FoundationalConceptsNeuroscience

ing of the hairs initiates a signal from the hair cell to the nerves carry-
ing signal information to the brain.
Here is how that neural signal is believed to come about. The hairs
are interconnected by tiny molecular cables, only a few billionths of
a meter in diameter, barely visible with high-resolution electron mi-
croscopy. These tiny cables appear to be coupled to positive ion chan-
nels, so that as the hairs bend, the cables tug on channels and cause
them to open. When the channels open, K* ions, which in the cochlear
fluid are more concentrated outside the cells than inside, flow into the
hairs and make the normally negative interior of the hair cell more
positive. This depolarization of the hair cell’s membrane potential
causes voltage-gated calcium channels to open and Ca ions to flow
into the cell. Then, just as in the axon terminals of neurons, the influx
of Ca triggers the fusion of synaptic storage vesicles with the outer
membrane of the cell, and neurotransmitter molecules are released
into the synaptic cleft. The neurotransmitter diffuses across the
synaptic cleft and activates postsynaptic receptors located on the den-
dritic fiber of the auditory nerve. If the auditory nerve fiber receives
sufficient stimulation, an action potential occurs there and a signal is
sent to the brain.