Vestibular System Flashcards
Where is the peripheral receptor apparatus found and what does it do?
Found in inner ear; responsible for transducing head motion/position
Where is the central vestibular nuclei found and what does it do?
Brainstem; inegrating and distributing info that controls motor activities and spatial orientation
Where is the vestibuloocular network found and what does it do?
Found in vestibular nuclei; involved in the control of eye movements
What does the vestibulospinal network do?
Coordinates head movements, axial musculature, and postural reflexes
What does the vestibulothalamocortical network do?
Conscious perception of movement/spatial orientation
What are semicircular canals?
Function in horizontal, anterior, posterior rotational head movements (angular accelerations)
What are otolith organs of utricle and saccule?
Function in translational head movements (linear accelerations)
What are receptor cells in vestibular organs innervated by?
Primary afferent fibers of vestibular ganglion (Scarpa ganglion)
Where do the central processes of the bipolar cells travel and terminate?
Enter brainstem and terminate in ipsilateral vestibular nuclei and cerebellum
What is perilymph? What is endolymph?
Peri: located between membranous and bony labyrinths; similar to CSF
Endo: located within membranous labyrinth; bathes receptors of auditory and vestibular systems
What maintains the different ion concentrations in the peri and endolymph?
Maintained by secretory cells in membranous labyrinth and endolymphatic sac
What causes vestibular disease?
Disturbances in volume or ionic content of endolymph
What supplies blood to the labyrinth?
Labyrinthine artery, a branch of the AICA; enters temporal bone via internal auditory meatus
Stylomastoid artery also feeds semicircular canals but is not their primary source
What happens when you interrupt blood supply to the labyrinth?
Vertigo, nystagmus, and/or unstable gait due to a compromised vestibular and cochlear function
What is Meniere’s Disease?
Disruption of normal endolymph volume, resulting in endolymphatic hydrops (abnormal distention of membranous labyrinth)
What are the symptoms of Meniere’s?
Fluctuating hearing loss, vertigo, positional nystagmus, and nausea; also unpredictable attacks of auditory and vestibular symptoms, including vomiting, tinnitus, and inability to make head movements or even stand passively
What are the treatments for Meniere’s?
Diuretic and salt restricted diet to reduce hydrops; implantation of a small shunt into swollen endolymphatic sac
Describe hair cells (type 1 and 2)
Contain 60-100 stereocilia and single kinocilium; CN VIII innervation; movement of stereocilia towards or away from kinocilium will de- or hyperpolarize the cell
Bundled in vestibular organs with specific orientations, responsive various head displacements
What is the ampullae?
Semicircular ducts; hair cells embedded in cristae, extends across the base of ampulla and extend into cupula; function in rotational/angular accelerations -> displaces endolymph -> bends cupula to one side or other -> displaces stereocilia
What is the maculae?
Found in utricle and saccule; stereocilia extend into gelatinous otolith membrane covered by otoconia; function in gravity/linear accelerations -> displace otoconia -> bend underlying hair cell stereocilia
What are vestibulovestibular fibers?
Reciprocal connections w/ analogous contralateral nucleus
What are spinovestibular fibers?
Arise from all SC levels to provide proprioceptive input
What do primary vestibulocerebellar fibers do?
Target dentate nucleus and terminate as mossy fibers
What do secondary vestibulocerebellar fibers do?
Target flocculonodular lobe and fastigial and dentate nuclei
What do cerebellovestibular fibers do?
Target vestibular nuclei and provide regulatory mechanisms for control of eye movements, head movements, and posture
What is dizziness?
Nonspecific term that generally means a spatial disorientation; may/may not involve feelings of movement; may be accompanied by nausea or postural instability; not exclusively vestibular in origin
What is vertigo? Subjective vertigo? Objective vertigo?
Vertigo: illusion of body motion, often spinning or turning, experienced when no real motion is taking place
Subjective: patient experiences the sensation of spinning while things in the environment are not moving
Objective: sensation is one of objects spinning while patient is not moving
What is benign paroxysomal positional vertigo?
Characterized by brief episodes of vertigo that coincide with particular changes in body position; triggered by turning over in bed, getting up in the morning, bending over, or rising from bent position
What is a vestibular schwannoma?
Benign tumor that originates from schwann cells of the vestibular root (90% of cases and 5-10% of all intracranial tumors); typically found within cerebellopontine angle -> impinges on structures traversing internal acoustic meatus (VII, VIII, labyrinthine artery); slow growing
What is vestibular neuritis? What are some treatments?
Thought to involve edema of the vestibular nerve/ganglion; patients present with severe vertigo, nausea, and vomiting, but no accompanying hearing loss or other CNS deficits
Treatments: antiemetics, vestibular suppressants, corticosteroids, and antiviral agents
Where do all vestibular nuclei project to?
Ventral posterior nuclear complex
What do lesions in parietoinsular vestibular cortex cause?
Vertigo, unsteadiness, and loss of visual vertical
What do lesions in posterior parietal cortex cause?
Confusion in spatial awareness
What is the vestibuloocular reflex?
When turning your head, your eyes turn the opposite direction; at this moment, the eyes are at the back of the orbit and can’t go back further; eyes intermittently and rapidly reset back to a central position
What causes spontaneous nystagmus?
Silencing of output from damaged side = net differences in firing rates of CN VIII whe head is stationary; caused by unilateral damage to vestibular system; causes spontaneous nystagmus and may include vertigo, falls, and vomiting
Describe peripheral damage as it relates to spontaneous nystagmus
Damage is to vestibular labyrinth and/or CN VIII; causes imbalance between left/right vestibular input and causes nystagmus; does not affect voluntary saccades and smooth pursuit
Describe central damage as it relates to spontaneous nystagmus
Damage is to brainstem or cerebellar structures; causes nystagmus with impaired voluntary saccades and/or smooth pursuit; involvement of multiple pathways or cortical areas
What is the caloric test?
Tests vestibular labyrinth function/VOR (vestibuloocular reflex) without moving the head; uses water to alter convection currents in endolymph that alters CN VIII firing rate
What does warm water cause in the caloric test?
Generates currents that mimic turning head to irrigated side; nystagmus that beats toward irrigated ear
What does cold water cause in the caloric test?
Opposite effect of warm water; nystagmus that beats away from irrigated ear
What is the oculocephalic reflex/head thrust test?
Rotating the head back and forth horizontally that induces compensatory eye movements that are dependent on visual and vestibular function; reflex occurs when eyes move in opposite direction of the head movements -> doll’s eyes
What happens in conscious patients when testing the oculocephalic reflex? Comatose patients?
Conscious: doll’s eyes are absent due to voluntary eye movements that mask the reflex
Comatose: reflex is used to assess brainstem function; if the brainstem is intact the doll’s eye reflex is present; if brainstem contains a lesion, doll’s eye reflex is absent
