What causes Benign Paroxysmal Positional Vertigo (BPPV)?
Debris collected in the inner ear (otoconia/CaCO) shift when head moves causing false vertigo.
What is AC-BPPV?
BPPV in the anterior semicircular canal. Causes down-beating positional nystagmus.
What is LC-BPPV?
BPPV in the lateral/horizontal canal. Causes horizontal nystagmus.
What is PC-BPPV?
BPPV in the posterior canal. Causes rotatory, up-beating nystagmus.
How is PC-BPPV treated?
What is the vestibulo-ocular reflex?
Reflex eye movement caused by vestibular stimulation.
What is the role of the vestibulo-ocular reflex?
Keeps retinal image stable, maintaining visual processing.
Describe the direct mechanism of the vestibulo-ocular reflex.
Rotation of head bends hair cells in semicircular canals in the opposite direction. Hair cells depolarise and send signal to medial and lateral recti (muscles) of the eye via vestibular nuclei.
Describe the indirect mechanism of the vestibulo-ocular reflex.
Head movement info travels to granule cells of cerebellum via mossy fibres. Parallel fibres of granule cells synapse onto Purkinje cells inhibiting vestibular nucleus. Climbing fibres send feedback from the eye muscles, inhibiting the vestibular nucleus.
How is vestibular learning achieved?
Long term potentiation/depression at the parallel-purkinje synapses. Altered purkinje output via climbing fibre feedback.
Describe the different distribution of cerebellar defects.
Cerebellar hemisphere syndrome - Affects half of body. Rostral vermis syndrome - Chest down (not hands). Caudal vermis syndrome - Affects chest to knees. Pan cerebellar syndrome - affets whole body.
How is a lesion of one of the vestibular organs perceived by the brain?
As a head-turn in the opposite direction.
What is the neurological term for the lesion of the vestibule?
Unilateral vestibular ganglionectomy.
Describe the chemical mechanisms of vestibular compensation.
Downregulation of Glycine and GABA at ipsilesional vestibular nucleus causing smaller response to commissural inhibition of ipsilesional VN by contralesional VN. Upregulation of excitability of ipsilesional VN.
Describe the physical mechanisms of vestibular compensation.
Cerebellar cortical plasticity reduces amount of inputs to contralesional VN. Reduces inhibition of ipsilesional VN by commissural fibres from contralesional VN.