ANATOMY and PHYSIOLOGY of CHRONIC OTITIS MEDIA

Ear

• The ear is divided into 3 main regions: the external ear, which collects soundwaves and channels them inward; the middle ear, which convey soundvibration to the oval window; and the internal ear, which houses thereceptors for hearing and equilibrium

External ( Outer) ear

• The external or outer ear consist of the auricle, external auditory canal, andeardrum. The auricle (pinna) is a flap of elastic cartilage shape like the flaredand of a trumpet and covered by skin. The rim of the auricle is the helix; theinferior portion of the lobule. Ligaments and muscles attach the auricle tothe head.

• The external auditory canal (audit=hearing) is a curved tube about2.5 cm long that lies in the temporal bone and leads from the auricle to theeardrum. The eardrum or tympanic membrane (tympan= a drum) is a thin,semitransparent partition between the external auditory canal and middleear.

• The eardrum is covered by epidermis and lined by simple cuboidalepithelium. Between the epithelial layers is connective tissue composed ofcollagen, elastic fibers, and fibroblasts. Near the exterior, the externalauditory canal contains a few hairs and specialized sebaceous glands calledceruminous glands that secret earwax or cerumen. The combination of hairsand cerumen prevent dust and foreign objectives from entering the ear.

• Cerumen usually dries up and falls out of the ear canal. Some people,however, produce a large amount of cerumen, which can become impactedand can muffle incoming sounds.

Middle ear

• The middle is a small, air-filled cavity in the temporal bone that is lined byepithelium. It is separated from the external ear by the eardrum and fromthe internal ear by a thin bony partition that contains 2 small membrane-covered opening: the oval window and the round window.

• Extending across• the middle ear and attached to it by ligament are the 3

smallest bone in thebody, the auditory ossicles, which are connected by synovial joints.

• The bones, named for their shapes, are the malleus, incus, and stapes-commonlycalled the hammer, anvil and stirrup, respectively. The “handle” of themalleus attaches to the internal surface of the eardrum.

• The head of themalleus articulates with the body of the incus.

• The incus, the middle bone inthe series, articulates with the head of the stapes. The base or footplate ofthe stapes fits into the oval window. Directly below the oval window isanother opening, the round window, which is enclosed by a membrane,called the secondary tympanic membrane.

• Besides the ligaments, 2 tinyskeletal muscles also attach to the ossicles. The tensor tympanimuscle,which is innervated by the mandibular branch of the trigeminal nerve orcranial nerve number 5, limits movement and increases tension on theeardrum to prevent damage to the inner ear from loud noises.

• The stapedious muscle, which is innervated by the facial nerve (cranial nerveVII), is the smallest of all skeletal muscle. By dampening large vibrations ifthe stapes due to loud noises, it protect s the oval window but it alsodecrease the sensitivity of hearing. For this reasons, paralysis of thestapedious muscle is associated with the hyperacusia (abnormally sensitivehearing).

• Because it takes a fraction of a second for the tensor tympanic andstapedious muscles to contract, they can protect the inner from prolongedloud noises, but not from brief ones such as a gun shot. The anterior wall ofthe middle ear contains an opening that leads directly into the auditory(pharyngotympanic) tube, commonly known as the Eustachian tube.

• The auditory tubes, which consist of both bone and hyaline cartilage, connect themiddle ear with the nasopharynx (upper portion of the throat). It is normallyclosed at its medial (pharyngeal) end. During swallowing and yawning, itopens, allowing air to enter or leave the middle ear until the pressure in themiddle equals the atmospheric pressure.

• When the pressures are balanced,the eardrum vibrates freely as sound waves strike it. If the pressure is notequalized, intense pain, hearing impairment, ringing in the ears, and vertigo

• could develop. The auditory also is route whereby pathogens may travel from

• the nose and throat to the middle ear.

Internal (inner) ear

• The internal ear is also called the labyrinth because of its complicate d seriesof canals. Structurally, it consists of 2 main divisions; an outer bony labyrinththat encloses an inner membranous labyrinth.

. The only labyrinth is a seriesof cavities in the temporal bone divided into 3 areas:

• 1. the semi circularcanals,

• 2. the vestibule, both if which contain receptors for equilibrium, and

• 3. The choclea, which contains receptors for hearing.

• The bony labyrinth islined with periosteum and contains perilymph. This fluid, which is chemicallysimilar to cerebrospinal fluids, sorrounds the membranous labyrinth, a seriesof sacs and tubes inside the bony labyrinth and having the same generalform. The membranous labyrinth is lined by epithelium and contains

• endolymph.

• The level of potassium ions in endolymph is unusually high foran extracellular fluid, and potassium ions play a role in the generation ofauditory signals. The vestibule is the oval central portion of the bonylabyrinth. The membranous labyrinth in the vitibule consists if 2 sacs calledthe utricle (little bag) and the saccule (little sac), which are connected by asmall duct.

• Projecting superiorly and posteriorly from the vestibule is the 3bony semicircular canals, each of which lies at approximately right angles tothe other 2. Based on their positions, they are named the anterior, posterior,and lateral semi circular canals.

• The anterior and posterior semi circularcanals are vertically oriented; the lateral one is horizontally oriented. At oneend of each canal is a swollen enlargement called the ampulla (saclike duct).The portions of the membranous labyrinth that lie inside the bony semicircular canals are called the semi circular ducts.

• These structurescommunicate with the utricle of the vestibule. The vestibular branch of thevestibulochoclear nerve (cranial nerve VIII) consists of ampullary, utricular,and saccular nervers. These nerves contain both first order sensory neurons and motor neurons that synapse with receptors for equilibrium.

• The firstorder sensory neurons carry sensory information from the receptors, and themotor neurons carry feedback signals to the receptors, apparently to modifytheir sensitivity. Cell bodies Of the sensory neurons are located in thevestibular ganglia. Anterior to the vestibule is the choclea (snail shape) abony spiral canal that resembles a snail’s shell and makes almost 3 turnsaround a central bony core called the modiolus.

• Sections through the choclea reveal that it is divided into 3 channels.Together, the partitions that separate the channels are shaped like the letterY. the stem of the Y is bony shell that protrudes into the canal; the wing ofthe Y are composed mainly of membranous labyrinth. The channel above thebony partition is the scala vestibule, which ends at the oval window; thechannel below is the scala tympani, which ends at the round window.

• Thescala vestibule and scala tympani both contain perilymph and are completelyseparated, except for an opening at eh apex of the cochlea, the helicotrema.The cochlea adjoins the wall of the vestibule, into which the scala vestibuleopens. The perilymph in the vestibule is continuous with that of the scalavestibule. The third channel is the cochlear duct or scala media.

• Thevestibular membrane separate the cochlear duct from the scala vestibule,and the basilar membrane separates the cocholear from the scala tympani.Resting on the basilar membrane is the spiral organ of corti. The spiral cortiis a coiled sheet of epithelial cells, including supporting cells and about 16thousands hair cells, which are the receptors for hearing.

There are 2 groupsof hair cells:

• the inner hair cells are arranged in a single row whereas theouter hair called are arrange in 3 rows. At the apical tip of each hair cell is ahair bundle, consisting of 30-100 streocilia that extends into the endolymphof the cochlear duct. Despite their name, stereocilia are actually long hairlike microvili arrange in several rows of graded height.

• . At their ends, innerand outer cells synapse both with first-order sensory neurons and with motorneurons from the cochlear branch of the vestibulucocholear. Cells bodies ofthe sensory neurons are located in the spiral ganglion. Although outer haircells outnumber them by 3-1 the inner hair cells synapse 90-95% of the firstorder sensory neurons in the cochlear nerve that relay auditory informationto the brain.

• By contrast 90% of the motor neurons in the cochlear nervesynapse in the outer hair cells. Projecting over and in contact with hair cellsof the spiral organ is the tectorial membrane, a flexible gelatinousmembrane.

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