Sensory Organ: Ear - BIOL 221-001 PDF

Chapter 16 Andrews University Sensory Organ: Ear BIOL 221-001 Part 2 Professor: Brian Y.Y. Wong, Ph.D.  Hearing and Equilibrium Hearing— a response to vibrating air molecules Equilibrium— the sense of motion, body orienta...

Chapter 16 Andrews University Sensory Organ: Ear BIOL 221-001 Part 2 Professor: Brian Y.Y. Wong, Ph.D.  Hearing and Equilibrium Hearing— a response to vibrating air molecules Equilibrium— the sense of motion, body orientation, and balance Both senses reside in the inner ear, a maze of fluid- filled passages and sensory cells Fluid is set in motion and the sensory cells convert this motion into an informative pattern of action potentials 16-2  The Nature of Sound Sound— any audible vibration of molecules – A vibrating object (e.g., a tuning fork) pushes on air molecules – These, in turn, push on other air molecules – Air molecules hitting the eardrum cause it to vibrate  Pitch Pitch— our sense of whether a sound is “high” or “low” – Determined by vibration frequency: hertz (Hz) or cycles/second – Human hearing range is 20 to 20,000 Hz Infrasonic frequencies below 20 Hz Ultrasonic frequencies above 20,000 Hz – Speech is 1,500 to 5,000 Hz, where hearing is most sensitive – Most hearing loss with age is in the range of 250 to 2,050 Hz  Loudness Loudness— the perception of sound energy, intensity, or amplitude of the vibration – Expressed in decibels (dB) – Prolonged exposure to sounds > 90 dB can cause damage  Anatomy of the Ear The ear has three sections: outer, middle, and inner ear – The first two are concerned only with the transmission of sound to the inner ear – Inner ear: vibrations converted to nerve signals Outer Ear  Outer Ear Outer ear— a funnel for conducting vibrations to the tympanic membrane (eardrum) – Auricle (pinna) directs sound down the auditory canal Shaped and supported by elastic cartilage – Auditory canal (external acoustic meatus): passage leading through temporal bone to the tympanic membrane Slightly S-shaped tube that begins at the external opening and courses for about 3 cm Guard hairs protect the outer end of the canal Cerumen (earwax)— a mixture of secretions of ceruminous and sebaceous glands and dead skin cells 16-8  Middle Ear Middle ear— located in the air-filled tympanic cavity in the temporal bone – Tympanic membrane (eardrum) closes the inner end of the auditory canal (separates it from the middle ear) About 1 cm in diameter Suspended in a ring-shaped groove in the temporal bone Vibrates freely in response to sound Innervated by sensory branches of the vagus and trigeminal nerves – Highly sensitive to pain – Tympanic cavity is continuous with mastoid air cells Space only 2 to 3 mm wide between outer and inner ears Contains auditory ossicles 16-9  Middle Ear – Auditory (eustachian) tube connects middle-ear to the nasopharynx Equalizes air pressure on both sides of the tympanic membrane Normally closed, but swallowing or yawning open it Allows throat infections to spread to the middle ear – Auditory ossicles Malleus: has a long handle attached to the inner surface of the tympanic membrane Incus: articulates with malleus and stapes Stapes: shaped like a stirrup; footplate rests on oval window— where inner ear begins – Stapedius and tensor tympani muscles attach to stapes and malleus 16-10  Middle-Ear Infection Otitis media (middle-ear infection) is common in children – Auditory tube is short and horizontal – Infections easily spread from throat to the tympanic cavity and mastoid air cells Symptoms – Fluid accumulates in the tympanic cavity producing pressure, pain, and impaired hearing – Can spread, leading to meningitis – Can cause fusion of ear ossicles and hearing loss Tympanostomy— lancing tympanic membrane and draining fluid from the tympanic cavity 16-11 Inner Ear  Inner Ear Bony labyrinth— passageways in the temporal bone Membranous labyrinth— fleshy tubes lining bony labyrinth – Filled with endolymph: similar to intracellular fluid – Floating in perilymph: similar to cerebrospinal fluid  Inner Ear 40 to 70 stereocilia and one true cilium kinocilium Motion Hearing Gravity Balance (A U S) Equilibrium Labyrinth— vestibule and three semicircular ducts Cochlea— organ of hearing (Organ of Corti) – Winds 2.5 coils around a screw-like axis of spongy bone, the modiolus – Threads of the screw form a spiral platform that supports the fleshy tube of the cochlea  Inner Ear Cochlea has three fluid-filled chambers separated by membranes – Scala vestibuli: superior chamber Filled with perilymph Begins at the oval window and spirals to the apex – Scala tympani: inferior chamber Filled with perilymph Begins at the apex and ends at the round window – Secondary tympanic membrane: covers round window – Scala media (cochlear duct): middle chamber Filled with endolymph Separated from: – Scala vestibuli by vestibular membrane – Scala tympani by the thicker basilar membrane Contains spiral organ— Organ of Corti: acoustic organ that converts vibrations into nerve impulses 16-15  Stapes hits on Oval window (a) Inner Ear Vestibular membrane Cochlear duct (b) Spiral ganglion (scala media) Cochlear nerve Scala vestibuli (with perilymph) Organ of Corti Vestibular Tectorial membrane membrane Cochlear duct (with Hairs (stereocilia) endolymph) Outer hair cells (fr.pr) Scala tympani (with perilymph) Supporting cells Tectorial Basilar membrane membrane Sprial organ Inner hair Cell (hearing) Basilar membrane Fibers of cochlear nerve (c)  Inner Ear Spiral organ has epithelium composed of hair cells and supporting cells Hair cells have long, stiff microvilli called stereocilia on the apical surface – Gelatinous tectorial membrane rests on top of stereocilia Spiral organ has four rows of hair cells spiraling along its length – Inner hair cells: a single row of about 3,500 cells Provides for hearing – Outer hair cells: three rows of about 20,000 cells Adjusts response of cochlea to different frequencies Increases precision 16-17  Outer hair cells Inner hair cells 20,000 3,500 cells cells adjusts the for response of hearing the cochlea to different frequencies Increases precision Outside of cell is +80 mV and 10 μm SEM inside of cell is near −40 mV  The Physiology of Hearing Tympanic membrane – Has 18 times the area of the oval window – Ossicles concentrate the energy of the vibrating tympanic membrane on an area 1/18 that size – Ossicles create a greater force per unit area at the oval window and overcome the inertia of the perilymph – Ossicles and their muscles have a protective function Lessen the transfer of energy to the inner ear 16-19  The Physiology of Hearing Tympanic reflex – During loud noise, the tensor tympani pulls the tympanic membrane inward and tenses it – Stapedius muscle reduces motion of the stapes – Muffles the transfer of vibration from the tympanic membrane to the oval window – Middle-ear muscles also help to coordinate speech with hearing Dampens the sound of your speech 16-20  Stimulation of Cochlear Hair Cells Vibration of ossicles causes vibration of the basilar membrane under hair cells – As often as 20,000 times per second – Hair cells move with the basilar membrane  Stimulation of Cochlear Hair Cells Stereocilia of outer hair cells – Bathed in high K+ fluid, the endolymph Creating an electrochemical gradient Outside of cell is +80 mV and inside of cell is near −40 mV – Tip embedded in tectorial membrane  Potassium Channels K+ flows in — depolarization highK+ fluid causes release of the endolymph neurotransmitter stimulates sensory dendrites and generates action potential in the cochlear nerve +80 mV -40 mV  Stimulation of Cochlear Hair Cells Stereocilium on inner hair cells – Single transmembrane protein at the tip functions as a mechanically gated ion channel Stretchy protein filament (tip link) connects the ion channel of one stereocilium to the sidewall of the next The tallest stereocilium is bent when the basilar membrane rises toward tectorial membrane Pulls on tip links and opens ion channels K+ flows in— depolarization causes release of neurotransmitter Stimulates sensory dendrites and generates action potential in the cochlear nerve  Sensory Coding Variations in loudness (amplitude) cause variations in the intensity of cochlear vibrations – Soft sound produces relatively slight up-and-down motion of the basilar membrane – Louder sounds make the basilar membrane vibrate more vigorously Triggers higher frequency of action potentials brain interprets this as a louder sound 16-25  Sensory Coding Pitch depends on which part of the basilar membrane vibrates – At the basal end, membrane attached, narrow and stiff Brain interprets signals as high-pitched – At distal end, 5 times wider and more flexible Brain interprets signals as low-pitched 16-26  Tympanic membrane (vibrating) Stapes footplate Scala vestibuli (vibrating) Scala Cochlear Basilar tympani duct Membrane Frequency Response Secondary Basilar Helicotrema tympanic membrane (a) membrane (vibrating) Low-frequency sound (20–800 Hz) Medium-frequency sound (1,500–4,000 Hz) High-frequency sound (7,000–20,000 Hz) (b) Distal Proximal end end (attached) (free) 20,000 5,000 1,000 500 200 Hz (c)  Cochlear Tuning Increases ability of the cochlea to receive some sound frequencies Outer hair cells shorten, (10% to 15%) reducing the basilar membrane’s mobility – Fewer signals from that area allow the brain to distinguish between more and less active areas of the cochlea Pons has inhibitory fibers that synapse near the base of inner hair cells – Inhibits some areas and increases the contrast between regions of the cochlea 16-28  Deafness Deafness— hearing loss – Conductive deafness: conditions interfere with the transmission of vibrations to the inner ear Damaged tympanic membrane, otitis media, blockage of auditory canal, and otosclerosis – Otosclerosis: fusion of auditory ossicles that prevents their free vibration – Sensorineural (nerve) deafness: death of hair cells or any nervous system elements concerned with hearing Factory workers, musicians, construction workers 16-29  The Auditory Projection Pathway Sensory fibers begin at the bases of hair cells – Somas form the spiral ganglion around the modiolus – Axons lead away from the cochlea as the cochlear nerve – Joins with the vestibular nerve to form the vestibulocochlear nerve (cranial nerve VIII) – Each ear sends nerve fibers to both sides of the pons – End in cochlear nuclei Synapse with second-order neurons that ascend to the nearby superior olivary nucleus – Superior olivary nucleus issues efferent fibers back to the cochlea to tune the cochlea – Superior olivary nucleus also functions in binaural hearing— comparing signals from the right and left ears to identify the direction from which a sound is coming 16-30  The Auditory Projection Pathway Other cochlear nucleus fibers ascend to the inferior colliculi of the midbrain – Helps to locate the origin of the sound, processes fluctuation in pitch, and mediates the startle response and rapid head turning in response to loud noise Third-order neurons begin in the inferior colliculi and lead to the thalamus Fourth-order neurons from the thalamus to primary auditory cortex at the superior margin of the temporal lobe – Functions in conscious perception of sound – Auditory system has extensive decussations, so damage to the one side of cortex does not cause unilateral hearing loss 16-31 The Auditory Projection Pathway The Auditory Projection Pathway (4)  Equilibrium Equilibrium— coordination, balance, and orientation in three- dimensional space Vestibular apparatus— constitutes receptors for equilibrium – Three semicircular ducts Detect only angular acceleration – Two chambers Anterior saccule and posterior utricle Responsible for static equilibrium and linear acceleration 16-34  Equilibrium Static equilibrium— the perception of the orientation of the head when the body is stationary Dynamic equilibrium— perception of motion or acceleration – Linear acceleration— change in velocity in a straight line (elevator) – Angular acceleration— change in rate of rotation (car turns a corner) 16-35  The Saccule (M) and Utricle (G) Macula— a 2 by 3 mm patch of hair cells and supporting cells in the saccule and utricle – Macula Sacculi: lies vertically on the wall of Saccule →Motion – Macula Utriculi: lies horizontally on the floor of Utricle →Gravity Motion Gravity  The Saccule and Utricle kinocilium 40 to 70 stereocilia and one true cilium  The Saccule and Utricle Each hair cell has 40 to 70 stereocilia and one true cilium—- kinocilium embedded in a gelatinous otolithic membrane – Otoliths: calcium carbonate–protein granules that add to the weight and inertia and enhance the sense of gravity and motion Static equilibrium— when head is tilted, heavy otolithic membrane sags, bending the stereocilia and stimulating the hair cells Dynamic equilibrium— in car, linear acceleration detected as otoliths lag behind, bending the stereocilia and stimulating the hair cells Because macula sacculi is nearly vertical, it responds to vertical acceleration and deceleration  The Semicircular Ducts C A S U Rotary movements detected by the three semicircular ducts Bony semicircular canals of temporal bone hold membranous semicircular ducts Each duct is filled with endolymph and opens up as a dilated sac (ampulla) next to the utricle Each ampulla contains crista ampullar— a mound of hair cells and supporting cells  The Semicircular Ducts (CAC →SRP) Crista Ampullaris – Consists of hair cells with stereocilia and a kinocilium buried in a mound of gelatinous membrane called the Cupula (one in each duct) Spatial orientation of canals causes ducts to be stimulated by Rotation in different Planes  The Semicircular Ducts As head turns, endolymph lags behind, pushes cupula, stimulates hair cells  Vestibular Projection Pathways (5 target areas) Integrates Awareness of vestibular position and motor information into control of head its control of and body head and eye 5. movements, muscle tone, and 2. posture CN III, IV, and VI) to produce 1. vestibulo– ocular reflex: Keeps vision fixed on distant object 4. while walking Innervate extensor 3. (antigravity) Adjust blood circulation and muscles breathing to postural changes Figure 16.21  Vestibular Projection Pathways Hair cells of macula sacculi, macula utriculi, and semicircular ducts synapse on the vestibular nerve (part of CN VIII) Fibers end in a complex of four vestibular nuclei on each side of the pons and medulla – Left and right nuclei receive input from both ears Process signals about the position and movement of the body and relay information to five target areas 16-43  Vestibular Projection Pathways Five target areas 1. Cerebellum: integrates vestibular information into its control of head and eye movements, muscle tone, and posture 2. Nuclei of oculomotor, trochlear, and abducens nerves (CN III, IV, and VI) to produce vestibulo–ocular reflex: keeps vision fixed on distant object while walking 3. Reticular formation: thought to adjust blood circulation and breathing to postural changes 4. Spinal cord: descend through two vestibulospinal tracts of the spinal cord and innervate extensor (antigravity) muscles 5. Thalamus: thalamic relay to the cerebral cortex for awareness of position and motor control of head and body 16-44