the ear II

vestibular system

important for balance, equilibrium, posture, head, body, and eye movement

vestibular labyrinth

3 semicircular canals + 2 otolithic organs on each side of the head

endolymph

produced continuously by specialized cells in several locations in the membranous labyrinth

flows from the semicircular ducts into the utricle, and from the cochlear duct into the saccule

meniere’s disease

defective circulation or absorption of endolymph

swelling of the membranous labyrinth

transient attacks of vertigo, nausea, hearing loss, ringing in the ears

semicircular canals

sense head rotation in different planes

utricle and saccule

otolithic organs that detect static balance and linear acceleration

hair cells

has 40-70 stereocilia and one larger kinocilium

mechanoreceptors in the vestibule and semicircular canals, have synaptic vesicles

activated by the deflection or shearing of the stereocilia

utricle

located at both ends of each semicircular duct and is used to communicate

most sensitive to the movement in a horizontal plane

ampulla

dilation located at one end of each semicircular duct

contains a crista that is the sensory organ

crista

part of the ampulla that contains sensory hair cells and supporting cells

cupula

gelatinous mass where the hair cells are embedded

covers the crista and extends across the ampulla

macula

receptor organ

patches on the walls of the utricle and saccule where the supporting and hair cells are

has gelatinous substance containing otoconia

utricular macula

lies at the bottom and is roughly horizontal

saccular macula

on the medial wall and is roughly vertical

otoconia

calcium carbonate crystals in a gelatinous substance in the macula

saccule

most sensitive to the movement in the sagittal plane

vestibulo-ocular reflex (VOR)

line of sight fixed on visual target

senses rotations of head, commands compensatory movement of eyes in opposite direction

connections from semicircular canals to vestibular nucleus to cranial nerve nuclei → excite extraocular muslces

static VOR

maintain position of eyes in orbit with head tilts

horizontal VOR

generate eye movements that compensate for the brief head movements

a person’s gaze can stay fixed on an object even when the head is moving or turning

counter-rotation of the eyes is generated, so the direction of gaze would not change

nystagmus

fast eye movement following the slow movements of the VOR

in the same direction of head rotation and its function is to return the eyes to the center of the orbit

can be horizontal or vertical

caloric nystagmus

instilling cool or warm water into a subject’s ear causes endolymphatic convection currents that will induce nystagmus

used to diagnose vestibular disorders

spontaneous nystagmus

symptom of damage to vestibular system or cerebellum

optokinetic nystagmus

induced by moving visual stimuli

clinically demonstrated by rotating a striped drum or a moving piece of striped cloth

vertigo

a sensation of turning or rotation in space without actual rotation