The basic element of the nervous system is the nerve cell, or neuron. In combination, neurons form nerves, fibers that transmit impulses throughout the body. A protective covering of myelin, a fatty substance, insulates parts of the fibers.
The action of nerve cells is both electrical and chemical. At the ends of each nerve cell there are specialized regions called synaptic terminals, which contain large numbers of tiny membranous sacs that hold neurotransmitter chemicals. These chemicals transmit nerve impulses from one nerve cell to another. After an electrical nerve impulse has traveled along a neuron, it reaches the terminal and stimulates the release of neurotransmitters from their sacs.
The neurotransmitters travel across the synapse (the junction between the neighboring neurons) and stimulate the production of an electrical charge, which carries the nerve impulse forward. This process is repeated over and over again until a muscle is moved or relaxed or a sensory impression is noted by the brain. These electrochemical events can be considered the "language" of the nervous system, by which information is transmitted from one part of the body to another.
There are two major divisions of the nervous system: the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and the spinal cord. The brain lies within the skull and governs body functions by sending and receiving messages through the spinal cord. Protecting the brain and spinal cord are bones, layers of tissue, and cerebrospinal fluid.
Once messages leave the central nervous system, they are carried by the peripheral nervous system. The peripheral system includes the cranial nerves (nerves branching from the brain) and the spinal nerves (nerves branching from the spinal cord). These nerves convey sensory messages from receptor cells in the body to the central nervous system. They also transport motor impulses from the central system out to the body, where muscles and glands can respond to the impulses.
The autonomic nervous system, which is part of the peripheral nervous system, reg-ulates all activity that is involuntary but necessary for life, including activity of the internal organs and glands.
Working together, these divisions coordinate adjustment and reaction of the body to internal and external environmental conditions.
Now that we've covered the nervous system, let's discuss the brain, cerebrospinal fluid, and other related elements in the next section.
The Brain and Cerebrospinal Fluid
The brain is the body's control center. The brain sends messages to and receives stimulation from all parts of the body. More than 10 billion interlinked brain cells regulate the functioning of the body during sleep and wakefulness.
Different areas of the brain control different body functions. At the back of the skull is the cerebellum, which controls coordination of movements, balance, and posture. Deep inside the brain is the thalamus, which is the relay station for incoming impulses from the rest of the body, conveying sensations of pain, touch, and temperature to other parts of the brain.
Around the thalamus is the hypothalamus, which governs involuntary (automatic) body operations, such as heartbeat and blood circulation. The pituitary gland is attached to the hypothalamus by a thin stalk. Because the pituitary gland controls most of the hormones in the body, the hypothalamus is considered a major influence on primary drives governed by hormones, such as hunger, thirst, and sexual desire.
Covering the inner parts of the brain is the cerebral cortex, which consists of two cerebral hemispheres. Located in these hemispheres are the nerve centers that regulate thought and voluntary action. Connecting the left and right cerebral hemispheres is a broad band of fibers called the corpus callosum. Because nerve fibers from the two cerebral hemispheres cross one another in a structure called the medulla at the base of the brain before progressing down the spinal cord, each hemisphere generally controls functions in the opposite side of the body. For example, a region in the left hemisphere governs movement of the right arm.
The brain is the most complex organ in the body. Although research has identified many of its capabilities in memory, reasoning, and creative thought processes, many functions of the brain continue to remain a mystery.
Cerebrospinal fluid (CSF) is a clear, colorless fluid that surrounds the brain and spinal cord, cushioning them against injury.
The CSF is made of water containing small amounts of minerals and organic substances (especially protein). It is continually being produced by a specialized network of capillaries (tiny blood vessels) known as the choroid plexus, located in the ventricles (chambers) of the brain. About one pint is produced every 24 hours, and approximately five ounces is circulating at any one time.
From the two lateral ventricles, the CSF flows into the third and fourth ventricles of the brain. It then passes into the space between the innermost and second layers of the tissue covering the brain, bathing the entire outer surface of the brain in fluid before passing downward around the spinal cord. Eventually the fluid returns upward, is absorbed into special tissue between the linings of the brain, and passes into the blood vessels.
Samples of CSF (drawn from around the spinal cord with a needle inserted in the lower back -- a procedure known as a lumbar puncture) can be valuable in diagnosing disorders of the brain and spinal cord. The samples may indicate a hemorrhage or blood clot in the brain, various types of meningitis, a brain abscess, or a tumor of the brain or spinal cord.
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