Central Nervous System

In humans, that portion of the nervous system that lies within the brain and spinal cord; it receives impulses from nerve cells throughout the body, regulates bodily functions, and directs behavior.

The central nervous system contains billions of nerve cells, called neurons, and a greater number of support cells, or glia. Until recently, scientists thought that the only function of glial cells—whose name means "glue"—was to hold the neurons together, but current research suggests a more active role in facilitating communication. The neurons, which consist of three elements— dendrites, cell body, and axon—send electrical impulses from cell to cell along pathways which receive, process, store, and retrieve information. The dendrites are the

The brain and spinal cord comprise the central nervous system. At the right is a magnified view of the spinal cord showing the individual nerves. The inset shows an individual axon covered with a myelin sheath. (John Bavosi. Photo Researchers, Inc. Reproduced with permission.)

message-receiving portions of the neuron and the axons are the message-sending 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 and the rest of the body. It is the origin of 31 bilateral pairs of spinal nerves which 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 sense 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 brain is a mass of neural tissue that occupies the cranial cavity of the skull and functions as the center of instinctive, emotional, and cognitive processes. Twelve pairs of cranial nerves enter the brain directly. It is composed of three primary divisions: the forebrain, midbrain, and hindbrain, which are divided into the left and right hemispheres and control multiple functions such as receiving sensory messages, movement, language, regulating involuntary body processes, producing emotions, thinking, and memory. The first division, the forebrain, is the largest and most complicated of the brain structures and is responsible for most types of complex mental activity and behavior. It is involved in a huge array of responses, including initiating movements, receiving sensations, emoting, thinking, talking, creating, and imagining. The forebrain consists of two main divisions: the diencephalon and the cerebrum. The cerebrum is the larger part of the forebrain. Its parts, which are covered by the cerebral cortex, include the corpus callosum, striatum, septum, hippocampus, and amygdala.

The midbrain, or mesencephalon, is the small area near the lower middle of the brain. Its three sections are the tectum, tegmentum, and crus cerebri. Portions of the mid-brain have been shown to control smooth and reflexive movements, and it is important in 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 which 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 pons and medulla, because of their shape and position at the base of the brain, are often referred to as the brainstem.