NEURONS 



431 





protective 

 sYiecctli- 



The body of the nerve cell is usually irregular in shape, and differs 

 from other cells by possessing several delicate, branched, proto- 

 plasmic projections called dendrites. One of these processes, the 

 axon, is much longer than the others and ends in a muscle or in a 

 network of endings around another nerve cell. It is not certain that 

 these two nerve cells are actually in u 



contact, but a stimulus is transmitted 

 from one cell to the other by means 

 of this network. Such a communica- 

 tion is called a synapse (si-naps'). 

 The axon forms the pathway over 

 which nervous impulses travel to and 

 from the nerve centers. _ 



A nerve consists of a bundle of 

 tiny axons, bound together by con- 

 nective tissue. As a nerve ganglion 

 is a center of activity in the nervous 

 system, so a cell body is a center 

 of activity of the neuron and may 

 send an impulse over the thin strand 

 of protoplasm (the axon) prolonged 

 many hundreds of thousands of times 

 the length of the cell body. Some 

 neurons in the human body, although 

 visible only under the compound 

 microscope, give rise to axons several 

 feet in length. 



Because some axons originate in 

 organs that receive stimuli and send 

 them to the central nervous system, 

 they are called sensory axons. Other axons originate in the central 

 nervous system and pass outward, producing movement of muscles. 

 These are called motor axons. The neurons possessing these axons 

 are either sensory or motor neurons. When neurons connect sen- 

 sory with motor neurons they are called associative neurons. 



Reflexes and their place in our lives. We have seen that 

 reflexes play a very important part in the responsive life of simple 



terminal .branches 



A neuron. Where might such a cell 

 be found? Where might the termi- 

 nal branches be ? 



