There are three general classes of neurons (fig. 12.3) corresponding to the three major aspects of nervous system function listed earlier:
- Sensory (afferent) neurons are specialized to detect stimuli such as light, heat, pressure, and chemicals, and transmit information about them to the CNS. These neurons can begin in almost any organ of the body and end in the CNS; the word afferent refers to signal conduction toward the CNS. Some sensory receptors, such as pain and smell receptors, are themselves neurons. In other cases, such as taste and hearing, the receptor is a separate cell that communicates directly with a sensory neuron.
- Interneurons6 (association neurons) lie entirely within the CNS. They receive signals from many other neurons and carry out the integrative function of the nervous system—that is, they process, store, and retrieve information and "make decisions" that determine how the body responds to stimuli. About 90% of our neurons are interneurons. The word interneuron refers to the fact that they lie between,
6inter = between and interconnect, the incoming sensory pathways and the outgoing motor pathways of the CNS.
3. Motor (efferent) neurons send signals predominantly to muscle and gland cells, the effectors that carry out the body's responses to stimuli. These neurons are called motor neurons because most of them lead to muscle cells, and efferent neurons to signify the signal conduction away from the CNS.
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