Synapses - Communication to and from the Brain
Hair cells have synapses located at the end of the cell opposite the stereocilia bundle. Synapses are structures that permit communication between neuronal cells. One side of the synapse is presynaptic and the other postsynaptic. A chemical known as a "neurotransmitter" is secreted from the presynaptic cell and changes the membrane potential of the postsynaptic cell. There are two types of synapses associated with hair cells. "Afferent" synapses convey information into the central nervous system by exciting "action potentials" in the afferent nerve fibers that enter the brain. Action potentials are essentially pulses that travel down the fiber and carry information in a type of digital code. "Efferent" synapses modulate the membrane potential of the hair cell in response to neurotransmitter release from their presynaptic element which is the terminal of a nerve fiber that originates deep in the brainstem. The neural signals from the brain conveyed by these efferent fibers may be viewed as having the ability to change the "gain" (amplification) of the hair cells they innervate. The brain cannot change the gain of chemoreceptor or photoreceptor sensory cells directly because, in contrast to hair cells, they are not innervated by nerve fibers from the central nervous system.
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