NEURONS AND NEURON-CHAINS 



The Neuron as a Trophic Unit. All parts of a cell are interdependent, and a 

 continuous interaction between the nucleus and cytoplasm is a necessary con- 

 dition for life. Any part which is detached from the portion containing the 

 nucleus will disintegrate. In this respect the nerve-cell is no exception. When 

 an axon is divided, that part which is separated from its cell of origin and 

 therefore from its nucleus dies, while the part still connected with the cell 

 usually survives. The degeneration of the distal fragment of the axon extends 

 to its finest ramifications, but does not pass the synapse nor involve the next 

 neuron. 



It must not be supposed, however, that the part of the neuron containing the 

 nucleus remains intact, for as a result of the division of an axon important 



Motor neuron 



Synapse 



Sensory neuron 



Fig. 30. Diagram of a reflex arc to illustrate the law of dynamic polarity. The arrows indicate 



the direction of conduction. 



changes occur in the cell body. The Nissl bodies undergo solution, the cell 

 becomes swollen, and the nucleus eccentric. This phenomenon is known as 

 chromatolysis, or the axon reaction, and is illustrated in Fig. 27, B. If the 

 changes have been very profound the entire neuron may completely disin- 

 tegrate; but, as a rule, it is restored to normal again by reparative processes. 

 The nucleus becomes more central, the Nissl bodies reform and usually become 

 more abundant than before, while from the cut end of the axon new sprouts 

 grow out to replace the part of the axon which has degenerated. From what 

 has been said it will be apparent that the nucleus presides over the nutrition of 

 the entire neuron, that the latter responds as a whole to an injury of even a 

 distant part of its axon, that the changes produced by such a lesion are limited 

 to the neuron directly involved, and that nerve-fibers are unable to maintain 



