The spinal cord is a neural cable about as thick as your little finger that extends from the base of the brain to the hips, protected by the bones of the vertebral column. Between the vertebrae, nerves called dorsal roots, carrying axons of sensory neurons, and ventral roots, carrying axons of motor neurons, arise from the dorsal and ventral portions of the spinal cord, respectively; these merge to form the peripheral nerves of the spinal cord, which are part of the peripheral nervous system. In the center of the spinal cord are neuron cell bodies, which form a butterfly-shaped area of gray matter. These are surrounded by bundles of axons called white matter owing to their white insulating myelin coating. The spinal cord relays signals between the brain and the rest of the boy, and it contains the neural circuitry for certain behaviors, including reflexes.

To illustrate some of the functions of the parts of the spinal cord, let's examine a simple spinal reflex, the pain-withdrawal reflex, which involves neurons of both the central nervous system and the peripheral nervous system. The cells bodies of the sensory neurons from the skin are just found outside the spinal cord in a row of ganglia. Each of these dorsal roots ganglia is located on a spinal nerve and nestled close to the vertebral column. Both association and motor neuron cell bodies are found in the Grey matter in the center of the spinal cord. The axons in the surrounding white matter communicate with the brain. Associationneurons for the pain withdrawal reflex, for example, not only synapse on motor neurons but also have axons extending up to the brain. Signals carried along these axons alert the brain to the painful event. The brain, in turn, sends impulses down axons in the white matter to cells in the Grey matter. These signals can modify spinal reflexes. With sufficient motivation, you can suppress the pain-withdrawal reflexes; to rescue a child from a burning building, for example you can reach into the flames.

In addition to simple reflexes, the entire program for operating some complex activities also resides within the spinal cord. All the neurons and interconnections needed to walk and run, for example, are found within the cord. In these cases, the role of the brain is to initiate and guide the activity of spinal neurons. The advantage of this partially independent arrangement is probably an increase in speed and coordination, since messages do not have to travel all the way up the cord to the brain and back down again merely to swing forward of your legs. The motor neurons of the spinal cord also control the muscles involved in conscious, voluntary activities such as eating, writing, or playing tennis. Axons of the brain cells directing these activities carry signals down the cord and stimulate the apporiate motor cells.