The peripheral and central nervous systems of vertebrates are highly integrated. One can’t function without the other. The role of the peripheral nervous system is to conduct impulses between the central nervous system and the sensory receptors and effectors scattered throughout the body.

In humans, the major signaling highways of the peripheral nervous system are the cranial and spinal nerves. Twelve pairs of cranial nerves connect directly to the brain; 31 pairs of spinal nerves arise from different points along the spinal cord. The peripheral nervous system extends further, beyond these nerves, to virtually every part of the body.

Remember that nerves are bundles of parallel axons, and that each axon belongs to a different neuron. Some axons within a particular nerve may conduct impulses toward the central nervous system, while others conduct impulses away from it. A few nerves carry signals in only one direction. Thus, nerves may function as either one-way or two-way passages. But each neuron within a nerve conveys signals in only one direction.

The neurons of the peripheral nervous system may be classified into two functional categories, depending on the direction of the signals they carry. The sensory division includes neurons that conduct impulses from sensory receptors to the central nervous system. The motor division encompasses neurons that transmit signals from the central nervous system to effectors, such as muscle cells.

Sensory neurons not only transmit signals we consciously perceive as sensations, such as the smell of food. They also monitor conditions within our body that are important for physiological regulation and for maintaining homeostasis, such as blood pressure and blood oxygen level. We don’t become consciously aware of this sensory information.

Neurons of the motor division can be classified into two groups depending on the category of effector they stimulate. These subdivisions are known as the somatic nervous system and the autonomic nervous system.

Motor neurons of the somatic nervous system stimulate skeletal muscles. They control the movement of the limbs, torso, and head. Most of these neurons are under our voluntary, conscious control, but some movements they induce are involuntary. An example of this is the knee-jerk reflex. The neural circuit for this reflex extends from stretch receptors, stimulated by a tap on the knee, through the spinal cord, to motor neurons, and finally to the muscle cells that move the leg. The circuit doesn’t travel through the brain, so the act is not under conscious control.

Motor neurons of the autonomic nervous system control the activities of glands, muscles, and other effector cells associated with internal organs, such as the heart, stomach, and kidneys.

See if you can identify the part of the motor division of the peripheral nervous system illustrated on the screen. Drag the correct label to its proper place under the image, then click Submit.

The rapid withdrawal of your hand from a hot object is a reflex action involving the somatic nervous system, like the knee-jerk reflex. The autonomic nervous system is responsible for the involuntary release of digestive fluids.

The peripheral nervous system of humans and other vertebrates requires a powerful centralized system to coordinate its signals. We’ll look at that system—the central nervous system—next.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education