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1/24/2016 The Ear • The organ of hearing and equilibrium The Special Senses – Cranial nerve VIII - Vestibulocochlear – Regions • External ear • Middle ear • Internal ear (labyrinth) Hearing and Equilibrium External Ear External ear • Two parts – Pinna or auricle (external structures) – External auditory meatus (car canal) • Site of cerumen (earwax) production Middle Internal ear ear (labyrinth) Auricle (pinna) – Waterproofing, protection • Separated from the middle ear by the tympanic membrane (eardrum) Helix – Vibrates in response to sound waves Lobule External Tympanic acoustic membrane meatus (a) The three regions of the ear Pharyngotympanic (auditory) tube Figure 15.25a Middle Ear Middle Ear Malleus Superior Incus Epitympanic recess Lateral • Tympanic cavity Anterior – Air-filled chamber – Openings View • Tympanic membrane – covers opening to outer ear • Round and oval windows – openings to inner ear • Epitympanic recess – dead-end cavity into temporal bone of unknown function • Auditory tube – AKA Eustachian tube or pharyngotympanic tube Pharyngotympanic tube Tensor tympani muscle Tympanic membrane (medial view) Stapes Stapedius muscle Figure 15.26 1 1/24/2016 Middle Ear • Auditory tube (Eustachian tube) Middle Ear • Otitis Media – Connects the middle ear to the nasopharynx • Equalizes pressure – Opens during swallowing and yawning Middle Ear • Contains auditory ossicles (bones) – Malleus – Incus – Stapes • Middle ear is air-filled; inner ear is fluid-filled • Sound is mostly reflected from a liquid medium Middle Ear • Sound waves cause tympanic membrane to vibrate • Ossicles help transmit vibrations into the inner ear – Reduce the area where force is applied – Increases the pressure of the force enough to transfer most of the energy into the liquid – Reflexive muscle action restricts the movement of the bones during loud noises Inner Ear Superior vestibular ganglion Inferior vestibular ganglion • Contains functional organs for hearing & equilibrium – Bony labyrinth - filled with perilymph – Membranous labyrinth – functional component • Filled with endolymph • Location of various inner ear receptors Temporal bone Semicircular ducts in semicircular canals Facial nerve Vestibular nerve Anterior Posterior Lateral Cochlear nerve Maculae Cristae ampullares in the membranous ampullae Spiral organ (of Corti) Cochlear duct in cochlea Utricle in vestibule Saccule in vestibule Stapes in oval window Round window Figure 15.27 2 1/24/2016 Inner ear - Labyrinth Superior vestibular ganglion Inferior vestibular ganglion • Labyrinth is modified to form 3 distinct regions – Vestibule • Gravity • Head position • Linear acceleration and deceleration (changes in speed) – Semicircular canals • Angular acceleration and deceleration (changes in direction) – Cochlea • Vibration Temporal bone Semicircular ducts in semicircular canals Facial nerve Vestibular nerve Anterior Posterior Lateral Cochlear nerve Maculae Cristae ampullares in the membranous ampullae Spiral organ (of Corti) Cochlear duct in cochlea Utricle in vestibule Saccule in vestibule Stapes in oval window * Note: It is always the membranous labyrinth that contains the receptors Round window Figure 15.27 Inner Ear • The cochlea – A spiral, conical, bony chamber Cochlear nerve, division of the vestibulocochlear nerve (VIII) Modiolus – Still 2 portions of bony labyrinth enclosing a portion of membranous labyrinth Bony labyrinth Spiral ganglion Membranous labyrinth Osseous spiral lamina Vestibular membrane Bony labyrinth Cochlear duct (scala media) Helicotrema (a) Figure 15.28a Inner ear Vestibular membrane Osseous spiral lamina Tectorial membrane • Cavity of the cochlea is divided into 3 chambers – Vestibular canal (scala vestibuli) • Vestibular membrane – Cochlear duct (scala media) • Basilar membrane supporting Organ of Corti – Organ of hearing – Tympanic canal (scala tympani) Cochlear duct (scala media; contains endolymph) Scala vestibuli (contains perilymph) Spiral ganglion Stria vascularis Spiral organ (of Corti) Basilar membrane Scala tympani (contains perilymph) (b) Figure 15.28b 3 1/24/2016 Organ of Corti Tectorial membrane Inner hair cell Hairs (stereocilia) Afferent nerve fibers Outer hair cells Organ of Corti Supporting cells Fibers of cochlear nerve Basilar membrane (c) Figure 15.28c Physiology of hearing Physiology of Hearing in a Nutshell Sounds set up vibrations in air that beat against the eardrum that pushes a chain of tiny bones that press fluid in the internal ear against membranes that set up shearing forces that pull on the tiny hair cells that stimulate nearby neurons that give rise to the impulses that travel to the brain – and you hear. • Transduction of sound Mechanical energy in middle ear Fluid pressure wave in inner ear Nerve impulse Wavelength Area of high pressure (compressed molecules) Area of low pressure (rarefaction) High frequency (short wavelength) = high pitch Low frequency (long wavelength) = low pitch Pressure Air pressure (This is from your textbook) Crest Trough Time (s) (a) Frequency is perceived as pitch. Distance Amplitude A struck tuning fork alternately compresses and rarefies the air molecules around it, creating alternate zones of high and low pressure. Pressure High amplitude = loud Low amplitude = soft (b) Sound waves radiate outward in all directions. Figure 15.29 Time (s) (b) Amplitude (size or intensity) is perceived as loudness. Figure 15.30 4 1/24/2016 Incus Malleus Stapes vibrating in oval window Helicotrema Auditory ossicles Cochlea Malleus Incus Stapes Cochlear nerve Sound waves Perilymph Scala vestibuli Oval window 3 7 4 Scala tympani Scala vestibuli Basilar membrane 8 5 1 2 6 9 External auditory canal Helicotrema Scala tympani Cochlear duct 2 3 Basilar membrane 1 8 Spiral organ (organ of Corti) Tectorial membrane Sounds with frequencies below hearing travel through the helicotrema and do not excite hair cells. Vestibular membrane Cochlear duct (contains endolymph) Tympanic membrane Secondary tympanic membrane vibrating in round window Tympanic Round membrane window (a) Route of sound waves through the ear 3 Pressure waves created by 1 Sound waves vibrate Middle ear Auditory tube the tympanic membrane. 2 Auditory ossicles vibrate. Pressure is amplified. the stapes pushing on the oval window move through fluid in the scala vestibuli. Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane and deflecting hairs on inner hair cells. Figure 15.31a Tectorial membrane Inner hair cell Hairs (stereocilia) Afferent nerve fibers Basilar membrane Outer hair cells High-frequency sounds displace the basilar membrane near the base. Supporting cells Fibers of cochlear nerve Fibers of basilar membrane Medium-frequency sounds displace the basilar membrane near the middle. Low-frequency sounds displace the basilar membrane near the apex. Apex (long, floppy fibers) Base (short, stiff fibers) Frequency (Hz) (b) Different sound frequencies cross the basilar membrane at different locations. Basilar membrane (c) Figure 15.31b •Movement of the basilar membrane bends the hair cells Figure 15.28c Medial geniculate nucleus of thalamus Primary auditory cortex in temporal lobe Inferior colliculus Lateral lemniscus Superior olivary nucleus (pons-medulla junction) Midbrain Cochlear nuclei Vibrations Medulla Vestibulocochlear nerve Vibrations Spiral ganglion of cochlear nerve Bipolar cell Spiral organ (of Corti) Figure 15.33 5 1/24/2016 Localization of Sound • Timing comparison Abnormalities of hearing • Conductive deafness – Interference in movement of middle ear bones – Side nearest sound detects sound first • Comparison of volume – High pitched = blocked by head • Impacted earwax, perforated eardrum or otosclerosis of the ossicles (overgrowth of bone) • Sensorineural deafness – Damage to the neural structures • Perceived as loudest in ear nearest the source • Aging, prolonged exposure to loud sounds – Low pitched = curve around head • Perceived as equally loud in both ears Abnormalities of hearing • Otitis media Abnormalities of hearing • Otosclerosis Abnormalities of hearing • Meniere’s Disease Episodes of vertigo Progressive hearing loss Tinnitus Feeling of fullness or pressure in ear – Usually only in one ear – – – – Abnormalities of hearing • Tinnitus – – – – Ringing, clicking, hissing, or roaring Sometimes can be heard with a stethoscope (objective tinnitus) May occur in the same rhythm as the heartbeat Caused by noise-induced hearing loss, ear infections, diseases of blood vessels, head injury, neurological problems, brain tumors, earwax… 6 1/24/2016 Vestibular Apparatus Physiology of Equilibrium Kinocilium Stereocilia • Information about position and movement of head – Vestibular apparatus Otoliths Otolithic membrane Hair bundle Macula of utricle Macula of saccule • Utricle Vesibule • Saccule • Semi-circular canals Hair cells Maculae are perpendicular to one another Supporting cells Vestibular nerve fibers Figure 15.34 Vestibule Semi-Circular canals – Crista ampullaris • Macula = receptor • Sensory receptor for dynamic equilibrium –Utricle – One in the ampulla of each semicircular canals – Major stimuli are rotatory movements • Horizontal movements • Tilting the head side to side –Saccule • Vertical movements Equilibrium Cupula Crista ampullaris Endolymph Hair bundle (kinocilium plus stereocilia) Hair cell Crista Membranous ampullaris labyrinth Fibers of vestibular nerve (a) Anatomy of a crista ampullaris in a semicircular canal • Motion Sickness – Conflicts between eye movements and equilibrium • Nystagmus • Physiologic Supporting cell – Involuntary eye movement, part of a reflex – Preserves clear vision during rotation of the head • Pathologic – Abnormal eye movement – eyes move as though the head is rotating while the head is still – Caused by damage to any part of the vestibular system Cupula (b) Scanning electron micrograph of a crista ampullaris (200x) Figure 15.36a–b 7