The organ of Corti, shown in Figures 52-2, 52-3, and 52-7, is the receptor organ that generates nerve impulses in response to vibration of the basilar membrane. Note that the organ of Corti lies on the surface of the basilar fibers and basilar membrane. The actual sensory receptors in the organ of Corti are two specialized types of nerve cells called hair cells—a single row of internal (or "inner") hair cells, numbering about 3500 and measuring about 12 micrometers in diameter, and three or four rows of external (or "outer") hair cells, numbering about 12,000 and having diameters of only about 8 micrometers. The bases and sides of the hair cells synapse with a network of cochlea nerve endings. Between 90 and 95 per cent of these endings terminate on the inner hair cells, which emphasizes their special importance for the detection of sound.
The nerve fibers stimulated by the hair cells lead to the spiral ganglion of Corti, which lies in the modiolus (center) of the cochlea. The spiral ganglion neuronal cells send axons—a total of about 30,000—into the cochlear nerve and then into the central nervous system at the level of the upper medulla. The relation
Outer hair cells
Outer hair cells
Cochlear nerve of the organ of Corti to the spiral ganglion and to the cochlear nerve is shown in Figure 52-2.
Excitation of the Hair Cells. Note in Figure 52-7 that minute hairs, or stereocilia, project upward from the hair cells and either touch or are embedded in the surface gel coating of the tectorial membrane, which lies above the stereocilia in the scala media. These hair cells are similar to the hair cells found in the macula and cristae ampullaris of the vestibular apparatus, which are discussed in Chapter 55. Bending of the hairs in one direction depolarizes the hair cells, and bending in the opposite direction hyperpolarizes them. This in turn excites the auditory nerve fibers synaps-ing with their bases.
Figure 52-8 shows the mechanism by which vibration of the basilar membrane excites the hair endings. The outer ends of the hair cells are fixed tightly in a rigid structure composed of a flat plate, called the reticular lamina, supported by triangular rods of Corti, which are attached tightly to the basilar fibers. The basilar fibers, the rods of Corti, and the reticular lamina move as a rigid unit.
Upward movement of the basilar fiber rocks the reticular lamina upward and inward toward the modi-olus. Then, when the basilar membrane moves downward, the reticular lamina rocks downward and outward. The inward and outward motion causes the hairs on the hair cells to shear back and forth against the tectorial membrane. Thus, the hair cells are excited whenever the basilar membrane vibrates.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.