Gross Anatomy: Ear Notes 2023 PDF

Gross Anatomy: Ear Page 1 of 14 Dr. Paul Walker Session Objectives By the end of this session, students will be able to accurately: 1. Discuss the functional anatomy of the external ear and apply anatomical knowledge to...

Gross Anatomy: Ear Page 1 of 14 Dr. Paul Walker Session Objectives By the end of this session, students will be able to accurately: 1. Discuss the functional anatomy of the external ear and apply anatomical knowledge to clinical problems of the external ear. 2. Discuss the functional anatomy of the middle ear and apply anatomical knowledge to clinical problems of the middle ear. 3. Discuss the anatomy of the internal ear and apply anatomical knowledge to clinical problems of the internal ear. Supplemental Reading Gray’s Anatomy for Students, 4th Ed (2020) Drake, Vogl, Mitchell (Elsevier). Ch 8 Scroll to section on Ear. Gross Anatomy: Ear Page 2 of 14 Dr. Paul Walker Session Outline I. Anatomy of the External Ear A. Auricle B. External Acoustic Meatus C. Sensory Innervation D. Tympanic Membrane (TM) E. Function of the External Ear F. Clinical Application of External Ear Anatomy 1. External Ear Disorders 2. Otoscopic TM Assessment 3. TM injury & Myringotomy II. Anatomy of the Middle Ear A. Bony Tympanic Cavity B. Boundaries/Walls C. Structural Relationships D. Functions of the Middle Ear 1. Functional Anatomy of the Ear Ossicles & Middle Ear Musculature 2. Functional Anatomy of the Auditory Tube E. Clinical Applications of Middle Ear Anatomy 1. Otosclerosis 2. Otitis Media 3. CN VII Paralysis 4. Tonic Tensor Tympani Syndrome III. Anatomy of the Internal Ear A. Bony & Membranous Labyrinths B. Anatomy of the Cochlea C. Function in Hearing: Fluid Movement & Transfer of Signal D. Clinical Applications of Internal Ear Anatomy 1. Sensorineuronal Deafness 2. Imaging Approaches Gross Anatomy: Ear Page 3 of 14 Dr. Paul Walker Overview The ear provides the mechanism for sound transmission to the CNS, and houses the apparatus for balance & equilibrium. This lecture focuses primarily on the anatomy of the external and middle ear compartments as related to function in sound transmission and application to clinical disorders. There will also be a brief overview of the internal ear containing the vestibular and cochlear structures. More details will be provided in histology lecture and lab for the inner ear and the auditory and vestibular neural pathways will be learned in neuroscience lecture and lab. Ear Compartments External Middle Internal Fig 1 (Gray’s Anatomy for Students) Summary Relating Anatomy to Function for Sound Transmission External Ear- collector/channel for sound waves directed to the tympanic membrane Middle Ear- signal transduced through air-filled chamber amplified by a chain of 3 bones Internal Ear- signal propagated through fluid-filled chambers and converted to electrical transmission sent to CNS via CN VIII (vestibulocochlear nerve). Fig 2 (Gray’s Anatomy for Students) Conductive hearing loss can result from any abnormality of the external or middle ear cavities that prevents the transmission of sound information to the internal ear. Sensorineural hearing loss results from damage to structures of the internal ear or the auditory CNS pathways learned in the neuroscience part of this course. Gross Anatomy: Ear Page 4 of 14 Dr. Paul Walker I. Anatomy of the External Ear Includes auricle (pinna) and external acoustic meatus (EAM). Bordered medially by the tympanic membrane (ear drum)- divides external vs. middle ear. A. Auricle Fig 3 (Gray’s Anatomy for Students) Single elastic cartilage covered on both sides by hairy skin (Fig 3). The cartilage continues medially with the cartilage of the EAM. The ear lobe (lobule) contains no cartilage but has small blood vessels making it a potential site for blood sampling. B. External Acoustic Meatus Fig 4 (Gray’s Anatomy for Students) Extends from the deepest part of the concha to the tympanic membrane (about 1 inch in adults) (Fig 4). Outer 1/3 is cartilaginous and inner 2/3 is a bony tunnel (except in infants where the entire EAM is cartilaginous). Covered with skin and contains modified sweat glands for the production of ear wax (cerumen). The outer EAM is approximately the diameter of a pencil. Where the EAM becomes bony, the diameter narrows to 5mm and is called the isthmus. The isthmus is where foreign bodies become lodged in children. C. Sensory Innervation of External Ear Fig 5 (Gray’s Anatomy for Students) There is considerable overlap of sensory innervation from the auricle & EAM (Fig 5). Auricle: Great auricular n, lesser occipital n, auriculotemporal nerve (CN V3), auricular branch of facial (CN VII), auricular branch of vagus (CN X). EAM: Auriculotemporal nerve (CN V3), auricular branch of facial (CN VII), auricular branch of vagus (CN X). Gross Anatomy: Ear Page 5 of 14 Dr. Paul Walker D. Tympanic Membrane Fig 6 (Gray’s Anatomy for Students) Thin semitransparent fibrous membrane divides external/middle ear compartments (Fig 6). Covered externally by skin of the EAM and internally by mucous membrane of the middle ear compartment. Sensory innervation of external surface of tympanic membrane is same as EAM. Internal surface sensory innervation is CN IX (as discussed below). Peripheral surface attached to temporal bone by fibrocartilaginous ring. Fig 7 (Gray’s Anatomy for Students) Central concavity is formed by attachment of the malleus handle. This central depression is called the umbo (Figs 6-7). TM is thin and slack superiorly as related to the malleus. The rest of the TM is thick and taut. Its lateral surface is oriented inferior and anterior. E. Function of the External Ear Sound Collection & Channeling to TM The auricle collects sound and the EAM channels it to the tympanic membrane. Sound wave contact causes the TM to vibrate and the signal is propagated across the middle ear cavity by the ossicle chain of bones (reviewed below in middle ear cavity). Auricular muscles can produce movements of the auricle in humans, but are not considered functional in the localization of sound as they are in other animals. These muscles receive their motor innervation from CN VII and are considered ‘other muscles of facial expression’. Can you wiggle your auricles? Gross Anatomy: Ear Page 6 of 14 Dr. Paul Walker F. Clinical Applications of External Ear Anatomy 1. External Ear Disorders Fig 8 (Google) The cartilaginous auricle can be easily damaged by repeated physical contact resulting in an auricular hematoma. Enlargement of the hematoma compromises blood supply and produces fibrosis in the overlying skin that deforms the auricle (Fig 8). This is commonly known as ‘cauliflower ear’ observed in wrestlers, boxers, UFC, etc. Bacterial infections of the EAM are common in swimmers and produce pain in the external ear. This is called otitis externa or ‘swimmer’s ear’. Surfer’s ear is different and occurs in individuals exposed repeatedly to cold water that results in a bony growth of the EAM. This constricts the EAM and reduces hearing. Many cases of conductive hearing loss involving the EAM result from excessive ear wax. 2. Otoscopic TM Assessment Fig 9 (Gray’s Anatomy for Students) The helix of the auricle must be tugged on slightly to permit insertion of the otoscope and to straighten the EAM for viewing the TM. When viewing through an otoscope, the TM is normally translucent and a gray-reddish color. A bright area called the ‘cone of light’ can be seen radiating anteroinferior from the umbo (Figs 7 & 9). This is normal and is called the light reflex (don’t confuse with the ‘pupillary light reflex’). 3. TM Injury & Myringotomy Perforation of the TM occurs from foreign bodies, middle ear infection, excessive pressure, or skull fracture. Severe bleeding or cerebral spinal fluid drainage through the TM and EAM is indicative of skull fracture and illustrates the close proximity of the external and middle ear compartments to the cranial cavity. Minor TM ruptures that occur during middle ear infection heal spontaneously and usually do not require surgical repair. Fig 10 (Google) Surgery to insert ear tubes into the tympanic membrane is a common outpatient procedure (myringotomy) to drain pus from the middle ear cavity due to chronic middle ear infection (otitis media). Incisions are made posteroinferiorly through the TM to avoid the ear ossicles & chorda tympani nerve (Fig 10). Ear tubes usually fall out after several months and the TM heals spontaneously. Gross Anatomy: Ear Page 7 of 14 Dr. Paul Walker B. Anatomy of the Middle Ear Fig 11 (Gray’s Anatomy for Students) 1. Bony Tympanic Cavity (Fig 11) Narrow cavity in the petrous portion of the temporal bone. The middle ear cavity is also called the tympanic cavity. Connected anteriorly to the nasopharynx by the auditory tube. The auditory tube is also called the pharyngotympanic tube or Eustachian tube. Connected posteriorly to air cells of the mastoid bone via the mastoid antrum (and aditus). Fig 12 (Gray’s Anatomy for Students) B. Boundaries/Walls Roof is the tegmen tympani- the bony surface of the temporal bone that separates the tympanic cavity from the middle cranial fossa. Find this on the dry skull and see also Fig 10 above. Floor separates the tympanic cavity from the internal jugular vein (Fig 12). Lateral wall is formed by the tympanic membrane (Fig 12). Medial wall separates the tympanic cavity from the internal ear compartment (Fig 12). Anterior wall is related to the carotid canal (not shown) & auditory tube (Fig 12). Posterior wall is related to the mastoid portion of the temporal bone (not shown). Gross Anatomy: Ear Page 8 of 14 Dr. Paul Walker C. Structural Relationships Medial Wall Structures: Fig 13a (Gray’s Anatomy for Students) Promontory (Fig 13a) formed by the basal portion of the cochlea Tympanic plexus (CN IX) (Fig 13a) Round (cochlear) window (Fig 13a) Oval (vestibular) window (Fig 13a) covered by footplate of stapes Prominence of facial canal containing CN VII Prominence of lateral (horizontal) semicircular canal (of vestibular apparatus) The tympanic nerve (branch of CN IX) passes through a hole in the floor of the tympanic cavity and spreads out across the round promontory as the tympanic plexus. This plexus supplies sensory innervation to the middle ear and also contains preganglionic PANS axons that exit the middle ear cavity as the lesser petrosal nerve that innervates the otic ganglion (reviewed in ANS Head/Neck lecture later). Lateral Wall Structures: Fig 13b (Gray’s Anatomy for Students) tympanic membrane (Fig 13b) malleus, incus (Fig 13b) chorda tympani nerve (Fig 13b) tendon of the tensor tympani muscle attaching to malleus (Fig 13b) Gross Anatomy: Ear Page 9 of 14 Dr. Paul Walker Figs 12 & 13 above show structures related to the posterior and anterior walls of the tympanic cavity. It may be easier to understand these if the tympanic cavity is drawn schematically, as below in Fig 14. The lateral wall containing the TM is removed allowing the other walls of the tympanic cavity to be viewed. Fig 14 (Gray’s Anatomy for Students) Posterior Wall Structures (Fig 14) Aditus to mastoid antrum (entrance to mastoid air cells) located superiorly. Small projection of bone (pyramid) that contains the stapedius muscle. Facial nerve descending in facial canal. Gives off branch to stapedius and chorda tympani n. Anterior Wall Structures (Fig 14) Tensor tympani muscle Opening of the pharyngotympanic tube Exiting lesser petrosal n. and chorda tympani n. D. Function of the Middle Ear Cavity The main function of the middle ear cavity is to provide an environment that regulates the transmission sound waves traveling through the air of the external ear to impact the fluid-filled chambers of the internal ear. To do this, the middle ear cavity serves to amplify the signal and also to regulate its intensity in order to preserve hearing function. Fig 15 (Gray’s Anatomy for Students) Gross Anatomy: Ear Page 10 of 14 Dr. Paul Walker 1. Functional Anatomy of the Ear Ossicles & Middle Ear Musculature Fig 16 (Gray’s Anatomy for Students) The ear ossicles are the malleus, incus, & stapes (Fig 16). They act as a series of levers to amplify (20X) the impact of sound waves. The handle of the malleus is attached to the tympanic membrane and moves when sound waves vibrate the membrane. The movement of the malleus handle is transmitted to its head, which is in contact with the incus. The incus, in turn, contacts the stapes with its base embedded in the oval window (Fig 16). Ear ossicle movements are regulated by 2 tiny muscles of the middle ear cavity (Fig 17 below) that contract to reduce sound amplification promoted by ear ossicle movements. Otherwise some sounds would sound too loud and could damage decibel-sensitive hair cells of the internal ear. This mechanism is called the acoustic (attenuation) reflex. Fig 17 (Gray’s Anatomy for Students) The tensor tympani m. (innervated by CN V3) inserts into the malleus (Fig 17) and tenses the tympanic membrane thus dampening its vibrations. The tensor tympani m. is activated during mastication to dampen chewing sounds. The stapedius m. (innervated by CN VII) inserts into the stapes and prevents. excessive movements at the oval window. As such it is effective at dampening sounds from our environment and is thought to be the primary regulator of the acoustic reflex. Although the acoustic reflex prevents excessive exposure to high-frequency sounds that could be damaging, it is also activated just prior to the contraction of laryngeal and facial muscles used during speech. Otherwise our own voices would sound too loud. There is delay in the initiation of the acoustic reflex, so the damaging effects of explosions and other sudden loud sounds are not suppressed by the acoustic reflex. Gross Anatomy: Ear Page 11 of 14 Dr. Paul Walker 2. Functional Anatomy of the Auditory Tube Fig 18 (Gray’s Anatomy for Students) Equalizes middle ear pressure to atmospheric pressure, which promotes free movements of the ear ossicles Connects middle ear to the nasopharynx, and thus to the external environment Funnel-shaped with its wide end directed toward the nasopharynx (Fig 18) Posterior 1/3 bone, anterior 2/3 cartilage Lined with the same mucosa as middle ear & nasopharynx (Fig 18) Cartilaginous auditory tube closed except during swallowing or yawning when tensor veli palatini and salpingopharyngeus muscles contract to open the auditory tube (‘makes your ears click’). E. Clinical Applications of Middle Ear Anatomy 1. Otosclerosis (otospongiosis)- this disease accounts for half of all cases of conductive hearing loss cases that arise from problems of the middle ear cavity. This disorder results in abnormal temporal bone growth that interferes with ossicle movements. 2. Otitis media- the approximation of pharyngeal (adenoid) and tubal tonsils near the pharyngeal orifice of the auditory tube provides a convenient route for infections to spread into the middle ear. Children are more likely to develop otitis media because the auditory tube is positioned more horizontally, thus infections spread into the middle ear more easily. Mild otitis media causes the mucous membrane of the auditory tube to swell thus closing off its narrow passageway and lowering middle ear cavity air pressure. This prevents free movement of the tympanic membrane and results in diminished hearing during the infection (ears feel clogged). Chronic otitis media can spread to mastoid air cells via the mastoid antrum. Mastoid infections can be treated with antibiotics, but if the infection persists a mastoidectomy is performed through the posterior wall of the EAM. Care must be taken not to damage CN VII in the facial canal. 3. Paralysis of the Stapedius m.- Facial n. damage (e.g. Bell’s palsy) produces hyperacusis because the stapedius m. has lost its motor innervation. As such, the stapes footplate vibrations against the oval window are not dampened and sounds are perceived as excessively loud. 4. Tonic Tensor Tympani Syndrome- a recently identified anxiety disorder that produces spasms of the tensor tympani m. It is associated with tinnitus and CN V irritability (head/face pain), and appears in patients suffering from hyperacusis even when they think about a sound that bothers them. Gross Anatomy: Ear Page 12 of 14 Dr. Paul Walker III. Anatomy of the Internal Ear A. Bony & Membranous Labyrinths Fig 19 Contains the cochlea and vestibular apparatus as well as vestibular and cochlear divisions of CN VIII embedded in the petrous portion of the temporal bone. The internal ear consists of bone cavities called the bony labyrinth. Cavities within the bony labyrinth include- the vestibule, 3 semicircular canals, & the cochlea. These structures are lined with connective tissue mesh and filled with a fluid called perilymph that is similar in chemical composition to extracellular fluid (high Na+, low K+) Suspended within the bony labyrinth is the membranous labyrinth- membranous ducts and sacs that consist of the semicircular ducts, cochlear ducts, utricle, & saccule. The membranous labyrinth is filled with endolymph similar in chemical composition to intracellular fluid (low Na+, high K+). Fig 20 (Gray’s Anatomy for Students) The Vestibular Apparatus is the ‘organ of balance’ and consists of the semicircular canals, utricle and saccule (Fig 20). These structures detect linear and angular acceleration of the head and will be presented in more detail in the CNS course. The Cochlea is the ‘organ of hearing’ (Fig 20) for the detection of sound waves transferred to the internal ear via movements of the auditory ossicles against the oval (vestibular) window. The microanatomy of the cochlea is learned later, but below are some macroanatomy details. Gross Anatomy: Ear Page 13 of 14 Dr. Paul Walker B. Anatomy of the Cochlea Fig 21 (Google) The cochlea projects in an anterior direction from the vestibule. It is a bony structure that twists upon itself 2.5-2.75 turns (Fig 21 right) around a central axis of bone called the modiolus (Fig 22 below). The base of the cochlea faces posteromedially and the apex faces anterolaterally. The base of the modiolus is positioned next to the internal acoustic meatus and is penetrated by the cochlear division of CN VIII. As such the modiolus contains the cochlear division of CN VIII including its sensory ganglia- called spiral ganglia (Fig 22 below). Fig 22 (Gray’s Anatomy for Students) Each turn of the cochlea contains 3 tubular compartments (Fig 22): Scala vestibuli (osseous)- connects to oval window Scala tympani (osseous)- connects to round window Scala media (membranous)- is the cochlear duct The scala vestibuli and scala tympani are continuous with one another at the apex of the cochlea. The connection point is called the helicotrema. C. Function in Hearing: Fluid Movement & Transfer of Signal Fig 23 (Gray’s Anatomy for Students) Stapes compression of the oval window produces perilymph movement in the scala vestibuli. The wave of fluid movement continues through the helicotrema into the scala tympani until it reaches the round window, covered by membrane that bulges into the middle ear cavity (Fig 23 above and Fig 24 next page). Gross Anatomy: Ear Page 14 of 14 Dr. Paul Walker Fig 24 (Gray’s Anatomy for Students) Fluid movement that culminates in the outward bulging of the round window produces ‘traveling wave’ vibration of the basilar membrane located in the cochlear duct (scala media) and detected by hair cells innervated by CN VIII. This is where gross anatomy ends and cochlear microanatomy presented in histology can take you further into the mechanism of hearing. Additional neuroanatomical details of both vestibular and cochlear structure/function are presented in neuroscience lectures. D. Clinical Applications of Internal Ear Anatomy 1. Sensorineural Deafness Sensorineural deafness results from damage to the cochlea or the cochlear division of CN VIII. Causes include repeated exposure to loud noise, certain aminoglycoside antibiotics (e.g. streptomycin), infections (e.g. rubella, mumps), tumors. Infections of the inner ear are called otitis interna or labyrinthitis and usually produce noticeable vestibular symptoms such as vertigo (room spinning) and loss of equilibrium. Tinnitus (ringing, buzzing) can also occur as a result of cochlear damage or injury to CN VIII or the central auditory pathway. Fig 25 (Mayo) Cochlear implants (Fig 25) offer an exciting and evolving technology to restore hearing to patients with moderate to severe hearing loss. An electrode array is positioned in the cochlea next to CN VIII and transmits digitally coded sound detected by a sound processor (transmitter) worn behind the ear. 2. Imaging Approaches Fig 26 (Gray’s Anatomy for Students) The middle and internal ear cavities are best viewed with MRI or CT imaging. Fig 26 shows a high-resolution horizontal (axial) CT image of all 3 ear compartments. The ear ossicles can be assessed in the middle ear as well as the cochlea in the internal ear. Note the extent of the mastoid air cells.