464 H. M. JOHNSON 



bution is unfavorable, the "receptiveness" of the sense-organ is 

 diminished, and its latent period is increased. 



The rate of transmission of a neural impulse along a single 

 axone is fairly constant under constant conditions and is not 

 affected by continued repetition of the stimulus, provided the 

 intervening period is not excessively short. 3 



A relatively large, and variable, portion of the time required 

 for completing a neural arc is consumed in the synapses (i.e., 

 the connections between single neurones) and especially in the 

 central synapses, where the efferent pathways for a given afferent 

 impulse are selected or blocked. If the stimulation of the sense- 

 organ is not sufficiently intense and inadequacy may be due 

 either to insufficient intensity of the stimulus or to an unfavorable 

 condition of receptiveness of the sense-organ the first afferent 

 impulse reaching a given center from the sense-organ may be 

 too weak to be transmitted across the surfaces of separation of 

 the connected neurones until it is reinforced by succeeding 

 impulses. 4 The latter may be transmitted over the same afferent 

 pathway, or over a different one. The concept of "intensity" 

 of an impulse may be formulated so as to accord with the "all 

 or nothing" principle, on the assumption that the intensity of 

 stimulation of a set of ganglionic synapses is determined, other 

 factors being constant, by the number of afferent neurones 

 excited; that the receptors belonging to the several afferent 

 neurones may have different thresholds or different latent periods; 

 that their activity is intermittent; and that the more favorable 

 the receptiveness of the sense-organ the greater will be the number 

 of receptors excited at the same instant by a given stimulus. 

 If a neural impulse is conveyed to a synapse and not discharged 

 across it, the energy may be stored for a time in the synapse 

 instead of being immediately dissipated. Thus, the afferent 

 impulse, though inadequate at first, may become adequate by 

 reinforcement. Any factor, therefore, which may tend to reduce 



'Howell, W. H.: A textbook of physiology (6th ed.). Philadelphia, W. B. 

 Saunders Company, 1915. 



4 Sherrington, C. S. : Integrative action of the nervous system. New Haven, 

 Yale University Press, 1906. 



