The organ of hearing, the ear, consists of three major regions, shown in Figure 1.
- The outer (external) ear consists of the auricle (pinna), a flap of elastic cartilage that protrudes from the head, and the external auditory canal (meatus), a tube that enters the temporal bone. The canal is lined with ceruminous glands that secrete cerumen (earwax), a sticky substance that traps dirt and other foreign objects. The eardrum (tympanic membrane), at the internal end of the external auditory canal, vibrates in response to incident sound waves.
- The middle ear (tympanic cavity) is an air‐filled cavity within the temporal bone. It contains three small bones, the auditory ossicles. These bones, called the malleus, incus, and stapes, act as a lever system that amplifies and transfers vibrations of the eardrum to the inner ear. The malleus at one end connects to the eardrum, while the stapes at the other end attaches with ligaments to the oval window, a small, membrane‐covered opening into the inner ear. Synovial joints connect the incus, the center bone of the auditory ossicles, to the malleus and stapes on each side. A second membrane‐covered opening to the inner ear, the round window (secondary tympanic membrane), lies just below the oval window. A third opening leads to the auditory (Eustachian) tube, which connects the middle ear to the upper throat. The auditory tube allows pressure differences between the middle and outer ear to equalize, thus reducing tension on the eardrum. Two muscles in the middle ear, the tensor tympani and the stapedius, connect to the malleus and stapes, respectively. Contraction of these two muscles restricts the movement of the eardrum and auditory ossicles, reducing damage that may occur when they are exposed to excessive vibration from loud noises.
- The inner (internal) ear, also called the labyrinth, is a system of double‐walled canals. The canals consist of an outer bony (osseous) labyrinth that encloses an inner membranous labyrinth. Perilymph fills the space between the two labyrinths, and endolymph fills the inner labyrinth. This double‐layer labyrinth structure is found throughout the following inner ear structures. This labyrinth is made of three semicircular canals and a snail‐shaped cochlea (see Figure 1).
Three semicircular canals contain receptor cells for determining angular movements of the head. This information is used for establishing equilibrium.
The cochlea is a coiled canal that contains receptor cells that respond to vibrations transferred from the middle ear. The interior of the cochlea is divided into three regions, or scalas: the scala vestibuli, the scala tympani, and the cochlear duct (scala media). The scalas are tubular channels that follow the coiled curvature of the cochlea. At the middle ear, the scala vestibuli and the scala tympani connect to the oval and round windows, respectively. At the other end of the cochlea, in a region called the helicotrema, these two scalas join, allowing free movement of the perilymph within. The third scala, the cochlear duct, is separated from the scala vestibuli and the scala tympani by the vestibular membrane and the basilar membrane, respectively. The cochlear duct is filled with endolymph and internally lined with the organ of Corti. The organ of Corti contains numerous hair cells. The bases of the hair cells are attached to the basilar membrane, while hairlike microvilli called stereocilia project upward into an overlying gel, the tectorial membrane. The stereocilia are receptors for vibrations that are produced when the underlying basal membrane moves relative to the overlying tectorial membrane.
The process of hearing occurs as follows:
- Sound waves, funneled into the outer ear by the auricle, cause the eardrum to vibrate.
- Vibrations of the eardrum are amplified and transferred by the auditory ossicles of the oval window.
- Vibrations on the oval window produce pressure waves in the perilymph of the scala vestibuli and the scala tympani. These vibrations are transferred to the basilar membrane.
- Vibrations of the basilar membrane move the hair cells of the organ of Corti. The stereocilia of the hair cells bend when they move against the tectorial membrane. The bending generates a graded potential in the hair cell, which causes the release of a neurotransmitter at its base. The neurotransmitter in turn generates an action potential in dendrites of the cochlear nerve. Cell bodies of the cochlear nerve assemble in the spiral ganglia, and its axons merge with the vestibulocochlear nerve.
- Pressure waves in the perilymph of the scala tympani cause the round window to bulge into the middle ear. This allows vibrational movements of the perilymph (and indirectly the endolymph) that, as an incompressible fluid, would not otherwise be able to vibrate within the surrounding rigid temporal bone.