The nervous system coordinates behavior and helps to maintain the internal stability of animals. It may be as simple as the nerve net of Cnidarians or ascomplex as the centralized system of mammals. In all nervous systems the functional unit is the nerve cell or neuron, a cell specialized to transmit and receive a stimulus.
In humans, the brain and spinal cord are formed early in embryonic development. At the beginning of the third week of gestation, the embryo has already formed a neural plate on the dorsal surface that eventually folds together to form a hollow tube from which the brain and spinal cord develop. During this time the 100 billion neurons found in the brain are produced--the sum total ofall the neurons that the brain will ever contain in an individual's lifetime. The brain is one of the largest organs in the body and consists of three main regions: the forebrain, midbrain, and hindbrain. The cerebrum, which is the most important area for neural processing, together with the thalamus and hypothalamus, forms the forebrain. In the midbrain are centers for the receiptand integration of several types of sensory information, such as seeing andhearing. The information is then sent on to specific areas in the cerebrum tobe processed. The hindbrain consists of three parts: the medulla oblongata,the pons, and cerebellum, and it functions in maintaining homeostasis and coordinating movement. The pons and medulla of the hindbrain, together with themidbrain, form the brainstem, which is the location of reflex centers such asthose that control heart beat rate and breathing rate. The other part of thecentral nervous system, the spinal cord, serves as a pathway for nerve tracts carrying impulses to and from the brain. It acts as the site for simple reflexes such as the familiar knee jerk. If a slice were made into the spinal cord, it would show a cord with a small central canal surrounded by an area ofgray matter shaped like a butterfly surrounded by white matter. The gray matter is composed of large masses of cell bodies, dendrites and unmyelinated axons; the white matter is composed of bundles of axons that are called tracts,which send information to and from the brain.
The central nervous system operates through the peripheral nervous system, which is the "roadway" that links the central nervous system to the rest of thebody. The nerves that carry information to the central nervous system from sensory receptors such as the eye are called sensory nerves or afferent nerves; those that carry impulses away from the central nervous system to effectororgans such as the muscles are called motor nerves or efferent nerves. Commonly the fibers of sensory and motor neurons are bundled together to form mixednerves. There are 12 pairs of cranial nerves that run to or from the brain,such as the optic and vagus nerves. There are 31 pairs of nerves called spinal nerves that originate from the spinal cord, such as the sciatic nerve and ulnar nerve, the nerve that is stimulated when you hit your elbow. Specific areas of the body are served by each of the spinal nerves. All sensory nerves enter the cord through a dorsal root, and all motor nerves exit through a ventral root. If the dorsal section of a root is destroyed, sensation from that area is also destroyed, but the muscles are still able to function. In the opposite situation, damage to the ventral root destroys muscle function, but sensory information is still processed.
There are two main divisions to the peripheral nervous system, the somatic and the autonomic. The somatic system involves the skeletal muscles and is considered voluntary since there is control over movement such as writing or throwing a ball. The autonomic nervous system (ANS) affects internal organs and is considered involuntary since the processes such as heart beat rate and glandular secretions occur with usually little control on the part of the individual. The autonomic nervous system, in turn, is divided into two divisions, the parasympathetic and sympathetic. The parasympathetic system is most activein normal, restful situations and is dominant during quiet, relaxed periods.It acts to decrease the heartbeat and to stimulate the motility and secretions necessary for digestion. The sympathetic nervous system is most active during times of stress and arousal and is dominant when energy is required, whenit increases the rate and strength of the contractions of the heart and inhibits the motility of the intestine. Together with hormones, the autonomic system maintains homeostasis, the internal balance of the body.
The functional unit of the nervous system is the neuron, a cell specialized to receive and transmit impulses. Even though there are a variety of neurons,the essential structures are the same in each: a cell body containing the nucleus and two kinds of processes extending from it, the axon and dendrite. Axons transmit impulses away from the cell body to the dendrites of adjoining neurons. Some axons may be over 3 ft (1 m) in length, such as the sciatic nerve, which runs from the spinal cord to the lower leg. The axons of the peripheral nerves are enclosed in a fatty (myelin) sheath formed from specialized cells called Schwann cells. The myelin sheath acts to insulate the axon, which helps to accelerate the transmission of a nerve impulse. Gaps along the sheathexpose the axon fiber and are important in allowing nerve impulses to jump from one section of the axon to another. The speed at which nerve impulses travel depends on the diameter of the axon and the presence of the myelin sheath; some impulses from the large motor nerves to the leg muscles travel as fastas 394 ft (120 m) per second. Damage to the sheath in multiple sclerosis patients causes impaired muscle control and other symptoms, an indication of theimportance of the myelin sheath in the transmission of nerve impulses. Axonsare bundled together and enclosed by connective tissue to form nerves. Dendrites are usually highly branched extensions of the cell body that receive impulses from axons. In some cases they may be very small, as seen in some of the neurons of the brain, or long, as is the sensory dendrite that runs from the foot to the spinal cord.