NEURONS AND NEURON-CHAINS 



49 



a true network. The fibrillge can be traced to the terminations of the dendrites 

 and axons. They have been looked upon by many as the chief elements in- 

 volved in the conduction of the nerve impulse. 



Other elements such as pigment granules may be present. Mitochondria 

 have been described in nerve-cells by Cowdry (1914) and Rasmussen (1919). 



Interrelation of Neurons. In the 

 ccelenterates, as we have learned, a single 

 nerve-cell may receive the stimulus and 

 transmit it to the underlying muscle. 

 But in vertebrates the transmission of a 

 nerve impulse to an effector requires 

 a chain of at least two neurons, the im- 

 pulse passing from one neuron to the next 

 along the chain. One of the most im- 

 portant problems in neurology, there- 

 fore, is this: How are the neurons re- 

 lated to each other so that the impulse 

 may be propagated from one to the 

 other? The place where two such units 

 come into such functional relation is 

 known as a synapse. In a synapse the 

 axon of one neuron terminates on the 

 cell body or dendrites of another. Func- 

 tional connections are never established 

 between the dendrites of one neuron 

 and the cell body or dendrites of an- 

 other. In Fig. 29 the axon of a basket 

 cell of the cerebellum is seen giving off 

 collaterals which terminate about and 

 form synapses with the Purkinje cells. 

 Another type of synapse is illustrated in 

 Fig. 70. 



The processes of one nerve-cell are not directly fused with those of others, 

 but, on the contrary, each neuron appears to be a distinct anatomic unit. At 

 least the most detailed study of Golgi and Cajal preparations, in which the 

 finest ramifications of dendrites and axons are stained, has failed to demon- 

 strate a structural continuity between neurons. In especially favorable material 



Fig. 28. Neurofibrils in a cell from the 

 anterior gray column of the human spinal 

 cord: ax, Axon; lii, interfibrillar spaces; n, 

 nucleus; x, neurofibrils passing from one 

 dendrite to another; y, neurofibrils passing 

 through the body of the cell. (Bethe, Hei- 

 denhain.) 



