bones return to their normal position, forcing air from the lungs. THE PROCESS OF RESPIRATION The   rhythmical   movements   of   breathing   are controlled   by   the   respiratory   center   in   the   brain. Nerves from the brain pass down through the neck to the chest wall and diaphragm. The nerve that controls the diaphragm is called the  phrenic nerve; the nerve that  controls  the  larynx  is  the  vagus  nerve;  and  the nerves that control the muscles between the ribs are the intercostal nerves. The respiratory center is stimulated by chemical changes in the blood. When too much carbon dioxide accumulates in the blood stream, causing the blood to become acidic, the respiratory center signals the lungs to breathe faster to get rid of the carbon dioxide. The  respiratory  center  can  also  be  stimulated  or depressed  by  a  signal  from  the  brain.  For  example, changes in one's emotional state can alter respiration through laughter, crying, emotional shock, or panic. The   muscles   of   respiration   normally   act automatically, with normal respiration being 14 to 18 cycles per minute. The lungs, when filled to capacity, hold about 6,500 ml of air, but only 500 ml of air is exchanged   with   each   normal   respiration.   This exchanged air is called tidal air. The amount of air left in the lungs after forceful exhalation is about 1,200 ml and is known as  residual air. THE NERVOUS SYSTEM LEARNING   OBJECTIVE:   Identify    the components and function of a neuron, recall the process  of  impulse  transmission,  and  identify the   components   and   functions   of   the   central and 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. The interaction   of   the   nervous   and   endocrine   systems provides the needed control. The nervous system is specifically adapted to the rapid  transmission  of  impulses  from  one  area  of  the body  to  another.  On  the  other  hand,  the  endocrine system, working at a far slower pace, maintains body metabolism at a fairly constant level. In this section, you will study the neuron, the basic functional unit of the nervous system. Also, you will study  the  components  and  functions  of  the  different divisions of the nervous system. The nervous system is divided  into  two  major  groups,  the  central  nervous system  (CNS)  and  the  peripheral  nervous  system (PNS). Another division of the nervous system is the autonomic nervous system  (ANS), which is further subdivided   into   the    sympathetic    and    parasym- pathetic nervous systems. THE NEURON The  structure  and  functional  unit  of  the  nervous system  is  the  nerve  cell,  or  neuron,  which  can  be classified  into  three  types.  The  first  is  the   sensory neuron, which conveys sensory impulses inward from the receptors. The second is the  motor  neuron, which carries command impulses from a central area to the responding  muscles  or  organs.  The  third  type  is  the interneuron, which links the sensory neurons to the motor neurons. The neuron is composed of dendrites, a cyton, and an axon (fig. 1-42). The  dendrites  are thin receptive branches, and vary greatly in size, shape, and number with   different   types   of   neurons.   They   serve   as receptors, conveying impulses toward the  cyton. The cyton  is  the  cell  body  containing  the  nucleus.  The single,  thin  extension  of  the  cell  outward  from  the cyton  is  called  the  axon.  It  conducts  impulses  away from   the   cyton   to   its   terminal   branches,   which transmit   the   impulses   to   the   dendrites   of   the   next neuron. Large   axons   of   the   peripheral   nerves   are commonly enclosed in a sheath, called  neurilemma, composed of Schwann cells (fig. 1-42). Schwann cells wrap around the axon and act as an electrical insulator. 1-36 HM3F0142 MYELIN SHEATH AXON CYTON DENDRITES NODES OF RANVIER NEURILEMMA SCHWANN CELL TERMINAL BRANCHES Figure 1-42.—The neuron and its parts.