matter is shaped roughly like the letter H. It establishes sensory   communication   between   the   brain   and   the spinal nerves, conducting sensory impulses from the body parts. The spinal cord may be thought of as an electric cable   containing   many   wires   (nerves)   that   connect parts of the body with each other and with the brain. Sensations received by a sensory nerve are brought to the spinal cord, and the impulse is transferred either to the brain or to a motor nerve. The majority of impulses go to the brain for action. However, a system exists for quickly handling emergency situations. It is called the reflex arc. If  you  touch  a  hot  stove,  you  must  remove  your hand from the heat source immediately or the skin will burn very quickly. But the passage of a sense impulse to the brain and back again to a motor nerve takes too much time. The reflex arc responds instantaneously to emergency situations like the one just described. The sensation of heat travels to the spinal cord on a sensory nerve. When the sensation reaches the spinal cord, it is picked up by an interneuron in the gray matter. This reception   then   triggers   the   appropriate   nerve   to stimulate a muscle reflex drawing the hand away. An illustrated example of the reflex arc is shown in figure 1-45. The  reflex  arc  works  well  in  simple  situations requiring  no  action  of  the  brain.  Consider,  however, what action is involved if the individual touching the stove pulls back and, in so doing, loses balance and has to grab a chair to regain stability. Then the entire spinal cord  is  involved.  Additional  impulses  must  travel  to the brain, then down to the muscles of the legs and arms to enable the individual to maintain balance and to hold on to a steadying object. While all this activity is going on,  the  stimulus  is  relayed  through  the  sympathetic autonomic nerve fibers to the adrenal glands, causing adrenalin to flow, which stimulates heart action. The stimulus   then   moves   to   the   brain,   making   the individual   conscious   of   pain.   In   this   example,   the spinal  cord  has  functioned  not  only  as  a  center  for spinal  relaxes,  but  also  as  a  conduction  pathway  for other areas of the spinal cord to the autonomic nervous system and to the brain. PERIPHERAL NERVOUS SYSTEM The peripheral nervous system (PNS) consists of the nerves that branch out from the CNS and connect it to  the  other  parts  of  the  body.  The  PNS  includes  12 pairs of cranial nerves and 31 pairs of spinal nerves. Cranial  and  spinal  nerves  carry  both  voluntary  and involuntary impulses. Cranial Nerves The 12 pairs of cranial nerves are sensory, motor, or mixed (sensory and motor). Table 1-3 shows the 12 cranial nerves and parts of the body they service. Spinal Nerves There are 31 pairs of spinal nerves that originate from the spinal cord. Although spinal nerves are not named individually, they are grouped according to the level   from   which   they   arise,   and   each   nerve   is numbered   in   sequence.   Thus,   there   are   8   pairs   of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves, and 1 pair of coccygeal  nerves. See figure 1-46. Spinal   nerves   (mixed)   send   fibers   to   sensory surfaces  and  muscles  of  the  trunk  and  extremities. Nerve   fibers   are   also   sent   to   involuntary   smooth muscles   and   glands   of   the   gastrointestinal   tract, urogenital system, and cardiovascular system. 1-39 HM3F0145 CYTON OF SENSORY NEURON DORSAL ROOT SPINAL NERVE VENTRAL ROOT WHITE MATTER GRAY MATTER 4 4 4 3 2 2 1 5 6 Figure 1-45.—Cross section of the spinal cord and reflex arc—arrows and numbers show impulse pathway.