136 



THE HUMAN BODY 



Neurons. The nervous system as a whole is made up of struc- 

 lures callod neurons, each of which seems to be a single nerve- 

 cell. 



A typical neuron consists of a cell-body containing a nucleus and 

 from whose surface project many rather short branching proc- 

 esses called dendrites, and a single long process having few if any 

 branches and known as the axon (Fig. 55 A). Neurons which con- 

 vey impulses to muscles (motor neurons} have this structure (Fig. 



A B C 



FIG. 54. Types of neurons. A, motor; B, sensory; C, association. 



54A). The only branching of a motor neuron is at its very end, 

 where it is distributed to the muscle fibers of which it has control. 

 The number of muscle fibers thus innervated by one motor neuron 

 varies in different muscles, ranging from a half dozen to fifty or 

 more. 



The neurons which convey impulses from sensory regions to 

 the center (sensory neurons} have a structure which appears at 

 first view, to be altogether different from that of the typical neuron 

 just described. They have cell-bodies with nuclei but instead of a 

 single axon and numerous much-branched dendrites the cell-body 

 gives rise to two long axon-like processes, one connecting with the 



