90 Electrical Potentials of the Brain /5 : 2 



or efferent neurons. The great majority of the units within the central 

 nervous system start and end there ; these are called interneurons. Thus, 

 many neurons form links between other neurons. As was pointed out 

 in the last chapter, one neuron may receive impulses from several 

 neurons, and it may excite or inhibit more than one other neuron. 

 Each neuron follows an all-or-none law; that is, it either is or is not 

 conducting a spike potential. This assemblage of neurons connecting 

 with other neurons is very similar in form to a complex digital computer 

 whose units are in one of two possible states. 



In addition to the spike potentials, there are also more diffuse changes 

 in electrical potential in various areas of the brain. These may also play 

 an important role in the central nervous system function, for example, 

 by altering the synaptic transmission from one neuron to the next. 

 These diffuse, slower potential changes are analogous to what one 

 might expect to find in an analog computer. They indicate the diffi- 

 culty of trying to use any electronic model for the central nervous 

 system. 



The vertebrate central nervous system is easily divided into two major 

 parts: the brain and the spinal cord. Both are surrounded by three 

 membranes, or meninges, which serve to protect the central nervous 

 system from injury. Between the various meninges are layers of cerebro- 

 spinal fluid which cushion the central nervous system from shock. 

 There are also fluid-filled chambers within the central nervous system 

 itself: four ventricles in the brain and the central canal in the spinal 

 cord. All four ventricles and the spinal canal are interconnected. 



Various nerves leave (or enter) the central nervous system. Along the 

 spinal cord, a pair of nerves passes between each pair of vertebrae. 

 These supply sensory, motor, and autonomic fibers to all parts of the 

 body other than the head. In addition, 12 pairs of nerves originate in 

 the brain itself. 



The spinal cord and brain consist of white matter and gray matter. 

 The white color is due to the myelin around the large nerve fibers; the 

 white matter is made up of fiber tracts. The gray matter contains most 

 of the cell bodies. Some of these are arranged in compact volumes 

 referred to as nuclei. Many nuclei can be associated with specific 

 functions or actions, such as control of respiration, or conducting impulses 

 from muscular proprioceptors, and so forth. However, the over-all 

 action of the nervous system, particularly with respect to subjective 

 phenomena as thinking or memory, is still in the realm of specula- 

 don. 



Figure 1 shows the structure of a medial section through the human 

 brain. The portion of the brain joining the spinal column is called the 

 brain stem. In lower vertebrates, as fishes, there are two small bumps 



