4.1: Vestibular System

otolith organs

utricle and saccule which form labyrinth w/ semicircular canals

utricle

otolith w/ hair bundles oriented horizontally, which codes motion in horizontal place

saccule

otolith w/ horizontal hair bundles, which codes motion in vertical plane

striola

vestibular organ that divides hair cells into 2 populations w/ opposite orientation

macular

sensory epithelium in utricle and saccule where hair bundles project onto a gelatinous layer located below otolithic membrane, where hair cells are located

ampulla

bulbous expansions on each semicircular canal w/ crista

cupula

gelatinous mass of hair bundles bridging entire width of ampulla, preventing endolymph circulation, detects rotational and translational movements

x axis

roll - detects a backflip

y axis

pitch - detects a cartwheel

z axis

yaw - detects a pirouette

endolymph

fluid in apex part of hair cell at -45mV, high in K+ and low in Na+

perilymph

fluid in basal part of hair cell at +40mV, low in K+ and high in Na+

vestibular ocular reflex

mechanism for producing eye movements that counter head movements, permitting gaze to remain fixed on a particular point

oscillopsia

inability to fixate visual targets while head is moving, can be caused by VOR damage; fine if unilateral

Scarpa’s ganglion

location of vestibular nerve soma

spontaneous nystagmus

eye movements in absence of head rotation b/c of net difference in firing rates between 2 vestibular nerves

physiological nystagmus

slow eye movements counter direction of rotation & fast movements in the same direction to reset eye position

vestibulocervical reflex

neck muscles regulate head position by activating upon stimulation of the semicircular canals; semicircular canals → medial vestibular nucleus → neck muscles (also top down Purkinje input)

vesitibulospinal reflex

extensor muscles in neck and limbs provide muscle tone to stand upright; otolith organs → lateral vestibular nuclei → ipsilateral horn → extensors

parieto insular vestibular cortex

brain region activated by motion responsible for sense of self motion