Encyclopedia of Homeopathy

178 •SERIOUS AILMENTSNERVOUS SYSTEMThe nervous system is the human body’s central command center. It receivesand evaluates stimuli from inside and outside the body, and issues directivesto tissues and organs in response to these stimuli. It consists of the centralnervous system and the peripheral nervous system.How the system worksThe central nervous system—the brain andspinal cord—receives information from allover the body. This information takesthe form of electrical impulses that aretransmitted along a nerve network—theperipheral nervous system—which branchesoff the central nervous system. The brain thensends out instructions to internal organs,muscles, glands, and tissues throughout thebody via the peripheral nervous system.In terms of function, the parts of thenervous system consisting of nerves thatcontrol the muscles involved in voluntaryactions are known as the somatic nervoussystem. Nerves concerned with theunconscious control of bodily functionssuch as digestion, gland secretions, andtemperature regulation are part of theautonomic nervous system.NETWORK OF NERVES Nerves extendto all parts of the body in a vast andcomplex network. The peripheralnervous system spreads out from12 pairs of cranial nerves, which areconnected to the brain, and 31 pairsof spinal nerves, which are connectedto the spinal column. Peripheralnerves branch and divide so thatall organs, tissues, and farthestreaches of the limbsare contacted.Radial nerveMediannerveAxillarynerveSpinal cordNerves leavespinal cordthrough bonyoutlets betweenvertebraeFemoral nerveHow nerves functionA nerve is made up of strings of nerve cells,or neurons, which are the basic units of thenervous system. Some nerves carryinstructions from the brain or spinal cord tomuscles, glands, or other tissues throughoutthe body. Others carry information to thebrain from sensory receptors, sensory organssuch as the eyes and ears, and internalorgans. Each nerve cell (see right) has partsthat receive electrical messages from othernerve cells, and parts that transmit messagesto nerve cells or other tissues. The cell bodiesof neurons make up the gray matter of thebrain and spinal cord, while the long nervefibers, or axons, make up the white matter ofthe central nervous system.All but the smallest nerve fibers areinsulated and protected by a fatty substancecalled myelin, which also helps to conductnerve impulses quickly along the fibers.Multiple sclerosis is believed to occurbecause the myelin becomes damaged insome way. The brain and spinal column areprotected by delicate membranescalled meninges.Nerve fiber issurrounded byprotectivemyelin sheathTerminal fibersat end of nervefiber transmitnerve impulsesto other nervesSchwann cell maintains eachsection of myelin sheathCell body consistsof nucleus andseveral branchingdendritesSTRUCTURE OF A NERVE CELL A nerve cell,or neuron, has a cell body, anda long nerve fiber, or axon. Dendritesreceive nerve impulses from other nervecells. Impulses are carried away from thecell body by the nerve fibers, and cantravel at speeds of up to 300 miles(500 km) an hour.SciaticnerveTibial nerve