The hair cell transmits information about an acoustic signal by releasing a neurotransmitter to excite afferent nerve fibers. This results in discharge of the auditory nerve. The release of neurotransmitter by the hair cell is triggered by the entry of calcium into the hair cell through voltage-dependent calcium (Vca) channels. Even in the absence of acoustic stimulation, the small resting current entering through the transduction channels in the stereocilia always slightly depolarizes the hair cell. Because of this slight depolarization, the Vca current is always activated. Thus, transmitter is always being released, and the auditory nerve fiber is generally always firing. Further depolarization of the hair cell, for example by acoustic stimulation, increases the probability that transmitter will be released. In contrast, hyperpolarization of the hair cell, as during some phases of an acoustic stimulus, decreases the probability of transmitter release. Thus to a first approximation, the release of transmitter from the hair cell reflects the membrane potential of the hair cell and determines the rate of discharge in the auditory nerve fiber.