An afferent neuron is one of millions of cells making up a sensory nerve, a vessel of the peripheral nervous system. Linked end-to-end in a unidirectional chain, these cells conduct an electrical signal known as a nerve impulse from receptors in the body along the nerve toward the central nervous system: the spinal cord and brain. As the nervous system’s most basic unit, each afferent neuron is a single cell comprising a soma, or cell body, and an axon and a dendrite, projections that transmit the nerve impulse through the cell body and on to the next neuron. In doing so, they send information to the central nervous system about pain, temperature, muscle tension, and other sensory input, and it then coordinates a response to this information that is sent back out to the body via efferent or motor neurons.
Nerve cells can vary widely in their specific structure, but each contains a soma, a spherical cell body with a nucleus within; a single axon, which protrudes from the cell body like a tail and which can be more than 1 meter (3 feet) or only 1 micrometer (0.00003937 inch) in length; and any number of dendrites, which branch from either end of the neuron and attach either to an adjacent neuron or to the bodily tissue they innervate. An afferent neuron features a soma, a short axon, and a single long dendrite. The axon carries the electrical impulse away from the cell body and therefore points toward the central nervous system.
This electrical signal travels only a short distance from the soma before reaching the endpoint of the axon, known as the axon terminal, where it crosses the gap between adjacent neurons known as the synapse. On the other side of the synapse is the receiving dendrite of the next afferent neuron. While the dendrites of many of the body’s nerve cells resemble tree branches and conduct impulses through a network of similar cells instead of a unidirectional chain, the dendrite of the afferent neuron simply receives its signal and passes it through the soma and along the axon to the next cell. It does so very rapidly due to the presence of the myelin sheath, a mostly fatty layer that surrounds the dendrite like the rubber casing around an electrical cord, increasing the conductivity of the fiber.
Such a system of signal transmission allows information like whether a stove is hot or whether a muscle has been stretched to its limit to reach the central nervous system. Similar to the circulatory system in which blood cycles through the heart and lungs, out to the body, and back, this system of afferent and efferent nerves operates in a closed loop. In other words, a nerve impulse transmitting information about water temperature travels from sensory receptors in the fingertips, for instance, along an afferent neuron pathway up the hand and arm. Once the afferent nerve reaches the spinal cord near the base of the neck, it cycles through a network of interneurons that carry it to the relevant control center in the central nervous system. A response is quickly coordinated and sent out via similar efferent pathways to the muscles of the arm and torso responsible for snatching the hand back if the water is too hot.