Prof. J.P.N. Mishra
The nervous system and the endocrine system are responsible for regulation and coordination of body parts. Together, they maintain homeostasis. The nervous system permits a quick response to external and internal stimuli, while the endocrine system is slower to act but provides longer-lasting effects.
Nerve cells are called neurons, or nerve fibres. Whatever their specific functions, all neurons have the same physical parts. The cell body contains the nucleus (Fig. 1-15) and is essential for the continued life of the neuron. As you will see, neuron cell bodies are found in the central nervous system or close to it in the trunk of the body. In these locations, cell bodies are protected by bone. There are no cell bodies in the arms and legs, which are much more subject to injury. Dendrites are processes (extensions) that transmit impulses toward the cell body. The one axon of a neuron transmits impulses away from the cell body. It is the cell membrane of the dendrites, cell body, and axon that carries the electrical nerve impulse. In the peripheral nervous system, axons and dendrites are "wrapped" in specialized cells called Schwann cells (Fig. 1-15). During embryonic development, Schwann cells grow to surround the neuron processes, enclosing them in several layers of Schwann cell membrane. These layers are the myelin sheath; myelin is a phospholipid that electrically insulates neurons from one another. Without the myelin sheath, neurons would short-circuit, just as electrical wires would if they were not insulated. The spaces between adjacent Schwann cells, or segments of the myelin sheath, are called nodes of Ranvier (neurofibril nodes) (Fig 1-15). These nodes are the parts of the neuron cell membrane that depolarize when an electrical impulse is transmitted. The nuclei and cytoplasm of the Schwann cells are wrapped around the outside of the myelin sheath and are called the neurolemma, which becomes very important if nerves are damaged. If a peripheral nerve is severed and reattached precisely by microsurgery, the axons and dendrites may regenerate through the tunnels formed by the neurolemmas. The Schwann cells are also believed to produce a chemical growth factor that stimulates regeneration. Although this regeneration may take months, the nerves may eventually re-establish their proper connections, and the person may regain some sensation and movement in the once-severed limb. In the central nervous system, the myelin sheaths are formed by oligodendrocytes, one of the neuroglia (glial cells), the specialized cells found only in the brain and spinal cord. Because no Schwann cells are present, however, there is no neurolemma, and regeneration of neurons does not occur. This is why severing of the spinal cord, for example, results in permanent loss of function. Another kind of neuroglia are microglia, which are constantly moving, phagocytizing cellular debris, damaged cells, and pathogens. The nervous system is involved in some way in most body functions. Some of the major functions of the nervous system are:
- Sensory components monitor numerous external and internal stimuli, such as touch, temperature, taste, smell, sound, blood pressure, pH of body fluids, and body position.
- The brain is the center of mental activities, including consciousness, thinking, memory, and emotions.
- Nervous system controls the activities of muscles and various glands. Skeletal muscles normally contract only when stimulated by the nervous system, and the nervous system controls of the major movements of the body through the control of skeletal muscle. Some smooth muscle, such as in the walls of blood vessels, contracts only when stimulated by the nervous system or hormones. Cardiac muscle and some smooth muscle, such as in the wall of the stomach, contract auto rhythmically. That is, no external stimulation is necessary for contraction to occur. Although the nervous system does not initiate contraction in these muscles, it can cause the contractions to occur more rapidly or slowly. Finally, the nervous system controls the secretions from many glands, such as sweat glands, salivary glands, and glands of the digestive system.
Fig 1-15. Structure of Neuron
