bones return to their normal position, forcing air fromthe lungs.THE PROCESS OF RESPIRATIONThe rhythmical movements of breathing arecontrolled by the respiratory center in the brain.Nerves from the brain pass down through the neck tothe chest wall and diaphragm. The nerve that controlsthe diaphragm is called the phrenic nerve; the nervethat controls the larynx is the vagus nerve; and thenerves that control the muscles between the ribs are theintercostal nerves.The respiratory center is stimulated by chemicalchanges in the blood. When too much carbon dioxideaccumulates in the blood stream, causing the blood tobecome acidic, the respiratory center signals the lungsto breathe faster to get rid of the carbon dioxide.The respiratory center can also be stimulated ordepressed by a signal from the brain. For example,changes in one's emotional state can alter respirationthrough laughter, crying, emotional shock, or panic.The muscles of respiration normally actautomatically, with normal respiration being 14 to 18cycles per minute. The lungs, when filled to capacity,hold about 6,500 ml of air, but only 500 ml of air isexchanged with each normal respiration. Thisexchanged air is called tidal air. The amount of air leftin the lungs after forceful exhalation is about 1,200 mland is known as residual air.THE NERVOUS SYSTEMLEARNING OBJECTIVE: Identify thecomponents and function of a neuron, recall theprocess of impulse transmission, and identifythe components and functions of the centraland peripheral nervous systems.The activities of the widely diverse cells, tissues,and organs of the body must be monitored, regulated,and coordinated to effectively support human life. Theinteraction of the nervous and endocrine systemsprovides the needed control.The nervous system is specifically adapted to therapid transmission of impulses from one area of thebody to another. On the other hand, the endocrinesystem, working at a far slower pace, maintains bodymetabolism at a fairly constant level.In this section, you will study the neuron, the basicfunctional unit of the nervous system. Also, you willstudy the components and functions of the differentdivisions of the nervous system. The nervous system isdivided into two major groups, the central nervoussystem (CNS) and the peripheral nervous system(PNS). Another division of the nervous system is theautonomic nervous system (ANS), which is furthersubdivided into the sympathetic and parasym-pathetic nervous systems.THE NEURONThe structure and functional unit of the nervoussystem is the nerve cell, or neuron, which can beclassified into three types. The first is the sensoryneuron, which conveys sensory impulses inward fromthe receptors. The second is the motor neuron, whichcarries command impulses from a central area to theresponding muscles or organs. The third type is theinterneuron, which links the sensory neurons to themotor neurons.The neuron is composed of dendrites, a cyton, andan axon (fig. 1-42). The dendrites are thin receptivebranches, and vary greatly in size, shape, and numberwith different types of neurons. They serve asreceptors, conveying impulses toward the cyton. Thecyton is the cell body containing the nucleus. Thesingle, thin extension of the cell outward from thecyton is called the axon. It conducts impulses awayfrom the cyton to its terminal branches, whichtransmit the impulses to the dendrites of the nextneuron.Large axons of the peripheral nerves arecommonly enclosed in a sheath, called neurilemma,composed of Schwann cells (fig. 1-42). Schwann cellswrap around the axon and act as an electrical insulator.1-36HM3F0142MYELINSHEATHAXONCYTONDENDRITESNODES OFRANVIERNEURILEMMASCHWANNCELLTERMINALBRANCHESFigure 1-42.—The neuron and its parts.
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