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The

• The organ of and equilibrium – Cranial VIII - Vestibulocochlear – Regions The Special • External ear • Hearing and • Internal ear (labyrinth) Equilibrium

External Ear

Middle Internal ear • Two parts External ear (labyrinth) ear – Pinna or (external structures) – External auditory meatus (car canal) Auricle • Site of cerumen (earwax) production (pinna) – Waterproofing, protection • Separated from the middle ear by the tympanic membrane () – Vibrates in response to sound waves Lobule External acoustic Tympanic Pharyngotympanic meatus membrane (auditory) tube (a) The three regions of the ear

Figure 15.25a

Middle Ear Epitympanic Middle Ear Superior recess Lateral • Anterior – Air-filled chamber – Openings View • Tympanic membrane – covers opening to • Round and oval windows – openings to • – dead-end cavity into temporal of unknown function • Auditory tube – AKA or pharyngotympanic tube Pharyngotym- panic tube

Tensor Tympanic Stapedius tympani membrane muscle muscle (medial view)

Figure 15.26

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Middle Ear Middle Ear

• Auditory tube (Eustachian tube) • Media – Connects the middle ear to the nasopharynx • Equalizes pressure – Opens during and yawning

Middle Ear Middle Ear

• Contains auditory () • Sound waves cause tympanic membrane to vibrate – Malleus • Ossicles help transmit vibrations into the inner ear – Incus – Reduce the area where force is applied – Stapes – Increases the pressure of the force enough to transfer most of the • Middle ear is air-filled; inner ear is fluid-filled energy into the liquid – Reflexive muscle action • Sound is mostly reflected from a liquid medium restricts the movement of the bones during loud noises

Inner Ear Superior vestibular ganglion Inferior vestibular ganglion • Contains functional organs for hearing & equilibrium – - filled with Semicircular ducts in – – functional component Vestibular • Filled with nerve • Anterior Location of various Posterior inner ear receptors Lateral Cochlear Cristae ampullares nerve in the membranous Maculae ampullae Spiral organ in (of Corti) vestibule in in vestibule Stapes in Round window

Figure 15.27

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Inner ear - Labyrinth Superior vestibular ganglion Inferior vestibular ganglion • Temporal Labyrinth is modified to form 3 distinct regions bone Semicircular –Vestibule ducts in Facial nerve semicircular • Gravity canals • Head position Anterior • Linear acceleration and deceleration (changes in speed) Posterior –Semicircular canals Lateral Cochlear • Angular acceleration and deceleration (changes in direction) Cristae ampullares nerve –Cochlea in the membranous Maculae ampullae • Spiral organ Vibration Utricle in (of Corti) vestibule Cochlear duct Saccule in in cochlea vestibule Stapes in Round * Note: It is always the membranous labyrinth that contains the receptors oval window window

Figure 15.27

Inner Ear

• The cochlea – A spiral, conical, bony chamber , – Still 2 portions of bony labyrinth enclosing a portion of membranous division of the labyrinth Bony vestibulocochlear labyrinth nerve (VIII) Membranous labyrinth Bony labyrinth Cochlear duct (scala media) (a)

Figure 15.28a

Inner ear Vestibular membrane Osseous spiral lamina Scala Spiral • Cochlear duct vestibuli ganglion Cavity of the cochlea is divided into 3 chambers (scala media ; – (contains Vestibular canal (scala vestibuli) contains perilymph) • Vestibular membrane endolymph) – Cochlear duct (scala media) Stria vascularis • supporting – Organ of hearing Spiral organ – Tympanic canal (scala tympani) (of Corti) Scala tympani Basilar (contains membrane perilymph)

(b)

Figure 15.28b

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Organ of Corti Tectorial membrane Inner Organ of Corti Hairs () Afferent nerve fibers Outer hair cells

Supporting cells

Fibers of cochlear nerve

Basilar membrane (c)

Figure 15.28c

Physiology of hearing Physiology of Hearing in a Nutshell

• Transduction of sound Sounds set up vibrations in air that beat against the eardrum that pushes a chain of tiny bones Mechanical energy in middle ear that press fluid in the internal ear against membranes that set up shearing forces that pull on the tiny hair cells that stimulate nearby Fluid pressure wave in inner ear neurons that give rise to the impulses that travel to the brain – and you hear.

Nerve impulse (This is from your textbook)

Area of High frequency (short wavelength) = high pitch high pressure Low frequency (long wavelength) = low pitch (compressed molecules) Area of Wavelength low pressure (rarefaction)

Crest Pressure

Trough Time (s)

Air pressure Air (a) Frequency is perceived as pitch.

Distance Amplitude High amplitude = loud A struck tuning fork alternately compresses Low amplitude = soft and rarefies the air molecules around it, creating alternate zones of high and low pressure. Pressure (b) Sound waves radiate outward in all directions. Time (s) (b) Amplitude (size or intensity) is perceived as loudness.

Figure 15.29 Figure 15.30

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Malleus Incus Stapes vibrating Helicotrema Cochlea Auditory ossicles in oval window Malleus Incus Stapes Cochlear nerve Sound waves Perilymph Scala vestibuli Oval Helicotrema window Scala tympani 3 8 Scala 7 tympani Cochlear duct 4 2 Scala 3 5 Basilar vestibuli membrane 6 Basilar 1 2 9 membrane 1 External auditory 8 canal Spiral organ (organ of Corti) Tectorial membrane Vestibular membrane Sounds with frequencies below hearing travel through Cochlear duct the helicotrema and do not Tympanic (contains endolymph) Tympanic Round excite hair cells. membrane window membrane Sounds in the (a) Route of sound waves through the ear Secondary tympanic go through the cochlear duct, membrane vibrating 1 Sound waves vibrate 3 Pressure waves created by vibrating the basilar membrane Middle ear Auditory tube in the tympanic membrane. the stapes pushing on the oval and deflecting hairs on inner 2 Auditory ossicles vibrate. window move through fluid in hair cells. Pressure is amplified. the scala vestibuli.

Figure 15.31a

Tectorial membrane Inner hair cell

Basilar membrane Hairs (stereocilia) Afferent nerve fibers Outer hair cells High-frequency sounds displace the basilar membrane near the base. Supporting cells

Fibers of basilar membrane Fibers of Medium-frequency sounds displace the basilar membrane near the middle. cochlear Base Apex (short, (long, nerve stiff floppy fibers) fibers)

Low-frequency sounds displace the basilar membrane near the apex. Frequency (Hz) (b) Different sound frequencies cross the basilar membrane at different locations. Basilar membrane (c)

Figure 15.31b Figure 15.28c

• Movement of the basilar membrane bends the hair cells Medial geniculate nucleus of

Primary in temporal lobe Superior olivary nucleus (-medulla junction) Cochlear nuclei

Medulla Vibrations

Vibrations Spiral ganglion of cochlear nerve Bipolar cell Spiral organ (of Corti)

Figure 15.33

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Localization of Sound Abnormalities of hearing

• Conductive deafness • Timing comparison –Interference in movement of middle ear bones – Side nearest sound detects sound first • Impacted earwax, or of the ossicles (overgrowth • Comparison of volume of bone) – High pitched = blocked by head • Sensorineural deafness – • Perceived as loudest in ear nearest the source Damage to the neural structures • Aging, prolonged exposure to loud sounds – Low pitched = curve around head • Perceived as equally loud in both

Abnormalities of hearing Abnormalities of hearing

• • Otosclerosis

Abnormalities of hearing Abnormalities of hearing

• Meniere’s Disease • – Episodes of – Ringing, clicking, hissing, or roaring – Progressive – Sometimes can be heard with a stethoscope (o bjective tinnitus) – – Tinnitus May occur in the same rhythm as the heartbeat – – Caused by noise-induced hearing loss, ear infections, diseases of blood Feeling of fullness or vessels, head injury, neurological problems, brain tumors, earwax… pressure in ear – Usually only in one ear

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Vestibular Apparatus Physiology of Equilibrium Otolithic Stereocilia membrane • Information about position and movement of head Hair bundle – Vestibular apparatus Macula of • Utricle utricle • Saccule Vesibule Macula of saccule • Semi-circular canals

Hair cells Maculae are Supporting perpendicular cells to one another Vestibular nerve fibers

Figure 15.34

Vestibule Semi-Circular canals

• Macula = receptor – –Utricle • Sensory receptor for dynamic equilibrium – One in the ampulla of each semicircular canals • Horizontal movements – Major stimuli are rotatory movements • Tilting the head side to side –Saccule • Vertical movements

Cupula Crista Equilibrium ampullaris Endolymph • Hair bundle (kinocilium plus stereocilia) – Conflicts between eye movements and equilibrium Hair cell • Nystagmus Membranous Crista • Physiologic labyrinth ampullaris Supporting Fibers of vestibular nerve cell – Involuntary eye movement, part of a reflex (a) of a crista ampullaris in a – Preserves clear vision during rotation of the head semicircular canal • Pathologic Cupula – Abnormal eye movement – eyes move as though the head is rotating while the head is still – Caused by damage to any part of the

(b) Scanning electron micrograph of a crista ampullaris (200x)

Figure 15.36a–b

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