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BALANCE NOTES
WCR May 2015
Marieb & Hoehn chapter 15; Martini & Ober chapter 15
Structure of Ear
The structure of the external and middle ear is not relevant for balance. Structures used for sensing
orientation and head acceleration (other than the eyes) are part of the inner ear.
 Internal. In cavities in temporal bone. Includes cochlea, vestibule, and semicircular canals. Of
these, only vestibule and semicircular canals are involved with balance.
o Vestibule anatomy. Cochlea is a coiled structure with 3 chambers or ducts (scalae)
running along its length: vestibular duct (scala vestibuli, connects to oval window),
tympanic duct (scala tympani, connects to round window, which is a membrane covered
opening between middle and inner ear), and most importantly the cochlear duct (scala
media). Vestibular & tympanic ducts filled with perilymph and connect to one another at
helicotrema, the connecting hole between them at apex of cochlea. Spiral ganglion
spirals around the axis of cochlea and contains cell bodies of the neurons whose fibers
come from organ of Corti (see next). Nerve fibers join to make cochlear nerve which
joins cranial nerve VIII.
o Cochlear duct (scala media) contains the spiral organ of Corti. Organ of Corti = basilar
membrane, inner & outer hair cells, overlying tectorial membrane, and nerve fibers going
to/from the outer/inner hair cells. Cochlear duct filled with endolymph (not perilymph).
Physiology of Balance Sense
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Hair cell excitation
 One row inner hair cells, 3 rows outer hair cells. All hair cells have stereocilia (“hairs”) sticking out
of their tops (apical ends) into the endolymph-filled space between hair cells & tectorial membrane.
Afferent nerve fibers connect to basal end of hair cells.
 Incoming sound causes BM vibration causes stereocilia movement. Stereocilia mechanically linked
to ion channels. When stereocilia move back/forth, ion channels open/close, cause cell membrane
voltage to get more positive/negative, causing more/less release of excitatory neurotransmitter onto
nerve fibers at base of hair cells.
 Most afferent nerve fibers are from inner hair cells. Outer hair cells receive some efferent
innervation and affect local mechanical responsiveness (hence sensitivity) of BM.
Auditory pathway to brain
 Hair cell depolarization releases glutamate from hair cell onto afferent fiber, exciting it.
 Nerve fibers from hair cells pass through spiral ganglion (where cell bodies are), merge to form
cochlear nerve, then join nerves from semicircular canals & vestibule to make cranial nerve VIII.
Afferents synapse in cochlear nucleus (medulla). Pathway leads eventually to medial geniculate

nucleus of thalamus. Thalamic MGN neuron axons go to primary auditory cortex (temporal lobe)
which gives rise to conscious awareness. Most but not all fibers go to opposite side of cortex.
Tonotopic mapping (different frequencies excite neurons in different places) is found at multiple
levels of processing, from organ of Corti to cochlear nucleus to primary auditory cortex.
Auditory processing
 Pitch perception: Based on which hair cells are activated. Basal end fibers (close to oval window) =
high pitch, apical end fibers = low pitch. Complex sounds have many component frequencies.
 Loudness perception: Based on frequency of nerve impulses on a given fiber, and how many fibers
are activated. Louder sound causes more frequent action potentials and more fibers recruited.
 Direction perception: Done by detecting differences in the intensity and timing of sounds arriving at
R & L ears.
Homeostatic Imbalances of Balance
Vertigo
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