74 



The Conduction of Impulses by Nerves /4 : 3 



,---!/, = — I- 



*~*# M (~) 



V 2 =jwL I 



V-* = RI 



inductors, L, which do not impede a direct current. An a-c circuit is 

 illustrated in Figure 2. Inductors are not as frequently encountered in 



biological systems as are cap- 

 acitors. Most biological mem- 

 branes act as capacitors in an 

 equivalent electrical circuit. As 

 such, they may be charged, 

 maintaining a fixed potential 

 difference between their two 

 sides, or they may conduct a 

 rapid change in potential. These 

 ideas are applied directly to 

 neurons in Section 4 of this 

 chapter. 



In addition, most membranes 

 can generate a potential differ- 

 ence between their two sides, thereby expending chemical energy. Such a 

 generator is called an electromotive force or emf. These generator pro- 

 perties of neuron and muscle membranes are discussed in this chapter 

 and in Chapter 8, as well as in Chapters 23 and 24. 



/ 



<L> 



V" 



Figure 2. An a-c series circuit. Note in the 

 symbolism used that <f, <f , /, I , V 1} V 2 , V 3 

 are all complex numbers and e is base of 

 natural logarithms. 



3. Anatomy and Histology of Neurons 



The functional unit of the nervous system is called the neuron. It con- 

 sists of a nerve cell body, small processes called dendrites, and one large 

 process called an axon. 2 Outside of the central nervous system, many 

 of the larger axons are surrounded by a thick, fatty myelin sheath. The 

 sheath is interrupted somewhat periodically at the nodes of Ranvier. 

 Along the side of the sheath are satellite cells called Schwann cells. Some 

 axons are more than a meter long. A diagram of such a neuron is 

 presented in Figure 3. Smaller axons, although not as thickly myelin- 

 ated, are always surrounded by a lipid layer, as well as by extremely small 

 satellite cells. 



The neuron 3 is a single cell. The dimensions of the cell body, of its 

 nucleus, and of the diameters of the axons and dendrites are all typical 

 of other cells, ranging for vertebrates from 1-100 /x. The length of the 

 axon is the outstanding exception, being far longer than typical cellular 



2 Some authors use axon and dendrite to indicate direction of transmission. 

 In this text, axon and nerve fiber are used as synonyms. Some neurons have the 

 cell body in the middle of the nerve fiber rather than at one end. 



3 The satellite cells along the myelinated fibers are actually separate cells but 

 are considered part of the neuron, nonetheless. 



