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Ears & Hearing 10-34 Ears & Hearing - Outer Ear • Sound waves funneled by pinna (auricle) into external auditory meatus • External auditory meatus channels sound waves to tympanic membrane Fig 10.17 10-47 Ears & Hearing - Middle Ear continued • Malleus (hammer) is attached to tympanic membrane – Carries vibrations to incus (anvil) – Stapes (stirrup) receives vibrations from incus, transmits to oval window Fig 10.18 10-49 Ears & Hearing - Middle Ear continued • Stapedius muscle, attached to stapes, provides protection from loud noises – Can contract & dampen large vibrations – Prevents nerve damage in cochlea Fig 10.18 10-50 Ears & Hearing - Cochlea • Consists of a tube wound 3 turns & tapered so looks like snail shell Fig 10.19 10-51 Ears & Hearing - Cochlea continued • Low frequencies can travel all way thru vestibuli & back in tympani • As frequencies increase they travel less before passing directly thru vestibular & basilar membranes to tympani Fig 10.20 10-54 Ears & Hearing - Cochlea continued • High frequencies produce maximum stimulation of Spiral Organ closer to base of cochlea & lower frequencies stimulate closer to apex Fig 10.20 10-55 Spiral Organ (Organ of Corti) • Is where sound is transduced • Sensory hair cells located on the basilar membrane Fig 10.22 – 1 row of inner cells extend length of basilar membrane – Multiple rows of outer hair cells are embedded in tectorial membrane 10-56 Neural Pathway for Hearing • Info from 8th nerve goes to medulla, then to inferior colliculus, then to thalamus, & on to auditory cortex Fig 10.23 10-58 Neural Pathways for Hearing • Neurons in different regions of cochlea stimulate neurons in corresponding areas of auditory cortex – Each area of cortex represents different part of cochlea & thus a different pitch Fig 10.24 10-59 Hearing Impairments • Conduction deafness occurs when transmission of sound waves to oval window is impaired – Impacts all frequencies – Helped by hearing aids • Sensorineural (perceptive) deafness is impaired transmission of nerve impulses – Often impacts some pitches more than others – Helped by cochlear implants • Which stimulate fibers of 8th in response to sounds 10-60 Vestibular Apparatus • Provides sense of equilibrium – =orientation to gravity • Vestibular apparatus & cochlea form inner ear • V. apparatus consists of otolith organs (utricle & saccule) & semicircular canals Fig 10.11 10-35 Semicircular Canals • Provide information about rotational acceleration • Project in 3 different planes • Each contains a semicircular duct • At base is crista ampullaris where sensory hair cells are located Fig 10.12 10-42 Vestibular Apparatus continued • Utricle and saccule provide info about linear acceleration • Semicircular canals, oriented in 3 planes, give sense of angular acceleration Fig 10.12 10-37 Vestibular Apparatus continued • Hair cells are receptors for equilibrium – Each contains 20-50 hair-like extensions called stereocilia • 1 of these is a kinocilium Fig 10.13 10-38 Vestibular Apparatus continued • When stereocilia are bent toward kinocilium, hair cell depolarizes & releases NT that stimulates 8th nerve • When bent away from kinocilium, hair cell hyperpolarizes – In this way, frequency of APs in hair cells carries information about movement Fig 10.13 10-39 Utricle & Saccule • Have a macula containing hair cells – Hair cells embedded in gelatinous otolithic membrane • Which contains calcium carbonate crystals (=otoliths) that resist change in movement Fig 10.14 10-40 Utricle & Saccule continued • Utricle sensitive to horizontal acceleration Fig 10.14 – Hairs pushed backward during forward acceleration • Saccule sensitive to vertical acceleration • Hairs pushed upward when person descends 10-41 Semicircular Canals • Provide information about rotational acceleration • Project in 3 different planes • Each contains a semicircular duct • At base is crista ampullaris where sensory hair cells are located Fig 10.12 10-42 Semicircular Canals continued • Hair cell processes are embedded in cupula of crista ampullaris • When endolymph moves cupula moves Fig 10.15 – Sensory processes bend in opposite direction of angular acceleration 10-43 Neural Pathways for Equilibrium & Balance Fig 10.16 10-44 Nystagmus & Vertigo • Vestibular nystagmus is involuntary oscillations of eyes that occurs when spinning person stops – Eyes continue to move in direction opposite to spin, then jerk rapidly back to midline • Vertigo is loss of equilibrium – Natural response of vestibular apparatus – Pathologically, may be caused by anything that alters firing rate of 8th nerve • Often caused by viral infection 10-45