Neurons can be divided into three functional classes: afferent neurons, efferent neurons, and interneurons. Afferent neurons convey information from the tissues and organs of the body into the central nervous system. Efferent neurons convey information from the central nervous system out to effector cells (particularly muscle or gland cells or other neurons); the cell bodies and dendrites of efferent neurons are within the central nervous system, but the axons extend out into the periphery. Interneurons connect neurons within the central nervous system. As a rough estimate, for each afferent neuron entering the central nervous system, there are 10 efferent neurons and 200,000 interneurons. Thus, the great majority of the neurons in the central nervous system are interneurons.
Afferent neurons are unusual in that they have only a single process, usually considered to be an axon. Shortly after leaving the cell body, the axon divides. One branch, the peripheral process, ends at the receptors; the other branch, the central process, enters the central nervous system to form junctions with other neurons.
At their peripheral ends (the ends farthest from the central nervous system), afferent neurons have sensory receptors, which respond to various physical or chemical changes in their environment by causing electrical signals to be generated in the neuron. The receptor region may be a specialized portion of the plasma membrane or a separate cell closely associated with the neuron ending. The term "receptor" has two distinct meanings, the one defined here and the other referring to the specific proteins with which a chemical messenger combines to exert its effects on a target cell.) Afferent neurons propagate electrical signals from their receptors into the brain or spinal cord.
Characteristics of the Three Classes of Neurons
I. Afferent neurons
A. Transmit information into the central nervous system
from receptors at their peripheral endings
B. Cell body and the long peripheral process of the axon
are in the peripheral nervous system; only the short
central process of the axon enters the central nervous
C. Have no dendrites (do not receive inputs from other
II. Efferent neurons
A. Transmit information out of the central nervous system
to effector cells, particularly muscles, glands, or other
B. Cell body, dendrites, and a small segment of the axon
are in the central nervous system; most of the axon is
in the peripheral nervous system
A. Function as integrators and signal changers
B. Integrate groups of afferent and efferent neurons into
C. Lie entirely within the central nervous system
D. Account for 99 percent of all neurons