The Ear 2023 PDF

The Ear Images from Human Anatomy 6th ed., © 2009, by Martini, Timmins and Talitsch, denoted by “Ma”. Images from Human Anatomy, 2nd ed., © 2008 by McKinley & O’Loughlin, denoted by “Mc”. Images from Neuroscience: Exploring the Brain by Bear, et al., 3rd Edition, © 2006. Sound Waves • when an obje...

The Ear Images from Human Anatomy 6th ed., © 2009, by Martini, Timmins and Talitsch, denoted by “Ma”. Images from Human Anatomy, 2nd ed., © 2008 by McKinley & O’Loughlin, denoted by “Mc”. Images from Neuroscience: Exploring the Brain by Bear, et al., 3rd Edition, © 2006. Sound Waves • when an object moves towards a patch of air, molecules in the air are compressed, i.e. made more dense • when an object moves away from a patch of air, molecules in the air are rarefied, i.e. made less dense • vibrating objects alternately rarefy and compress the air around it, creating alternating patches of compressed / rarefied molecules travelling through space • these sound waves and move away from the source at the speed of sound • when this mechanical energy meets the auditory apparatus, we “perceive sound” Rarefied air Air pressure Compressed air Distance One cycle Pitch of Sound • the frequency of sound waves is the number of compressed or rarefies patches of air that pass by our ears each second, expressed in cycles per second or Hz • pitch is proportional to the frequency of the changes in air pressure per unit time • our auditory system responds to sound waves over the range of 20 Hz – 20 kHz • range decreases with age and with damage to the inner ear due to loud noise • infrasound: sound waves below 20 Hz • ultrasound: sound waves above 20 kHz Lower frequency / pitch i.e. not detectable by our hearing apparatus From Neuroscience: Exploring the Brain, 3e, by Bear, et al., © 2006. Air pressure Higher frequency / pitch Distance One cycle Distance One cycle Intensity of Sound • intensity is the difference in pressure between the compressed and rarefied patches of air and determines the loudness • intensity is measured in decibels or db, a logarithmic scale that requires a ten-fold increase in intensity for a perceived doubling of “loudness” • this graphic illustrates two sounds at the same pitch, but differing in their intensity, and therefore perceived loudness From Neuroscience: Exploring the Brain, 3e, by Bear, et al., © 2006. Higher intensity / loudness Air pressure Lower intensity / loudness Distance One cycle From Neuroscence: Exploring the Brain by Bear, et al., 3e, © 2006. Distance One cycle The Ear: An Overview EXTERNAL EAR MIDDLE EAR AIR-FILLED SPACES GATHER, AMPLIFY, TRANSFER SOUND WAVES TO: Ma18.9 INNER EAR FLUID-FILLED SPACE CONTAINS SENSE ORGANS OF: 1. BALANCE 2. HEARING 5 A. pinna (auricle) gathers sound waves The External Ear EXTERNAL EAR – contributes to sound localization MIDDLE EAR Elastic cartilages – supported by elastic cartilage – the lobule (lobe) lacks cartilage – directs sound waves into the: B. external auditory canal – wall of lateral 1/3 cartilagenous, medial 2/3 bony A B C – lining skin contains ceruminous & sebaceous glands which produce cerumen (earwax) C. tympanic membrane (ear drum) – separates external, middle ear Petrous part of temporal bone Ma18.9 – vibration of air (sound waves) causes vibration of the “ear drum” – thin skin externally, mucous membrane internally why mucous membrane internally? 6 The Middle Ear MIDDLE EAR INNER EAR A. consists of the tympanic cavity, an airfilled space within the petrous temporal bone containing auditory ossicles B. tympanic cavity opens to the mastoid air cells by many variable channels; AUDITORY OSSICLES middle ear infections can lead to mastoiditis C. opens to the nasopharynx by the auditory (pharyngotympanic, Eustachian) tube ∴lined by mucus membrane EXTERNAL AUDITORY CANAL – walls 1/3 bone, 2/3 cartilagenous A – latter closed at rest, opens with muscular contraction during swallowing, yawning… B C TO NASOPHARYNX Ma18.9 – equalizes P in middle ear with atmosphere – ∴ prevents distortion of tympanic membrane with changes in atmospheric pressure – route for spread of infection from pharynx to middle ear “otitis media” 7 The Auditory Ossicles FOOTPLATE OF STAPES • span tympanic cavity from the tympanic membrane to the oval window • oval window separates the air-filled middle ear and the fluid-filled inner ear • footplate of stapes fits in oval window, sealed and secured by annular ligament Malleus Incus TYMPANIC MEMBRANE Ma18.10 • vibration of tympanic membrane moves the auditory ossicles • mvt of ossicles ↑ force of sound waves, transfers them to the oval window Stapes TYMPANIC CAVITY • stapes moves like a piston in the oval window, initiating pressure waves in the fluid of the inner ear 8 MIDDLE EAR INNER EAR VIII VESTIBULAR DIVISION VIII COCHLEAR DIVISION CN VIII VESTIBULAR APPARATUS COCHLEA The Inner Ear I • contains 2 special sense organs innervated by the vestibulocochlear N. (CN VIII): A. the vestibular apparatus which senses “equilibrium”, or the position & acceleration of the head in space • served by the vestibular division of the vestibulocochlear N. (CN VIII) B. the cochlea which senses sound • served by the cochlear division of the vestibulocochlear N. (CN VIII) Ma 18.9 9 The Bony Labyrinth Cast of right labyrinth (lateral aspect) Frontal section through left temporal bone (posterior aspect) Rohen, J. W. Anatomy: A Photographic Atlas, 8e, © 2015, Wolters Kluwer Health. Cast of right labyrinth (posteromedial aspect) 10 Bony labyrinth Cochlea Membranous labyrinth Cochlear duct Vestibule Utricle & saccule Semicircular Semicircular canals ducts Senses: The Inner Ear II Hearing • bony labyrinth: interconnected, fluid-filled spaces within the petrous temporal bone Position, linear acceleration of head Angular acceleration of head Vestibular apparatus SEMICIRCULAR CANALS VESTIBULE: UTRICLE SACCULE COCHLEA SEMICIRCULAR DUCTS Ma18.12 – 3 subdivisions: the cochlea, vestibule & semicircular canals • membranous labyrinth: suspended within the bony labyrinth, also fluid-filled – consists of the cochlear duct, utricle & saccule and semicircular ducts – the sense organs of hearing and balance are epithelial specializations of the membranous labyrinth, and are illustrated in purple KEY COCHLEAR DUCT Membranous labyrinth Bony labyrinth 11 The Cochlea • a subdivision of the bony labyrinth • a coiled, tubular space; walls made of compact bone • spirals around a peg of bone, the modiolus which contains Ns & BVs VIII B C B B A C A C Ma18.17 • the membranous cochlear duct is suspended within the cochlea; its contained space is the scala media (B) A • the cochlear duct spans the cochlea from the bony spiral lamina (SL) to the opposite wall; B A • cochlear duct subdivides the cochlear space into the scala vestibuli (A) and the scala B tympani (C) C A C • cochlear duct ends at the apex of the cochlea • scalae vestibuli and tympani continuous at SL the apex of the cochlea via the helicotrema Ns, BVs 12 The Spiral Organ Ma18.17 • basilar membrane (BM) separates the scala media from the scala tympani SCALA MEDIA • sense organ of hearing, the spiral organ is located on the basilar membrane SCALA VESTIBULI • spiral organ contains hair cells and support cells BM SPIRAL GANGLION SCALA TYMPANI • afferent nerve fibres monitor hair cells • cell bodies of sensory afferents in the spiral ganglion SPIRAL ORGAN • axons join cochlear branch of CN VIII within modiolus HAIR CELLS AFFERENT NERVE FIBRES SUPPORT CELLS BASILAR MEMBRANE 13 + + Hair Cells + + • sensory cells of inner ear • transduce sound energy into APs in the afferent nerves Reticular lamina Depolarization • “hairs” are stereocilia on the apices of the hair cells Voltage-gated calcium channel Hair cell • membrane potential of hair cells determines rate of NT release from basal surface of hair cells • bending of hairs in one direction opens channels, depolarizes hair cell, ↑ NT release • bending of hairs in the other Vesicle filled direction closes channels, with excitatory hyperpolarizes hair cell, ↓ NT neurotransmitter release Spiral ganglion neurite B11.15 • pulsatile release of NT causes bursts of APs in afferent Ns 14 The Cochlea Uncoiled • the oval window / footplate of stapes are at the base of the scala vestibuli • the membrane-enclosed round window is at the base of the scala tympani • with vibration of the tympanic membrane, the stapes moves in and out of the oval window like a piston • pressure waves initiated in the fluid of the scala vestibule are relieved at the round window • the membranous labyrinth / basilar membrane vibrates in response to these pressure waves • hairs on the hair cells vibrate → pulsatile release of NT A Helicotrema Malleus Incus Stapes Oval window Scala vestibuli Scala media Tympanic membrane Scala tympani Round window with membrane B C 33 mm Tympanic membrane Round window Oval window Compression Rarefaction From Principles of Neural Science, 5e, by Kandel, et al., © 2014 by McGraw-Hill. Basilar membrane The Perception of Pitch LOW-FREQUENCY SOUNDS SCALA MEDIA SCALA VESTIBULI HELIOCOTREMA BASILAR MEMBRANE SCALA TYMPANI MEDIUM-FREQUENCY SOUNDS • BM is narrow and stiff at it’s base, wide and floppy at its apex • this is because the size of the spiral lamina decreases from base to apex HIGH-FREQUENCY SOUNDS Base Apex BASILAR MEMBRANE HIGH FREQ Mc19.29 MED FREQ Hz Hz 20,000 1500 (high notes) LOW FREQ Hz 500 • the BM is optimally displaced at a certain point along its length by a specific frequency; this activates the population of neurons monitoring the spiral organ at that point • thus, the spiral organ is organized “tonotopically” Hz • 20 (low notes) pitch coded by the pop’n of neurons firing in response to a given sound 16 Auditory Pathways Auditory pathways project bilaterally to primary auditory cortex. Auditory cortex (temporal lobe) Low-frequency sounds What is the clinical implication of this wrt a unilateral lesion? Low-frequency sounds Thalamus High-frequency sounds Medial geniculate nucleus (thalamus) Inferior colliculus High-frequency sounds Cochlear nuclei Ma18.18 Cochlear N. 17 Deafness Conduction Deafness • involves the external or middle ear • e.g. “swimmer’s ear”, otosclerosis • inner ear is intact, so hearing aids are effective Sensorineural Deafness • involves the inner ear or cochlear nerve • e.g. destruction of hair cells by exposure to loud sounds, ototoxic drugs, lesion of CN VIII 18 Bony labyrinth Cochlea Membranous labyrinth Cochlear duct Vestibule Utricle & saccule Semicircular Semicircular canals ducts Senses: The Inner Ear II Hearing • bony labyrinth: interconnected spaces within the petrous temporal bone Position, linear acceleration of head Angular acceleration of head Vestibular apparatus SEMICIRCULAR CANALS VESTIBULE: UTRICLE SACCULE COCHLEA SEMICIRCULAR DUCTS Ma18.12 – 3 subdivisions: the cochlea, vestibule & semicircular canals • membranous labyrinth: suspended within the bony labyrinth – consists of the cochlear duct, utricle & saccule and semicircular ducts – the sense organs of hearing and balance are epithelial specializations of the membranous labyrinth, and are illustrated in purple KEY COCHLEAR DUCT Membranous labyrinth Bony labyrinth 19 The Vestibular Apparatus I: the Otolith Organs • are the utricle and the saccule • each contains a macula, an area of epithelium with hair cells and support cells • tips of stereocilia embedded in a gelatinous matrix, the otolith membrane • otolith membrane contains calcium carbonate crystals called otoconia (statoconia) • therefore, the otolith membrane has a higher density than the surrounding fluid Otolith membrane Gelatinous material otoconia Hair cells Nerve fibers Ma18.15 20 The Otolith Organs: The Utricle & Saccule • fxn: detect the static position and linear acceleration of the head • macula of utricle oriented horizontally; macula of saccule oriented vertically • tilting of head (see below) causes otolith membrane to shift due to gravitational pull; this distorts stereocilia, alters NT release • linear acceleration of head in plane of macula (not shown) causes otolith membrane to shift due to inertia; this also distorts stereocilia, alters NT release • popn of hair cells activated, 1° sensory neurons firing APs codes for head position, linear acceleration Head in Neutral Position Ma18.15 Gravity Head Tilted Posteriorly Receptor output increases Gravity Otolith moves “downhill,” distorting hair cell processes 21 The Vestibular Apparatus II: The Semicircular Ducts Semicircular ducts Ampulla • three arranged at right angles to each other • each has an ampulla containing the crista ampullaris with hair cells Utricle Saccule • tips of hair cells embedded in the gelatinous cupola • no otoconia, so density of cupola is the same as the surrounding fluid • ∴ cupola moves with fluid Ampulla filled with endolymph Cupula Anterior semicircular duct Hair cells Lateral semicircular duct Crista Supporting cells Ma18.13 Sensory nerve Ma18.14 Posterior semicircular duct 22 The Semicircular Ducts Detect Rotation • fxn: detect rotation of the head • with rotation of the head in the plane of a given duct, endolymph and cupola lag behind due to inertia, bending the hairs • this alters AP traffic along vestibular division of CN VIII • since at right angles to each other, each optimized to detect mvt around a different axis Direction of relative endolymph movement Direction of duct rotation Direction of duct rotation Semicircular duct Ampulla Ma18.13 Cupola At rest 23 Pathways for Equilibrium Sensations Information used to coordinate head mvt with eye, neck, trunk and limb mvt. To thalamus and cerebral cortex for conscious sensation Vestibular ganglion Semicircular canals N III Vestibulo -ocular reflex Vestibular branch N IV Vestibular nuclei N VI To cerebellum Vestibule Cochlear branch Rotate your owl Ma18.16 N XI Vestibulocolic reflex Vestibulocochlear nerve (N VIII) Vestibulospinal tract 24 The End Rohen, J. W. Anatomy: A Photographic Atlas, 8e, © 2015, Wolters Kluwer Health. 25