16.4c/d Physiology of Hearing/Physiology of Equilibrium

tympanic membrane

moves in air, vibrates easily; has 18 times area of oval window

ossicles

concentrate the energy of the vibrating tympanic membrane on an area 1/18 that size; have a protective function of lessening the transfer of energy to the inner ear

tympanic reflex

loud noise causes tensor tympani to contract, tensing the eardrum, and stapedius contraction reduces vibration of stapes on oval window

variations in amplitude

cause variations in the intensity of cochlear vibrations; Soft sound produces relatively slight up-and-down motion of the basilar membrane; Louder sounds make the basilar membrane vibrate more vigorously

pitch

depends on which part of basilar membrane vibrates; basal end interprets high _____, distal end interprets low _____

outer hair cells

innervated by some motor fibers; In response to sound, they send sensory signals to medulla, pons sends motor signals to them; they shorten, reducing basilar membrane mobility

pons

has inhibitory fibers that synapse at base of inner hair cells; Inhibits sensory neurons of some areas; enhances contrast between regions of cochlea

first order (sensory) neurons

Somas form the spiral ganglion around the modiolus; Axons begin at the bases of hair cells; Axons lead away from cochlea as the cochlear nerve and joins with the vestibular nerve to form CNVIII; Each ear sends nerve fibers to both sides of the medulla oblongata; Synapse with second-order neurons in cochlear nuclei

second order neurons

Cell bodies in the cochlear nucleus; Some fibers ascend to the superior olivary nucleus; Other fibers ascend to the inferior colliculi of the midbrain

binaural hearing

signals from two ears compared to help localize sound

third order neurons

cell bodies in inferior colliculi; axons travel to thalamus

fourth order neurons

Cell bodies in thalamus; Axons travel to primary auditory cortex; Conscious perception of sound; Auditory system has extensive decussation, so damage to one side of cortex does not cause unilateral hearing loss

deafness

hearing loss

conductive deafness

conditions interfere with transmission of vibrations to inner ear

sensorineural (nerve) deafness

death of hair cells or any nervous system elements concerned with hearing

equilibrium

sense of body orientation, movement, and balance

vestibular apparatus

contains receptors for equilibrium; Consists of three semicircular ducts and two chambers

static equilibrium

perception of orientation of head in space (erect or tilted); When head is tilted, heavy otolithic membrane sags, bending the stereocilia and stimulating the hair cells

dynamic equilibrium

perception of motion or acceleration; Linear acceleration detected as otoliths lag behind, bending the stereocilia and stimulating the hair cells

saccule and utricle

two chambers of vestibular apparatus; Each contain a macula—a 2×3 mm patch of hair cells and supporting cells; Contribute to both static and dynamic equilibrium

macula sacculi

lies vertically on wall of saccule

macula utriculi

lies horizontally on floor of utricle

macula hair cells

each one has 40-70 stereocilia and one true cilium

kinocilium

a single, specialized cilium on each hair cell; embedded in a gelatinous otolithic membrane

otoliths

calcium carbonate protein granules that weigh down the otolithic membrane; add to inertia of membrane, enhance sense of gravity and motion

semicircular ducts

three of them; detect rotary movements; Spatial orientation of canals causes ducts to be stimulated by rotation in different planes; filled with endolymph and opens up next to utricle

ampulla

dilated sac found next to utricle that opens when filled with endolymph; within semicircular ducts

crista ampullaris

each ampulla contains one; mound of hair cells and supporting cells

cupula

Stereocilia and kinocilium embedded in gelatinous cap; found in semicircular canals

projection pathways

All hair cells synapse on vestibular nerve

cerebellum

integrates vestibular information into its control of head and eye movements, muscle tone, and posture

reticular formation

thought to adjust blood circulation and breathing to postural changes

spinal cord

descend through two vestibulospinal tracts of spinal cord and innervate extensor (antigravity) muscles

thalamus

relay to cerebral cortex for awareness of position and motor control of head and body

vestibulo-ocular reflex

keeps vision fixed on distant object while walking