15 - Riverside City College

Transcript 15 - Riverside City College

THE EAR: HEARING AND BALANCE

The Ear: Hearing and Balance

• Three parts of the ear • The External (outer) Ear • Pinna- Composed of the Helix (rim) and lobule (earlobe) • External acoustic canal (meatus)-short, curved tube leading to eardrum • Lined with ceruminous glands • Tympanic membrane- eardrum; CT boundary btwn. external and middle ear; vibrates in response to sound Copyright © 2010 Pearson Education, Inc.

External ear Middle ear Internal ear (labyrinth) Auricle (pinna) Helix Lobule External acoustic meatus (a) The three regions of the ear Tympanic membrane Pharyngotympanic (auditory) tube

Figure 15.25a

Middle Ear

• The Middle Ear (tympanic cavity) • A small, air-filled, mucosa-lined cavity in the temporal bone; flanked laterally by the eardrum and medially by the oval and round windows • Three bones (ossicles)- Incus, Malleus,Stapes Copyright © 2010 Pearson Education, Inc.

Middle Ear

• Two tiny skeletal muscles prevent damage due to large vibrations • Tensor tympani-arises from wall of pharyngotympanic tube and inserts on the malleus • Stapedius muscle-runs from posterior wall of middle ear to the stapes Copyright © 2010 Pearson Education, Inc.

Middle Ear

• Pharyngotympanic (auditory) tubes • Connects middle ear to nasopharynx • Normally flattened closed; opens when yawning or swallowing • Equalizes pressure in the middle ear cavity with the external air pressure Copyright © 2010 Pearson Education, Inc.

Superior Lateral Malleus Incus Epitympanic recess Anterior View Pharyngotym panic tube

Tensor tympani muscle Tympanic membrane (medial view) Stapes Stapedius muscle Figure 15.26

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Inner Ear

Consists of the bony (osseous) and membranous labyrinth • Bony labyrinth • • • Tortuous channels in the temporal bone Filled with perilymph Three regions: • Vestibule • Contains two sacs • Saccule-continuous w/ the cochlear duct • Utricle-continuous w/ the semicircular ducts • Cochlea • Semicircular canals Copyright © 2010 Pearson Education, Inc.

The Inner Ear

Semicircular ducts in semicircular canals Anterior Posterior Lateral Cristae ampullares in the membranous ampullae Utricle in vestibule Saccule in vestibule Stapes in oval window

Superior vestibular ganglion Inferior vestibular ganglion

Temporal bone

Facial nerve Vestibular nerve Cochlear nerve Maculae Spiral organ (of Corti) Cochlear duct in cochlea Round window Figure 15.27

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The Maculae and Static Equilibrium

Maculae • Sensory receptors for static equilibrium (monitor the position of head in space, respond to linear acceleration) • One in each saccule wall and one in each utricle wall • Each maculae contains: • ET hair cells and supporting cells • Otolithic membrane- gel like membrane that overlies hair cells • Otoliths- calcium carbonate crystals; increase weight and its inertia Copyright © 2010 Pearson Education, Inc.

Macula of utricle Macula of saccule Stereocilia Kinocilium Otoliths Otolithic membrane Hair bundle

Vestibular nerve fibers Hair cells Supporting cells Figure 15.34

Steps of linear movement

• Hair cells are always releasing neurotransmitter • When hair cells bend towards kinocilium they depolarize, neurotransmitter release is increased • When hair cells bend away from kinocilium they hyperpolarize, slowing release of neurotransmitter • This change in neurotransmitter release informs the brain of the changing of the position of the head in space Copyright © 2010 Pearson Education, Inc.

Otolithic membrane Kinocilium Stereocilia Receptor potential Nerve impulses generated in vestibular fiber

Hyperpolarization Depolarization

When hairs bend toward the kinocilium, the hair cell depolarizes, exciting the nerve fiber, which generates more frequent action potentials.

When hairs bend away from the kinocilium, the hair cell hyperpolarizes, inhibiting the nerve fiber, and decreasing the action potential frequency.

Figure 15.35

The Crista Ampullaris and Dynamic Equilibrium

• Semicircular Canals • Three canals are located in each ear: • Located in all three planes of space • Anterior, posterior and lateral • Endolymph-fills the semicircular ducts • Ampulla- swellling at end of semicircular duct • Crista Ampullaris • Composed of hair cells and supporting cells • Structure and function of the crista ampullaris is basically the same as the hair cells of cochlea and maculae • Cupula in – gelled mass that cilia of hair cells are embedded Copyright © 2010 Pearson Education, Inc.

Crista ampullaris Membranous labyrinth Crista ampullaris Fibers of vestibular nerve (a) Anatomy of a crista ampullaris in a semicircular canal Cupula (b) Scanning electron micrograph of a crista ampullaris (200x)

Endolymph

Cupula Hair bundle (kinocilium plus stereocilia) Hair cell Supporting cell Figure 15.36a

–b

The Crista Ampullaris and Dynamic Equilibrium

• Steps of Rotational Movement • At rest the cupula stands upright • During rotational acceleration, hair cells are bent, they depolarize and impulses reach the brain faster • As movement slows, endolymph keeps moving, cilia are bent in opposite direction causing hyperpolarization and reduction of impulses to brain Copyright © 2010 Pearson Education, Inc.

Section of ampulla, filled with endolymph Cupula Fibers of vestibular nerve Flow of endolymph At rest, the cupula stands upright.

(c) Movement of the cupula during rotational acceleration and deceleration

During rotational acceleration, endolymph moves inside the semicircular canals in the direction opposite the rotation (it lags behind due to inertia).

Endolymph flow bends the cupula and excites the hair cells.

As rotational movement slows, endolymph keeps moving in the direction of the rotation, bending the cupula in the opposite direction from acceleration and inhibiting the hair cells.

Figure 15.36c

Equilibrium Pathway to the Brain

• Vestibular nerve-

Impulses travel to the vestibular nuclei in the brain stem or the cerebellum

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Pathways are complex and poorly traced

Sound and the Cochlea

• Sound is detected by: the cochlea • Cochlea • The cochlea is A spiral, conical, bony chamber and contains the cochlear duct • Cochlear duct- houses the spiral organ (of Corti) • Divides cochlea into three chambers: • Scala vestibuli-superior to cochlear duct (contains perilymph) • Scala tympani-inferior to cochlear duct; terminates at round window (contains perilymph) • Scala media (cochlear duct) -middle cavity; (contains endolymph) Copyright © 2010 Pearson Education, Inc.

(a) Modiolus Cochlear nerve, division of the vestibulocochlear nerve (VIII) Spiral ganglion Osseous spiral lamina Vestibular membrane Cochlear duct (scala media) Helicotrema Figure 15.28a

Vestibular membrane Tectorial membrane Cochlear duct ( scala media ; contains endolymph) Spiral organ (of Corti) Basilar membrane (b)

Scala vestibuli Osseous spiral lamina (contains perilymph) Scala tympani (contains perilymph) Spiral ganglion Figure 15.28b

Sound and the Cochlea

• Oval window-an opening on the medial wall of the middle ear (foot of stapes rests at oval window) • Round window-an opening on the medial wall of the middle ear (scala tympani terminates at round window) • Vestibular membrane-roof of cochlear duct that separates the scala media from scala vestibuli • Basilar membrane- fibrous floor of cochlear duct Copyright © 2010 Pearson Education, Inc.

Sound and the Cochlea

• Organ of Corti • Runs through center of cochlea • Has hair cells and supporting cells • Tectorial membrane- gel-like mass that cilia of hair cells are embedded in • Bending of the cilia: excites hair cells Copyright © 2010 Pearson Education, Inc.

Tectorial membrane Hairs (stereocilia) Outer hair cells Supporting cells (c)

Basilar membrane Inner hair cell Afferent nerve fibers Fibers of cochlear nerve Figure 15.28c

Sound Transmission

• Transmission of Sound to the Inner Ear • Sound waves enter the external acoustic canal and cause tympanic membrane to vibrate • Ossicles vibrate and amplify the pressure at the oval window • Pressure waves move through perilymph of the scala vestibuli • Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane Copyright © 2010 Pearson Education, Inc.

Auditory ossicles Malleus Incus Stapes Cochlear nerve Scala vestibuli Oval window Helicotrema 2 3 1 Scala tympani Cochlear duct Basilar membrane Tympanic Round membrane window (a) Route of sound waves through the ear 1 Sound waves vibrate 3 Pressure waves created by 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 with frequencies below hearing travel through the helicotrema and do not excite hair cells.

Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane and deflecting hairs on inner hair cells.

Figure 15.31a

Sound Transmission

• Resonance of the Basilar Membrane - fibers of the basilar membrane are “tuned” to a particular sound frequency • Vibrations of the basilar membrane causes cilia of hair cells to bend • Bending cilia towards kinocilium excites hair cells (increase neurotransmitter release) • Bending cilia away from kinocilium inhibits hair cells (slow release of neurotransmitter) Copyright © 2010 Pearson Education, Inc.

Sound Transmission

• Impulses from the cochlea pass via the spiral ganglion to the cochlear nuclei of the medulla • Eventually impulses are sent to the primary auditory cortex (temporal lobe) Copyright © 2010 Pearson Education, Inc.

Medial geniculate nucleus of thalamus Primary auditory cortex in temporal lobe Inferior colliculus Cochlear nuclei Vibrations Vibrations

Midbrain Medulla Vestibulocochlear nerve Spiral ganglion of cochlear nerve Spiral organ (of Corti) Figure 15.33

Deafness

• Hearing loss can be temporary or permanent • Common causes: • Middle ear infections • Conduction deafness • Can be caused by: • Impacted earwax • Ruptured eardrum • Middle ear inflammations • Otosclerosis Copyright © 2010 Pearson Education, Inc.

Deafness

• Nerve Deafness • Can be caused by: • Gradual loss of hair cells throughout life • Single explosively loud noise • Prolonged exposure to loud noise • Degeneration of cochlear nerve, tumors in auditory cortex, etc.

Tinnitus

• Ringing or clicking sound in ears in the absence of auditory stimuli • One of the first symptoms of cochlear degeneration • Can be caused by middle ear inflammation Copyright © 2010 Pearson Education, Inc.

Meniere’s Syndrome

• Labyrinth disorder • Affects all three parts of the internal ear • Symptoms are repeated attacks of vertigo, nausea and vomiting • Balance is severely disturbed and hearing is ultimately lost Copyright © 2010 Pearson Education, Inc.

15 - Riverside City College

Transcript 15 - Riverside City College

THE EAR: HEARING AND BALANCE