Central Nervous System

The central nervous system contains billions of nerve cells, called neurons, and a greater number of support cells, called glia, which hold the neurons together and aid in facilitating communication. The neurons, which consist of three elements—dendrites, cell body, and axon—send electrical impulses from cell to cell along pathways that receive, process, store, and retrieve information. The dendrites are the message-receiving portions of the neuron and the axons are the messagesending part of the cell. Both are branching fibers that reach out in many extensions to join the neuron to other neurons. The junction between the axon of one cell and the dendrite of another is a minute gap, eighteen-millionths of an inch wide, which is called a synapse.

The spinal cord is a long bundle of neural tissue continuous with the brain that occupies the interior canal of the spinal column and functions as the primary communication link between the brain, the extremities, and the rest of the body. It is the origin of 31 bilateral pairs of spinal nerves that radiate outward from the central nervous system through openings between adjacent vertebrae. The spinal cord receives signals from the peripheral senses and relays them to the brain. Its sensory neurons, which send sensory data to the brain, are called afferent, or receptor, neurons; motor neurons, which receive motor commands from the brain, are called efferent, or effector, neurons.

The midbrain, or mesencephalon, is the small area near the lower middle of the brain. Its three sections are the tectum, tegmentum, and cerebral peduncles. Portions of the midbrain have been shown to control smooth and reflexive movements, vision, hearing, temperature regulation, and the regulation of attention, sleep, and arousal. The hindbrain (rhombencephalon), which is basically a continuation of the spinal cord, is the part of the brain that receives incoming messages first. Lying beneath the cerebral hemispheres, it consists of three structures: the cerebellum, the medulla, and the pons, which control such vital functions of the autonomic nervous system as breathing, blood pressure, and heart rate. The cerebellum, a large convoluted structure attached to the back surface of the brain stem, receives information from hundreds of thousands of sensory receptors in the eyes, ears, skin, muscles, and joints, and uses the information to regulate coordination, balance, and movement, especially finely coordinated movements such as threading a needle or tracking a moving target. The medulla, situated just above the spinal cord, controls heartbeat and breathing and contains the reticular formation that extends into and through the pons. The pons, a band of nerve fibers connecting the midbrain, medulla (hindbrain), and cerebrum, controls sleep and dreaming. The midbrain, pons and medulla, because of their shape and position at the base of the brain, often are referred to as the brainstem.

See also Brain ; Nerve ; Nervous system ; Neuron .