110 



THE PHYSIOLOGY OF NERVE 



nervous tissue, are divided into reflexes and voluntary reactions. The 

 former are non- volitional and the latter volitional in their nature; 

 hence, any action which is performed without the intervention of the 

 will, is a reflex, while one requiring this amplification, is a voluntary 

 reaction. As this topic will be dealt with in greater detail in a later 

 chapter, it may suffice at this time to state that the production of a 

 reflex necessitates the union of at least one sensory with one motor 

 neuron. This union, however, is not effected by continuity, because 

 the distributing terminals of the former merely lie in close contact 

 with the receptive dendrites of the latter without becoming confluent. 



The place where two neighboring 

 neurons are in this way functionally 

 united is known as a synapse. Most 

 generally these synapses appear in 

 the form of short arborizations of the 

 sensory terminals around the bushy 

 dendrite of the adjoining motor cell. 

 In other cases, the distributing fila- 

 ments are prolonged into the im- 

 mediate vicinity of the neighboring 

 cell-body which they surround in the 

 form of a closely knitted reticulum. 

 In still other synapses, the sensory 

 terminals twine around the neigh- 

 boring dendrite and invest it closely 

 for some distance. ^ Attention should 

 also be called to the fact that the sen- 

 sory and motor neurons present cer- 

 tain general peculiarities which render 

 them better adapted for their manner 

 of conduction. Thus we find that 

 the cell-body of the former generally 

 occupies a central position, while that 

 of the latter is situated near the end of 

 the neuron. In fact, in certain sen- 

 sory neurons, the cell-body lies at 

 some distance from the main conducting path, this condition being 

 most clearly in evidence in the ganglia of the posterior spinal roots and 

 those of the cranial nerves. 



Under experimental conditions the reflex circuit may be stimulated 

 at almost any point, the resulting impulse being propagated from here 

 toward the axon terminals of the efferent neuron. Under normal con- 

 ditions, however, the excitation is most generally received by the 

 radicles of the afferent neuron which are modified into a sense-organ. 

 A stimulus brought to bear upon the latter gives rise to an impulse 



1 Ramon Y. Cajal, Histologie de syst. nerveux, Paris, 1909, and Barker, The 

 Nervous System and Its Const. Neurones, New York. 





SO 



Fig. 63. — Reflex Circuit. 

 SO, sensory end-organ, receptor; 

 MO motor end-organ, effector; AN, 

 afferent neuron; EN, efferent neu- 

 ron; C, center; S, synapse. 



