Auditory Receptors and Transduction

Hearing begins with the transduction of sound energy into electrical signals in the inner ear, through a sequence of events so fast and sensitive as to be limited only by the thermal noise of molecules moving in inner ear liquids. This entry reviews that sequence with particular focus on the remarkable properties of auditory receptor cells.

Sound originates in the motion of vibrating objects, such as the vocal cords during speech. The vibration energy—a form of mechanical energy—is transmitted to air molecules and thence to the eardrum. Vibrations of the eardrum in turn move the bones of the middle ear, which push in and out, piston-like, on the liquid-filled chambers of the hearing organ in the inner ear. The mammalian hearing organ, called the cochlea, is coiled in a spiral of several loops. Membranes running the length of the cochlea subdivide it into three tubes, or chambers, filled with liquid. The bone enclosing [Page 184]the upper and middle chambers has membranous, elastic windows in it. Middle ear bones push in and out on the upper chamber's window, creating pressure changes that deflect the elastic middle chamber and distend the window on the lower chamber.

The middle chamber (see color insert, Figure 3a) houses a long sensory epithelium (the organ of Corti) containing thousands of auditory receptor cells, called hair cells for their most striking feature: a “hair” bundle of fine, interlinked columnar structures. The hair cells form four long rows on top of the bottom (basilar) elastic membrane of the middle chamber. As illustrated in Figure 3(a) in the color insert, sound-driven motion of the basilar membrane bends the hair bundles against a delicate tectorial membrane. Viewed along the length of the cochlea, the motion of the basilar membrane takes the form of a traveling wave, which peaks at different places for different sound frequencies. The hair cells convert the mechanical energy of hair-bundle bending into electrical signals called receptor potentials. The electrical signals are transmitted from the inner ear to the brain via auditory nerve fibers (color insert, Figure 3a).

The mammalian cochlea has a single row of inner hair cells and three or more rows of outer hair cells (color insert, Figure 3a). Despite the small number of inner hair cells, it is their sound-evoked signals that are conveyed to brain neurons. Inner hair cells transmit signals to auditory nerve fibers across specialized points of close contact called synapses (color insert, Figure 3b), stimulating the brief electrical events (action potentials) that neurons commonly use to transfer information over long nerve fibers. The sound-evoked action potentials travel along auditory nerve fibers to neurons in hearing centers in the brain.

Outer hair cells are unique to mammals. Surprisingly, given their large numbers, they have weak contacts with auditory nerve fibers. Nevertheless, they are essential to good hearing. They amplify sound-evoked motions within the cochlea by a remarkable mechanism, electromotility, which refers to the fact that the outer hair cells change their length in response to movement of the cilia. The resulting amplification of the motion of the basilar membrane is considered critical for the unusual high-frequency capability of the mammalian ear. Other vertebrates can hear sound frequencies ranging from about 0.1 kilohertz (kHz, thousands of cycles per second) to as high as 10 kHz in some birds. Children can hear frequencies as high as 20 kHz (until hearing damage sets in), and other mammals can hear up to 50 or even 100 kHz. The entire frequency range is represented smoothly along the cochlear spiral such that the highest frequencies strongly activate hair cells at the base, next to the middle ear, and the lowest frequencies strongly activate hair cells at the top of the spiral. This systematic variation in the most effective sound frequency (best frequency or characteristic frequency) with cochlear location arises from smooth variation in the properties of the basilar membrane and hair cells.

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