Lecture 1 What are the major components of the outer middle and inner ear What membranes separate the middle ear from the outer ear and the inner ear Outer ear Middle ear Inner ear Pinna Ear canal Tympanic membrane border b t outer middle ear Ossicles Malleus Incus Stapes Oval window border b t middle inner ear Cochlea Semicircular canals Otolith organs fluid in ear that helps with coordination movement etc What is the purpose of the pinna collect sound and perform spectral transformations to incoming sounds which enable the process of vertical localization to take place Only mammals have pinna Varies greatly b t species little bit w in species Important in localizing sound What is the function of the ossicles and know their names too Why is this function necessary how is it accomplished Ossicles malleus incus stapes They conduct sound vibrations from the eardrum across the middle ear into the fluid filled inner ear Inside the inner ear these vibrations are converted to nerve signals that are carried by the auditory nerve to the brain Amplify sound vibrations Joints b t bones make them work like levers Concentrates energy from larger surface tympanic membrane to smaller surface oval window What are the muscles in the middle ear and what is their function What is the acoustic reflex Muscles tensor tympani stapedius Acoustic reflex muscles tense w loud sounds and self generated sounds such as chewing swallowing Takes about 1 5 second so not helpful w short sudden loud sounds like a gunshot only helpful w sustained loud sounds Could you describe the anatomy of the cochlea What are the 3 long fluid filled canals What membranes separate them Where are the oval window and the round window What is the helicotrema What makes up the cochlear partition 3 parallel fluid filled canals rolled up like a snail Vestibular canal middle canal and tympanic canal Organ of Corti separates Vestibular Canal Middle Canal Tympanic Canal separated lengthwise by Middle Canal cochlear BM more like a plate separates Middle Canal Tympanic Canal structure on Basilar Membrane composed of hair cells dendrites of Vestibular Canal partition Reissner s membrane Basilar membrane auditory nerve fibers overlain by Tectorial Membrane Cochlear partition TRANSDUCTION Vibrations sound waves thru tympanic membrane ossicles of middle ear push pull oval window in out of vestibular canal at base of cochlea combined basilar membrane tectorial membrane organ of Corti Movement of oval window causes traveling waves to flow thru fluid in vestibular canal Wave in the vestibular canal displaces some part of the cochlear partition causing it to flex back and forth as the wave passes Any pressure left at apex travels thru helicotrema thru tympanic canal and is absorbed at base of cochlea by round window Helicotrema connects the Vestibular Canal Tympanic Canal Cochlear partition is made up of the Tectorial membrane organ of corti and basilar membrane What are the differences physical and functional between the inner hair cells and the outer hair cells How are the hair cells stimulated What happens w in the cells when they are stimulated ie what ion comes in first and how is its channel opened what ion comes in next how is its channel opened What is the result of stimulation of a hair cell I ll tell you a receptor potential What s that Inner Hair Cells 1 row Most auditory info Afferents Outer Hair Cells 90 95 of auditory nerve fibers come from synapses w IHCs 3 rows mainly efferent innervation Stereocilia elongate amplify the movement of the basilar membrane in that region sharpen frequency response of IHCs Also inhibitory innervation of OHCs suppresses contractility to help filter noise Otoacoustic emissions OHCs oscillate vibrate basilar membrane backward propagation thru to vibrating tympanic membrane Ototoxic drugs that are selective for OHCs increase auditory threshold hearing loss of 60 80 dB Outer hair cells also responsible for 1 Otoacoustic emissions and 2 registering low amp weak sounds near the absolute threshold Neurons whose cell bodies lie in the spiral ganglion are strung along the bony core of the cochlea and send fibers axons into the central nervous system CNS These bipolar neurons are the first neurons in the auditory system to fire an action potential and supply all of the brain s auditory input Their dendrites make synaptic contact with the base of hair cells and their axons are bundled together to form the auditory portion of eighth cranial nerve The number of neurons in the spiral ganglion is estimated to be about 35 000 50 000 Two apparent subtypes of spiral ganglion cells exist Type I spiral ganglion cells comprise the vast majority of spiral ganglion cells 90 95 in cats and 88 in humans and exclusively innervate the inner hair cells They are myelinated bipolar neurons Type II spiral ganglion cells make up the remainder In contrast to Type I cells they are unipolar and unmyelinated in most mammals They innervate the outer hair cells with each Type II neuron sampling many 15 20 outer hair cells In addition outer hair cells form reciprocal synapses onto Type II spiral ganglion cells suggesting that the Type II cells K flows into hair cell cell nerve AP have both afferent and efferent roles Inhibitory input to OHCs still thru 8th cranial nerve and use Ach or GABA I think Vibration displacement of cochlear partition tectorial membrane sheers across width of cochlear partition bending of stereocilia back and forth Location of transduction Stereocilia connected to each other by tip links Stereocilia bend toward tallest tip links stretch pull open K channels Depolarization causes VG Ca2 channels in body of hair cell to open Ca2 into Neurotransmitter glutamate released from base of HC onto fiber of auditory Potassium flowing into the hair cell is different from what we learned about most neurons The fluid surrounding the cilia called endolymph has a very high concentration of potassium so this is what comes in to depolarize the cell The fluid surrounding the body of the hair cell is called perilymph The afferent fiber in the diagram is a fiber axon of the auditory nerve cranial nerve VIII Each hair cells synapses w 10 30 auditory nerve fibers The release of the neurotransmitter onto the auditory nerve fiber is called a receptor potential it is not an action potential although it results in an action potential traveling down the auditory nerve fiber A change in voltage across the membrane of a sensory receptor cell in
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