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The external ear collects sound waves and transports them through the external acoustic meatus to the tympanic membrane. The tympanic membrane vibrates, setting three tiny ear ossicles (malleus, incus, and stapes) in the middle ear into motion. The stapes attaches to the lateral wall of the inner ear, where the vibration is transduced into fluid movement. The fluid causes the basilar membrane in the cochlea to vibrate. The vestibulocochlear nerve [cranial nerve (CN) VIII] receives and conducts the impulses to the brain, where there is integration of sound and equilibrium.
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The external ear consists of the auricle, or pinna, which lies at the outer end of a short tube called the external acoustic meatus (Figure 19-1A). The auricle funnels sound waves through the external acoustic meatus to the tympanic membrane. The external ear receives general sensory innervation from the trigeminal, facial, and vagus nerves (cranial nerves (CNN) V, VII, and X, respectively) and from the great auricular nerve (cervical plexus C2–C3).
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The tympanic membrane, or “eardrum,” is a three-layered circular structure (Figure 19-1A–C).
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- Outer layer. Composed of modified skin that is continuous with the external acoustic meatus.
- Middle layer. Composed of connective tissue through which the chorda tympani nerve (CN VII) passes.
- Inner layer. Lined with the mucosa of the middle ear, and receives general sensory innervation via the tympanic plexus (CN IX).
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A physician uses an
otoscope to view the health of a patient's external and middle ear. One of the structures seen on the tympanic membrane is the where the
handle of the malleus attaches on its internal surface. When a physician shines the light of the otoscope onto a healthy tympanic membrane, the malleus causes a
cone of light to appear in the anterior–inferior quadrant (
Figure 19-1B).
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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 (Figure 19-1A). The middle ear consists of the tympanic cavity proper, auditory tube, ear ossicles, and branches of CNN VII and IX.
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Tympanic Cavity Proper
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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.
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- 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.
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The auditory (eustachian) tube is an osseous–cartilaginous tube that connects the nasopharynx and the middle ear (Figure 19-1A). 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.
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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.
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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.
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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 (Figure 19-1A–D). The ossicles function as amplifiers to overcome the impedance mismatch at the air–fluid interface of the middle and inner ear.
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- Malleus. The malleus is attached to the internal surface of the tympanic membrane and articulates with the incus (Figure 19-1B). 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.
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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.
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Branches of the Facial Nerve
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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 (Figure 19-1C and D):
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- 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.
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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.
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The bony labyrinth is structurally and functionally divided into the vestibule, the semicircular canals, and the cochlea (Figure 19-2A).
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The vestibule is the central portion of the bony labyrinth.
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- 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.
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The cochlea is a coiled tube divided into three chambers (Figure 19-2B).
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- 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.
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Vestibulocochlear Nerve
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The vestibulocochlear nerve (CN VIII) courses through the internal acoustic meatus and divides into the vestibular and cochlear nerves (Figure 19-2A).
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- 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.
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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).
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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.
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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?”
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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.