Parts of the Ear

§  The external ear

consists of the auricle and the external auditory canal, which leads to the tympanic membrane.

§  The middle ear

consists of the tympanic membrane and the tympanic cavity

connected to the nasopharynx through the auditory (Eustachian) tube.

§  The inner ear

consists of the bony labyrinth, which encloses the cochlea and the semicircular canals.

connected to the cranial cavity through the internal acoustic meatus.

§  Sound waves are collected by the auricle and conducted through the middle ear to the inner ear.

§  Specialized receptors in the inner ear convert the mechanical energy of sound (mechanical)à nervous impulses (electrical) à interpreted by the brain.

§  In the inner ear, specialized receptors convert mechanical energy produced by gravity à nervous impulses, providing information about the position and movement of the head.

Auricle

§  The most visible component of the external ear is the auricle.

§  The shape of the auricle is primarily determined by an irregular sheet of elastic cartilage. (cart has poor circulation)

§  The cartilage is covered by skin and a small amount of fat.

§  The auricle has several characteristic folds and spaces, including

the helix

scaphoid fossa

tragus

concha

antihelix

antitragus

lobule- lobule is the only part of the auricle unsupported by cartilage.

§ Clinical Correlates

Bleeding within the auricle from trauma may produce an auricular hematoma. If untreated, the hematoma could interrupt blood supply to       the auricular cartilage, resulting in fibrosis. This is the cause of “cauliflower ears” that occurs in wrestlers and boxers. Trauma from cosmetic procedures that pierce the skin may lead to infection (otitis externa). The auricles of swimmers who do not dry the meatus can            also become infected. Pain is felt in the auricle, and is increased by pulling.

External Auditory Canal

§  The external auditory canal (external acoustic meatus) is a canal from the auricle to the tympanic membrane.

§  It forms from the concha of the auricle, beneath the tragus.

§  The outer third of the canal is formed by elastic cartilage. It is lined by skin that contains hairs, as well as sebaceous and ceruminous glands (wax- catch big particles of dust)

§  The inner two-thirds of the canal is formed by bone. It is also lined by skin, but does not contain hairs, nor the specialized glands.

§  Landmarks on the skull in the area of the external acoustic meatus include

the tympanic part of temporal bone

the tympanomastoid fissure

the petrotympanic fissure (anterior, chorda tympani exits to join with lingual n.)

the suprameatal spine.

Muscles in the Auricle

§  There are a few small muscles in the auricle.

§  The intrinsic muscles attach one part of the auricular cartilage to another.

§  The extrinsic muscles attach the auricle to the cranium. These muscles are vestigial, and most people lack voluntary control of them. (innervated by facial n.)

Nerve Supply to the Auricle

§  All muscles in the auricle are innervated by the facial nerve (VII).

§  Sensory innervation is divided along a vertical plane approximately midway through the canal.

o   The anterior portion of the auricle and external auditory canal are innervated by the auriculotemporal nerve (CN V3-Trigeminial n.)

o   The posterior portion of the auricle is innervated by the great auricular nerve  (C2, C3, C4 – branch of cervical plexus).  Cervical nn

o   The posterior portion of external auditory canal is innervated by the great auricular nerve and the auricular branch of the vagus nerve (CN X).

Vascular Supply to the Auricle

§  The arterial supply to the auricle and external auditory canal is from branches of the external carotid, the superficial temporal and posterior auricular arteries.

§  The external canal also receives blood from the deep auricular artery, a branch of the maxillary artery.

§  Venous drainage from the auricle is through superficial temporal and posterior auricular veins, as well as the mastoid emissary veins.

§  The external auditory canal drains through branches of the maxillary and external jugular veins, as well as the pterygoid venous plexus.

Tympanic Membrane

§  The tympanic membrane has a trilaminar structure.

§  The outside surface is covered by skin.

§  Underneath is a layer of fibrous tissue; these fibers have radial, circular, or parabolic orientations. stiff

§  The inside surface is covered by mucous membrane.

§  The tympanic membrane is not a flat sheet; rather it is more like a shallow cone, the apex of which is called the umbo.

§  The tympanic membrane has two distinct regions.

The pars flaccida is small and located superiorly.  (no fibrous middle, not as stiff)

The pars tensa occupies most of the surface area. (stiff)

§  The fibrous annulus attaches the membrane to the temporal bone.

§  Clinical Correlates:

The external acoustic meatus and tympanic membrane are examined with an otoscope. This instrument is essentially a conically shaped magnifying glass with a light at the end. The external meatus is generally slightly curved. To provide the best access for the otoscope, the auricle is generally pulled posterosuperiorly before insertion. The light from the otoscope will reflect in a “cone of light” from the umbo through the anterior inferior quadrant. This reflection is usually called the “light reflex.” Shadows of the ossicles in the middle ear can be seen through the membrane. The manubrium of the malleus (“handle”) is visible particularly at its inferior attachment at the umbo. The long process of incus may also be seen posterior to the handle.

Perforation of the tympanic membrane may result from middle ear infection, trauma or excessive pressure. The membrane is also pierced to drain the middle ear (myringotomy). Incisions are generally made poseroinferiorly to minimize the risk of injury to the chorda tympani nerve and the ossicles. The inferior half of the membrane is also less vascular than the superior half, minimizing trauma to the membrane.          Small perforations of the membrane will heal spontaneously, but larger lesions may require surgical repair.

Tympanic Cavity

§  The tympanic cavity proper is the space deep to the tympanic membrane.

§  The region of the cavity above the level of the tympanic membrane is the epitympanic recess.

§  Posteriorly, the epitympanic recess is continuous with the mastoid air cells.

§   Anteriorly, the tympanic cavity gives rise to the auditory tube, which is continuous with the nasopharynx.

§  The following terms have the same meaning:

mastoid air cells hypotympanum
middle ear cavity mesotympanum
auditory tube epitympanum

Boundaries of the Middle Ear

§  Although its shape is in fact irregular, it is convenient to think of the tympanic cavity as a cube.

§  The roof of the cavity

Thin plate of bone called the tegmen tympani.

Superior to the roof is the middle cranial fossa.

§  The floor

Thin plate of bone that separates the cavity from the bulb of the jugular vein

it may be replaced in part by fibrous tissue.

§  Anteriorly

the styloid process is superficial to the floor of the tympanic cavity.

The anterior wall is a thin plate of bone that separates the tympanic cavity from the internal carotid artery.

There are two openings; one leads to the auditory tube, one to the canal for the tensor tympani muscle.

§  The posterior  wall

separates the tympanic cavity from the mastoid air cells.

The aditus to the mastoid antrum is in the superior part of the wall.

Beneath the aditus is a small conical projection called the pyramid which conducts the tendon for the stapedius muscle.

The belly of the muscle remains embedded in its canal.

Also related to the posterior wall is the mastoid portion of the facial canal.

The chorda tympani emerges through its canal, crossing between the malleus and incus.

§  The lateral wall

formed primarily by the tympanic membrane.

§  Superiorly

a small part of the squamous part of the temporal bone contributes as well.

§  The most prominent feature of the medial wall

is a large rounded projection called the promontory.: It is formed in part by the underlying cochlea.

§  There are two membrane covered openings to the bony labyrinth of the inner ear:

the oval window (fenestra vestibuli) is superior to the promontory

the round window (fenestra cochleae) is inferior to the promontory.

§  The oval window is the site of attachment of the footpad of the stapes.

§  A bony shelf supports the tensor tympani muscle, and ends in an upwardly curved pulley, called the processus cochleariformis.

§  A rounded ridge called the prominence of the facial canal runs superior to the promontory.

§  The geniculate ganglion is at the base of this canal.

§  Superior to this is the prominence of the lateral semicircular canal.

§  The mastoid portion of temporal bone has several features related to the middle ear cavity.

The aditus to the antrum is the opening to the mastoid antrum from the tympanic cavity.

The antrum is the air cell into which the aditus opens, and it is usually the largest of the air cells.

§  The sigmoid sinus in the posterior cranial fossa forms a ridge in the cavity of the mastoid process.

§  Tympanic plexus

§  Clinical Correlates:

One of the main causes of earache, especially in children, is otitis media. The middle ear is susceptible to infection through the auditory tube, which opens into the nasopharynx. In children, the auditory tube is short and straight, facilitating the spread of infection. In the otoscope, the sign of otitis media is a bulging red tympanic membrane. Occasionally, the fluid level of accumulated pus is visible. If untreated, hearing may be impaired due to scarring of the ossicles, restricting their ability to conduct sound to the inner ear. Tinnitus&HearingCenter diagnoses any of such conditions at an early stage and offers expert treatment.

Middle ear infections can spread into the mastoid antrum and air cells. From there, infections may spread superiorly into the middle cranial fossa through the petrosquamous fissure and infect the bone of the tegmen tympani. Surgical access to the mastoid process is generally through the posterior wall of the external acoustic meatus. A surgical approach to the middle ear is through the mastoid air cells. Care must be taken, however, not to damage CN VII as it traverses the facial canal.

Ossicles and Attached Muscles

§  The ossicles transmit vibrations from the tympanic membrane to the inner ear.

§  These three bones are tiny, but have identifiable regions.

§  The malleus is composed of a head, neck, and manubrium.

§  The incus is composed of a long process, a short process, and body.

§  The stapes is composed of a head, two crura, and a footplate.

§  Movement of the ossicles results in a hinged movement of the stapes, depressing the oval window.

§  Two small muscles dampen the conduction of vibrations upon contraction.

§  Prevent foot plate from damaging (puncture) surface of oval window

§  The stapedius muscle attaches to the head of the stapes.

§  The tensor tympani attaches to the manubrium of the malleus.

§ Clinical Correlate:

Damage to the nerves innervating the tensor tympani or stapedius muscles results in paralysis of the muscles. Consequently, the oval window membrane vibrates too strongly in response to loud tones, a condition called hyperacusis. Because the two muscles have different innervations (tensor tympani – V3, stapedius – VII), there is always some protection against loud tones.

Nerve Supply to the Middle Ear

§  Motor innervation of the tensor tympani is from CN V3.

§  The stapedius is innervated by CN VII.

§  Sensory innervation for the lining of the tympanic cavity arises from the tympanic plexus.

The plexus is formed from the tympanic nerve, a branch of CN IX.

The plexus also gives rise to the lesser petrosal nerve.

§  The chorda tympani passes through the tympanic cavity, but does not innervate it.

The nerve passes between the incus and malleus, and exits the cranium through the petrotympanic fissure.

Blood Supply to the Middle Ear

§  The anterior tympanic artery, a branch of the maxillary, supplies the anterior part of the cavity.

§  The stylomastoid artery, a branch of the posterior auricular, supplies the posterior part of the cavity and the mastoid air cells.

Auditory Tube

§  Anteriorly, medially, posteriorly

§  The auditory tube provides an opening for equalization of air pressure across the tympanic membrane.

§  Like the external auditory canal, there is both an osseous part (posterior one-third) and a cartilaginous part (anterior two-thirds).

§  Two muscles of the soft palate attach to the auditory tube, the tensor palati and, to a lesser extent, the levator palati.

When these muscles contract during swallowing, they open the tube.

§ Clinical Correlate:

Normally the walls of the tube are closely apposed, and it takes time for air pressure to equalize across the membrane. Consequently, relatively       sudden changes in external pressure (such as ascending a mountain in a car, or descent in an airplane) can result in a pressure differential    across the tympanic membrane. This results in limited movement of the membrane, difficulty hearing and a sensation that the ears are plugged. To liminate this sensation, the tube is opened by actions of the tensor and levator palati muscles, which are activated by yawning or swallowing. The most common complaint of divers is middle ear barotrauma. Inability to equalize pressure causes painful stress to the tympanic membrane. At deeper depths, the membrane may rupture. Subsequent exposure of the middle ear to cold water can induce dizziness through stimulation of the inner ear.

The auditory tube is a common pathway for the spread of infection from the nasopharynx to the middle ear, resulting in otitis media. Over time, residual air may be absorbed into the blood vessels of the mucosa lining the tympanic cavity, and a pressure differential may develop. Mucosal inflammation could impede opening of the auditory tube, preventing equalization and impeding hearing.

Bony Labyrinth

§  The bony labyrinth lies in the petrous portion of temporal bone.

§  The vestibule is the central part of the bony labyrinth.

§  The oval window (fenestra vestibuli) and round window (fenestra cochleae) also open into the bony labyrinth.

§  The vestibular aqueduct is a canal that opens into the posterior cranial fossa.

§  Three semicircular canals, anterior, posterior and lateral, open into the posterior part of the vestibule.

§  Each canal has a dilatation at one end, called the ampulla.

§  There are only 5 openings into the vestibule because the anterior and posterior canals join, forming the crus commune.

§  The cochlea opens into the anterior part of the vestibule.

§  The cochlear canal spirals for 2½ turns, ending in the cupula.

§  The cone-shaped central region is called the modiolus.

§  There are numerous small canals through the modiolus transmitting branches of the cochlear nerve.

§  Projecting from the modiolus is the spiral lamina, which partially separates the cochlear canal into vestibular and tympanic compartments.

§  The vestibular (scala vestibuli) and tympanic (scala tympani) compartments communicate at the apex of the spiral through a small opening called the helicotrema.

§  The canal terminates at the round window and at the cochlear aqueduct.

§  The aqueduct opens into the posterior cranial fossa.

Membranous Labyrinth

§  The membranous labyrinth follows the bony labyrinth, but is separated from it by perilymph, a CSF-like fluid.

§  The membranous labyrinth contains endolymph, which resembles an intracellular fluid.

§  In the vestibule are two sacs which contain sensory receptors, called maculae, specialized for detecting steady-state orientation of the head with respect to gravity.

§  The utricle is the largest of the two sacs. It is located posteriorly and superiorly in the vestibule.

§  The saccule is located anteriorly and inferiorly. The utricle and saccule give rise to short ducts that join to form the endolymphatic duct.

§  The semicircular ducts follow the semicircular canals.

§  Sensory receptors located in the ampullary crests are specialized for detecting changes in acceleration of the head.

§  The brain uses the output from the three canals, which are in the three unit planes, to infer the direction of head movement.

§  The endolymphatic duct arises from the saccule and travels through the vestibular aqueduct.

§  As the aqueduct opens into the posterior cranial fossa, the duct expands into the endolymphatic sac.

§  The opening is posterolateral to the internal auditory meatus.

§  The cochlear duct arises from the ductus reuniens, which itself arises from the saccule.

§  It follows the spiral, forming a third compartment, scala media, terminating at the apex of the cochlea.

§  The cochlear duct contains the spiral organ of Corti, which is the sensory organ for hearing.

§  The cochlear duct gives rise to the cochlear canaliculus, which communicates with the posterior cranial fossa through the cochlear aqueduct.

Nerve Supply to the Inner Ear

§  The vestibulocochlear nerve supplies the inner ear. It innervates the utricle, saccule and semicircular ducts.

§  The cochlear nerve innervates the cochlear duct.

§  CN VII passes through the inner ear, but does not supply it.

§  The sensory root, called the nervus intermedius, may sometimes be distinguished from the motor root.

§  The facial nerve travels in its own canal through the inner ear. Just medial to the tympanic cavity, the facial canal turns posteriorly.

§  At this point the geniculate ganglion is located.

§  The canal then travels inferiorly behind the posterior wall of the tympanic cavity, exiting the stylomastoid foramen.

§  There is a characteristic relationship among the nerves entering the internal acoustic meatus.

§  The upper left quadrant is CN VII.

§  The lower left quadrant is the cochlear division of CN VIII.

§  The upper right is the superior vestibular division of CN VIII, and the lower right is the inferior division.

§  Clinical Correlates:

It is important to distinguish between hearing deficits due to injury of inner ear and middle ear structures. The latter (called conductive hearing loss) can more successfully be remediated, e.g. by cleaning out earwax or replacing an ossicle. The former (called sensorineural hearing loss) is due to a deficit in the cochear duct or central neural structures. High tone deafness is one form of sensorineural hearing loss that    results from persistent exposure to excessively loud sounds (rock musicians commonly suffer this deficit). The deficit is due to degenerative changes in the spiral organ of the cochlear duct. Sensorineural deficits are difficult to repair, although significant advances have been made in development of prosthetic devices that directly stimulate the cochear nerve.

Damage to the semicircular canals results in a sensation of vertigo, which is a hallucination of movement such as spinning or swaying. The lack of information from vestibular organs does not by itself produce vertigo. Rather, it is the confused processing of all position information (including visual and kinesthetic sensation) within the brain that produces the symptoms.

Ménière syndrome can induce both hearing loss and vertigo, as well as tinnitus (persistent buzzing or ringing sensation). This syndrome is         caused by blockage of the cochlear aqueduct and the consequent increase in volume of endolymph. The increased pressure affects the ability of maculae, ampulae and the spiral organ to function properly.

Acoustic neuroma is a slow growing tumor of Schwann cells (i.e. those cells responsible for the “white matter”) that usually occurs at the base of the brain where the CN VII and VIII emerge. The tumor generally begins within the vestibular component of CN VIII, although loss of hearing is generally the first sign of tumor growth. Thus, while the vestibular and cochlear components of CN VIII are functionally independent, peripheral lesions to either component will generally involve both.

Vascular Supply to the Inner Ear

§  The inner ear is supplied by the labyrinthine vessels.

§  The labyrinthine artery is a branch of the anterior inferior cerebellar artery. (Netter’s Atlas shows it originating separately from the basilar artery.)

§  The labyrinthine vein drains into the superior petrosal sinus or the transverse sinus.