Nervous System

The human nervous system has two main divisions: the central nervous system (CNS), and the peripheral nervous system (PNS), which includes the somatic motor nervous system, and the sensory nervous system. The CNS consists of the brain and spinal cord. It acts as the central control region of the human nervous system, processing information and issuing commands. The autonomic nervous system (ANS) is the command network the CNS uses to maintain the body's homeostasis. It automatically regulates heartbeat and controls muscle contractions in the walls of blood vessels, digestive, urinary, and reproductive tracts. It also carries messages that help stimulate glands to secrete tears, mucus, and digestive enzymes.

The nerves that are easily visible to the unaided eye are not single cells. Rather, they are bundles of nerve fibers (neurons) each of which is itself a portion of a cell. The fibers are all traveling in the same direction and are bound together for the sake of convenience, though the individual fibers of the bundle may have widely differing functions. There are no cell bodies in nerves; cell bodies are found only in the CNS or in the ganglia. Ganglia are collections of cell bodies within the PNS.

The main portion of the neuron, the cell body, is not too different from other cells. It contains a nucleus and cytoplasm. Where it is most distinct from cells of other types is that out of the cell body, long threadlike projections emerge. Over most of the cell there are numerous projections that branch out into still finer extensions. These branching threads are called dendrites ("tree" in Greek). At one point of the cell, however, there is a particularly long extension that usually does not branch throughout most of its sometimes enormous length. This is the axon (the axis). Figure 01b shows the three parts of the neurons: dentrite(s), cell body, and axon. A dendrites conducts nerve impulses toward the cell body, the part of a neuron that contains the nucleus and other organelles. An axon conducts nerve implses away from the cell body. There are three types of neurons: sensory neuron, motor neuron, and interneuron. A sensory neuron takes a message from the recptors in the sense organ to the CNS. A motor neuron sends a message away from the CNS to an effector, a muscle fiber or a gland. An interneuron is always found completely within the CNS and conveys messages between parts of the system. In addition to neurons, nervous tissue contains glial cells such as the Schwann cells covering the neurons with sheath. These cells maintain the tissue by supporting and protecing the neurons. They also provide nutrients to neurons and help to keep the tissue free of debris. The neurons require a great deal of energy for the maintenance of the ionic imbalance between themselves and their surrounding fluids, which is constantly in flux as a result of the opening and closing of channels through the neuronal membranes. Neurotransmitters are chemicals that take a nerve signal across the synaptic gap between a sending neuron, and a receiving one.