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Auditory System

Terms in this set (12)

Auditory system pathway (ECOLI)
*** Hair cells in the organ of corti stimulate spiral ganglion cells, which project to the cortex via CN VIII and synapse on second order neurons at the cochlear nuclei in the rostral medulla

*** Second order neurons send both contralateral and ipsilateral projections to the superior olivary nuclei of the pons where they synapse on third order neurons

*** Third order neurons send both contralateral and ipsilateral projections to the inferior colliculus in the caudal midbrain via the lateral lemniscus and eventually project to the medial geniculate nucleus of the thalamus where they synapse on fourth order neurons

*** Fourth order neurons project to the transverse temporal gyri of the cortex via auditory radiations from the MGN

*** From the transverse temporal gyri, information moves to the superior temporal gyrus and eventually enters what and where pathways
Primary auditory cortex pic
Auditory association complex pic
Histological organization of the auditory system pathway
medulla: cochlear nucleus is is dorsolateral most area of the very rostral medulla

Thalamus: MGN is dorsomedial to the optic tracts in the most posterior part of the thalamus

Cortex: the transverse temporal gyri are beneath the posterior most part of the lateral sulcus and the superior temporal gyrus is just inferolateral to the lateral sulcus
Anatomy of middle ear and ossicle
*** Tympanic membrane against malleus, connects to incus and then stapes (all suspended by suspensory ligaments)

*** View of tympanic membrane shows long process of malleus, pars flaccida (superior to area where malleus is), pars tensa (drum), and tympanic annulus (ring)

*** Stapes footplate rests on oval window of cochlea
Acoustic property of middle ear
*** Vibrations of stapes footplate transfer mechanical energy to the fluids of the inner ear

*** Round window receives vibrations, displacing the fluid within the cochlea to shear against hair cells, causing electrical impulses to be sent to the brain for interpretation as sound
Middle ear amplifier
*** Amplifier concept: middle ear is air-filled while inner ear is fluid filled, thus sound will attenuate

*** Middle ear structures provide an impedance matching mechanism, as tympanic membrane is much larger than oval window

*** Typical amplification is by about 26 dB

*** Length of malleus compared to length of incus also factors in (1.3:1), 2 db gain
Cochlear amplifier
*** While inner hair cells are responsible for hearing, outer hair cells regulate frequency discrimination and aid in achieving a large dynamic range

*** These functions of outer hair cells are called the cochlear amplifier collectively
Cochlea anatomy
*** Cochlea is located anteriorly, coiled 2 ½ times and possessing three chambers

*** Scala tympani lies below the basilar membrane, communicates with round window

*** Scala media interacting intimately with organ of corti and basilar and tectorial membranes

*** Scala vestibuli also present, function not discussed
Cochlea physiology
*** Neuroepithelial cells called hair cells are responsible for the transduction of acoustic energy into action potentials

*** Damaged by aminoglycosides and cisplatin

*** Single kinocilium sits at the edge of a hair cell with stereocilia adjacent and connected

*** Stereocilia bending toward kinocilium = depolarization (bend away- hyperpolarization)

*** Cocholeotoxicity- loss of hearing

*** Vestibulotoxicity- loss of balance and function
Dead cochlea vs. live cochlea
*** Tonotopic organization of the cochlea, with high frequencies detected at the basal turn near the oval window and low frequencies travelling all the way to the apex

*** Dead cochlea - tuned like a musical instrument

*** Live cochlea - possesses exquisite frequency discrimination

*** Able to compress sound (when loud, not dampened when quiet sound) to allow for vast dynamic range

*** Neither of these features are present in a dead cochlea, indicating that outer hair cells achieve this function through a particular signaling mechanism that is an active process
Schematic of auditory pathway
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