The  membranes  of  the  Schwann  cell  are  composed largely of a lipid-protein called myelin, which forms a myelin sheath on the outside of an axon. The myelin sheath has gaps between adjacent Schwann cells called nodes of Ranvier. Nerve cells without Schwann cells also lack myelin and neurilemma sheaths. IMPULSE TRANSMISSION When   dendrites   receive   a   sufficiently   strong stimulus, a short and rapid change in electrical charge, or polarity, of the neuron is triggered. Sodium ions rush through the plasma membrane into the cell, potassium ions leave, and an electrical impulse is formed, which is conducted toward the cyton. The cyton receives the impulse and transmits it to the terminal filaments of the axon.  At  this  point  a  chemical  transmitter  such  as acetylcholine  is  released  into  the   synapse,  a  space between   the   axon   of   the   activated   nerve   and   the dendrite  receptors  of  another  neuron.  This  chemical transmitter activates the next nerve. In this manner, the impulse  is  passed  from  neuron  to  neuron  down  the nerve line to a central area at approximately the speed of a bullet. Almost   immediately   after   being   activated,   the chemical transmitter in the synapse is neutralized by the enzyme acetylcholinesterase, and the first neuron returns to its normal state by pumping out the sodium ions and drawing potassium ions back in through the plasma membrane. When these actions are completed, the nerve is ready to be triggered again. A particularly strong  stimulus  will  cause  the  nerve  to  fire  in  rapid succession,  or  will  trigger  many  other  neurons,  thus giving a feeling of intensity to the perceived sensation. NERVES A nerve is a cordlike bundle of nerve fibers held together with connective tissue. Each nerve fiber is an extension of a neuron. Nerves that conduct impulses into  the  brain  or  the  spinal  cord  are  called   sensory nerves, and those that carry impulses to muscles and glands   are   termed    motor  nerves.   Most   nerves, however, include both sensory and motor fibers, and they are called  mixed nerves. CENTRAL NERVOUS SYSTEM The central nervous system (CNS) consists of the brain  and  spinal  cord.  The  brain  is  almost  entirely enclosed in the skull, but it is connected with the spinal cord, which lies in the canal formed by the vertebral column. Brain The brain has two main divisions, the  cerebrum and the  cerebellum. The cerebrum is the largest and most   superiorly   situated   portion   of   the   brain.   It occupies most of the cranial cavity. The outer surface is called  the   cortex.  This  portion  of  the  brain  is  also called   “gray   matter”   because   the   nerve   fibers   are unmyelinated   (not   covered   by   a   myelin   sheath), causing them to appear gray. Beneath this layer is the medulla,  often  called  the  white  matter  of  the  brain because  the  nerves  are  myelinated  (covered  with  a myelin sheath), giving them their white appearance. CEREBRUM.—The  cortex  of  the  cerebrum  is irregular  in  shape.  It  bends  on  itself  in  folds  called convolutions, which are separated from each other by grooves,  also  known  as   fissures.  The  deep   sagittal cleft, a longitudinal fissure, divides the cerebrum into two hemispheres. Other fissures further subdivide the cerebrum into lobes, each of which serves a localized, specific  brain  function  (fig.  1-43).  For  example,  the frontal  lobe   is   associated   with   the   higher   mental processes   such   as   memory,   the    parietal  lobe    is concerned   primarily   with   general   sensations,   the occipital lobe is related to the sense of sight, and the temporal lobe is concerned with hearing. CEREBELLUM.—The   cerebellum   is   situated posteriorly to the brain stem (which is made up of the pons, mid-brain, and medulla oblongata) and inferior to  the  occipital  lobe.  The  cerebellum  is  concerned chiefly   with   bringing   balance,   harmony,   and coordination to the motions initiated by the cerebrum. PONS AND MEDULLA OBLONGATA.— Two smaller divisions of the brain vital to life are the pons and  the  medulla  oblongata.  Together,  the  pons  and medulla  form  the   brain  stem   (fig.  1-43).  The  pons consists chiefly of a mass of white fibers connecting the   other   three   parts   of   the   brain   (the   cerebrum, cerebellum, and medulla oblongata). The medulla oblongata is the inferior portion of the brain,  the  last  division  before  the  beginning  of  the spinal cord. It connects to the spinal cord at the upper level of the first cervical vertebra (C-1). In the medulla oblongata are the centers for the control of heart action, breathing, circulation, and other vital processes such as blood pressure. MENINGES.—The outer surface of the brain and spinal cord is covered with three layers of membranes called  the  meninges.  The   dura  mater   is  the  strong outer layer; the  arachnoid membrane  is the delicate 1-37