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.
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.
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.
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.
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The sound wave transfers its energy into the vibration of the tympanic membrane.
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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.
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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.
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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.
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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).
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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.
- 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.