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NSCI 1051 (6)

Auditory System Notes

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Temple University
Neuroscience - CLA
NSCI 1051

Auditory System Middle Ear Tympanic membrane • At end of auditory canal, aka eardrum • Moves the ossicles Ossicles: • The ossicles amplify pressure (and therefore sound) by making the pressure on the oval window greater than the pressure on the tympanic membrane • Sound is transformed from large movements of the tympanic membrane to smaller but stronger vibrations of the oval window • 3 small bones inside the ear that amplify sound by 20 dB, transfer movements from the tympanic membrane to the oval window Malleus: • The ossicle attached to the tympanic membrane Incus: • Middle ossicle forming rigid connection with the malleus and a flexible connection with the stapes Oval Window: • Comes after the ossicles • Membrane that covers a hole in the bone of the skull • Cochlea is behind it Cochlea: • Fluid filled apparatus for transforming the physical motion of the oval window membrane into a neuronal response ( triggers sensory neurons) • Vibration in oval window vibrates fluid in cochlea Stapedius Muscle: • Extends from a fixed anchor of bone and attaches to the stapes • Tensor tympani attaches to malleus • When these muscles contract, the chain of ossicles becomes much more rigid and sound conduction to the inner ear is greatly diminished • Attenuation reflex Basilar Membrane: • Seperates the scala tympani from the scala media (bottom and middle fluid filled chambers) in the cochlea • Flexible, bends in response to sound  flexing of the basilar membrane leads to the bending of the stereocilia in the organ of corti • When sound comes in  max. displacement of basilar membrane • Traveling wave down the basilar membrane starts at the base of the cochlea and moves towards the apex (gets wider and less stiff at the end/apex) • The position along the basilar membrane at which its amplitude is highest depends on the frequency of the stimulus • High frequencies have peak influence near base and stapes • Low frequencies travel further, have peak near apex • The differences in the traveling waves produced by different sound frequencies are responsible for the neural coding of pitch Organ of Corti: • Sits on the top of the basilar membrane • Contains auditory receptor neurons (hair cells) that convert mechanical energy into a change in the membrane polarization • Has hair cells (cilia) sticking out of it that move back and forth when the cochlea vibrates, transduction into neural impulses Inner Hair Cells: • Hair cells are sandwiched between the basilar membrane and the reticular lamina, rods of corti span these two membranes and provide structural support • Hair cells between the modiolus and the rods of corti Outer Hair Cells: • Hair cells farther out than the rods of corti Spiral Ganglion Cells: • Hair cells form synapses on neurons whose cell bodies are located in the spiral ganglion within the modiolus • Bipolar with neuritis extending to the bases and sides of the hair cells, where they receive synaptic input • Axons from these cells enter the auditory nerve Auditory nerve: • A branch of the auditory-vestibular nerve (cranial nerve VIII) which projects to the cochlear nuclei in the medulla Dorsal Cochlear Nucleus: • First synapse • A nucleus in the medulla that receives afferents from the spiral ganglion cells in cochlea (in the inner ear) Ventral Cochlear Nucleus: • A nucleus in the medulla that receives afferents from the spiral ganglion in the cochlea of the inner ear Superior Olive (superior olivary nucleus): • A nucleus in the caudate pons that receives afferents from the cochlear nuclei and sends efferents to the inferior colliculus • First structure containing binaural neurons, receives input from cochlear nuclei on both sides of the brain stem Lateral Lemniscus: • Lemniscus means a collection of axons • Axons of the olivary neurons ascend to the lateral lemniscus Inferior Colliculus: • A nucleus in the midbrain from which all ascending auditory signals project to the medial geniculate nucleus Tonotopy (analogous to retinotopy): • The systematic organization within an auditory structure on the basis of characteristic frequency • A systematic relationship exists between position in the cochlear nucleus and characteristic frequency -- there is a map of the basilar membrane within the cochlear nucleus • (characteristic frequency is the frequency at which neuron is
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