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Inner Ear

The inner ear contains the functional organs for hearing and equilibrium.

It consists of a series of bony cavities (bony labyrinth), within which is a series of membranous ducts (membranous labyrinth), all within the petrous part of the temporal bone.

The space between the bony and membranous labyrinths is filled with a fluid called perilymph.

The tubular chambers of the membranous labyrinth are filled with endolymph.

These two fluids provide a fluid-conducting medium for the vibrations involved in hearing and equilibrium.

Bony Labyrinth

The bony labyrinth is structurally and functionally divided into the vestibule, the semicircular canals, and the cochlea

A. Coronal section of the internal ear. B. Cross-section of the cochlea. C. Pathway for the transmission of sound.

Vestibule

The vestibule is the central portion of the bony labyrinth.

Vestibular window. The vestibular window serves as a membranous interface between the stapes from the middle ear and the vestibule of the inner ear.
Utricle and saccule. The membranous labyrinth within the vestibule consists of two connected sacs called the utricle and saccule. Both the utricle and saccule contain receptors that are sensitive to gravity and linear movements of the head.
Semicircular canals. The three bony semicircular canals of the inner ear are at right angles to each other. The narrow semicircular ducts of the membranous labyrinth are located within the semicircular canals. Receptors within the semicircular ducts are sensitive to angular acceleration and deceleration of the head, as occurs in rotational movement.

Cochlea

The cochlea is a coiled tube divided into three chambers (Figure 19-2B).

Scala vestibuli. Forms the upper chamber of the cochlea. The scala vestibuli begins at the vestibular window, where it is continuous with the vestibule, and contains perilymph.
Scala tympani. Forms the lower chamber of the cochlea. The scala tympani terminates at the cochlear window and contains perilymph.
- Helicotrema. The scala vestibuli and the scala tympani are separated completely, except at the narrow apex of the cochlea called the helicotrema, where they are continuous.
Cochlear duct. Forms the middle chamber of the cochlea. The roof of the cochlear duct is called the vestibular membrane, and the floor is called the basilar membrane. The cochlear duct is filled with endolymph and ends at the helicotrema. The cochlear duct houses the spiral organ (of Corti), where sound receptors transduce mechanical vibrations into nerve impulses.

Vestibulocochlear Nerve

The vestibulocochlear nerve (CN VIII) courses through the internal acoustic meatus and divides into the vestibular and cochlear nerves (Figure 19-2A).

Vestibular nerve. Special sensory innervation of the utricle and saccule of the semicircular canals (equilibrium and balance), with sensory cell bodies in the vestibular ganglion.
Cochlear nerve. Special sensory innervation of the spiral organ (of Corti) in the cochlea (hearing), with sensory cell bodies in the spiral ganglion.

Image not available. Sound waves travel in all directions from their source, similar to ripples in water after a stone is dropped (Figure 19-2C). Sound waves are characterized by their pitch (high or low frequency) and intensity (loudness or quietness).

A sound wave enters the external acoustic meatus and strikes the tympanic membrane.
The sound wave transfers its energy into the vibration of the tympanic membrane.
As the tympanic membrane vibrates, it causes the malleus to move medially, which in turn causes the incus and stapes to move sequentially, amplifying the sound wave.
The stapes is attached to the vestibular window; thus, the vestibular window also moves, resulting in a wave forming in the perilymph within the scala vestibuli of the cochlea.
The fluid wave in the perilymph progresses from the scala vestibuli of the cochlea, resulting in an outward bulging of the cochlear window at the end of the scala tympani.
This bulging causes the basilar membrane in the cochlea to vibrate, which in turn results in stimulation of the receptor cells in the spiral organ (of Corti).
The receptor cells conduct impulses to the brain through the cochlear division of CN VIII, where the brain interprets the wave as sound.

The difference between a sound wave and sound can best be explained by the age-old question, “If a tree falls in a forest and no one is around to hear it, does it make a sound?”

Sound, as we interpret it, results from transduction and perception of amplitude, frequency, and complexity of a sound wave by the brain. The falling tree produces sound waves, but there is no perception of sound without the brain interpreting the sound wave. Therefore, the tree does not make a sound unless someone's auditory apparatus is there to hear it. Image not available.

The internal ear consists of a membranous labyrinth containing endolymph; the membranous labyrinth is enclosed by the temporal bone's bony labyrinth which contains perilymph.

The membranous labyrinth has a central vestibule with two subdivisions: the utricle connects to the three semicircular ducts and the saccule connects to the cochlear duct.
The walls of the utricle and saccule each have a thickened area, the macula, which contains both sensory hair cells with synaptic connections to sensory nerves and supporting cells.
A bundle of rigid stereocilia and one rigid kinocilium project apically from each hair cell and are surrounded by endolymph containing a gel-like matrix with mineralized crystals called otoliths.
Head movements cause endolymph and the otolithic membrane to move, deforming the rigid apical structures of the hair cells, depolarizing them and producing nerve impulses.
Each of the semicircular ducts, oriented 90 degrees from one another, has a terminal ampulla region with a thickened crista ampullaris containing hair cells that contact a gel-like cupola.
Head movements displace endolymph and stereocilia of hair cells in the utricle, saccule, and semicircular ducts which together produce signals that contribute to the sense of equilibrium.
The cochlear duct is the middle compartment (scala media) of the cochlea and runs between two other long compartments that contain perilymph: the scala vestibuli and the scala tympani.
Along the base of the cochlear duct, the basilar membrane supports the spiral organ of Corti, which consists largely of hair cells connected to sensory fibers of cranial nerve VIII.
The cochlear hair cells include three to five rows of outer hair cells with stereocilia embedded in a gel-like tectorial membrane and one row of more heavily innervated inner hair cells.
Sound waves transmitted by the ossicles move the oval window and produce pressure waves in the cochlear perilymph which deflect the basilar membrane and organ of Corti, causing nerve impulses which the brain interprets as sounds.

Middle Ear
Inner Ear

The middle ear amplifies sound to facilitate conversion of the mechanical energy of the sound wave into an electrical signal by the inner ear hair cells, a process called mechanotransduction (Fig. 43-1).

This component lies in a cavity in the temporal bone. Its connection with the auditory meatus is closed by the tympanic membrane. It communicates with the nasopharynx through the auditory (eustachian) tube and also communicates with the mastoid air cells. The auditory tube is surrounded by elastic cartilage. Its walls are collapsed except during swallowing, when they separate to allow pressure in the middle ear cavity to equilibrate with the environment.

Windows. The medial bony wall of the middle ear cavity has two membrane-covered openings, the oval and round windows, which lie between the middle and internal ears.
Auditory ossicles. The main functional components of the middle ear are three uniquely shaped small bones that span the middle ear cavity from the tympanic membrane to the oval window membrane. The malleus (hammer), incus (anvil), and stapes (stirrup) transmit vibrations from the tympanic membrane to the fluid in the inner ear. Small muscles restrict ossicle movement and limit damage from loud noises.
Mucosa. The simple squamous to cuboidal epithelium that lines the middle ear contains mucous or seromucous secretory cells. A thin lamina propria binds the lining to the periosteum of the walls and ossicles. Near the auditory canal, the lining changes to the pseudostratified columnar epithelium of the canal and nasopharynx.

The middle ear is an air-filled chamber that transmits sound waves from air to the auditory ossicles and then to the fluid-filled inner ear. The middle ear consists of the tympanic cavity proper, auditory tube, ear ossicles, and branches of CNN VII and IX.

Tympanic Cavity Proper

The tympanic cavity proper is the space between the tympanic membrane and the vestibular window. Its mucosa receives general sensory innervation from the tympanic nerve and the tympanic plexus (CN IX) (Figure 19-1A–D). In addition, visceral motor preganglionic parasympathetic fibers from CN IX branch from the tympanic plexus to exit the middle ear as the lesser petrosal nerve on route to innervate the parotid gland.

The tympanic cavity of the middle ear opens to the nasopharynx via the auditory (eustachian) tube.
Within the tympanic cavity, an articulated series of three small bony ossicles (malleus, incus, and stapes) connects the tympanic membrane with the oval window in the wall of the internal ear.

Vestibular (oval) window. A membrane-covered opening between the middle ear and the vestibule of the inner ear. The oval window is pushed back and forth by the footplate of the stapes and transmits the vibrations of the ossicles to the perilymph at the origin of the scala vestibuli in the inner ear.
Cochlear (round) window. A membrane-covered opening that accommodates the pressure waves transmitted to the perilymph at the end of the scala tympani.

Auditory Tube

The auditory (eustachian) tube is an osseous–cartilaginous tube that connects the nasopharynx and the middle ear. The auditory tube receives general sensory innervation from the tympanic plexus (CN IX) and also serves as an attachment point for the tensor tympani muscle.

Image not available. The auditory tube normally is closed, but yawning or swallowing can open the tube, allowing air to enter, which equalizes the pressure between the middle ear and the atmosphere. When air enters, which can occur when in an airplane or at a high elevation, a soft “pop” sound may be felt. Image not available.

Image not available. A patient with otitis media (middle ear infection) will present with a red bulging tympanic membrane, which is usually due to a buildup of fluid or mucus. This inflammation is often the result of a pharyngeal infection transmitted via the auditory tube to the middle ear. Because the auditory tube is shorter and more horizontal in children, it is easier for infection to spread from the nasopharynx to the middle ear, resulting in a higher prevalence of otitis media in children compared to adults. Hearing may be diminished because of the pressure on the eardrum, and taste may be altered due to the effect on the chorda tympani nerve. Infection can easily spread from the tympanic cavity to the mastoid air cells, causing mastoiditis. Image not available.

Ear Ossicles

The ear ossicles are three small bones known as the malleus, incus, and stapes, which transmit vibrations from the tympanic membrane to the inner ear. The ossicles function as amplifiers to overcome the impedance mismatch at the air–fluid interface of the middle and inner ear.

Malleus. The malleus is attached to the internal surface of the tympanic membrane and articulates with the incus. The tensor tympani muscle attaches between the auditory tube and malleus and serves to reduce the movement of the tympanic membrane. It is innervated by CN V-3.
Incus. The incus articulates with the stapes.
Stapes. The footplate of the stapes attaches to the vestibular window, which separates the air environment of the middle ear from the fluid environment of the inner ear. The stapedius muscle is the smallest skeletal muscle in the body. The stapedius muscle prevents excess movement of the stapes and controls the amplitude of sound waves from the external environment to the middle ear. The stapedius is innervated by CN VII.

Image not available. Paralysis of the stapedius muscle is usually caused by a lesion of CN VII, resulting in wider oscillation of the stapes; consequentially, there is a heightened reaction of the auditory ossicles to sound vibration. This condition is known as hyperacusis and results in an increased sensitivity to loud sounds. Image not available.

Branches of the Facial Nerve

The facial nerve (CN VII) enters the internal acoustic meatus along with CN VIII. CN VII enters the facial canal and continues laterally between the internal and middle ear. It is at this point that the sensory geniculate ganglion forms a bulge in CN VII and gives rise to the following branches:

Greater petrosal nerve. Provides visceral motor innervation to the lacrimal, nasal, and palatal glands.
Branchial motor neurons. Provides innervation to the stapedius muscle.
Chorda tympani nerve. Arises before CN VII exits the stylomastoid foramen. The chorda tympani nerve ascends and courses through the posterior wall of the middle ear, passes through the middle layer of the tympanic membrane, continues between the malleus and stapes, and exits the skull at the petrotympanic fissure. The chorda tympani innervates the submandibular and sublingual salivary glands, and conveys taste sensation (special sensory) from the anterior two-thirds of the tongue.

Inner Ear

The inner ear contains the functional organs for hearing and equilibrium. It consists of a series of bony cavities (bony labyrinth), within which is a series of membranous ducts (membranous labyrinth), all within the petrous part of the temporal bone. The space between the bony and membranous labyrinths is filled with a fluid called perilymph. The tubular chambers of the membranous labyrinth are filled with endolymph. These two fluids provide a fluid-conducting medium for the vibrations involved in hearing and equilibrium.

Bony Labyrinth

The bony labyrinth is structurally and functionally divided into the vestibule, the semicircular canals, and the cochlea.

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