342 PHYSIOLOGY 



period of a muscle excited through its nerve as compared with the 

 latent period when excited directly, and we ascribed this latent period 

 to a delay in the motor end-plate. We should expect therefore to find 

 that the delay or latent period in the case of a reflex action, i.e. the 

 lost time in the conversion of an afferent into an efferent impulse 

 in the central nervous system, would be appreciable and would 

 increase with the complexity of the response that is, with the number 

 of neurons involved in the reaction. Such is indeed the case. 



In determining the actual ' lost time ' in the central nervous 

 system for any given reflex, it is necessary to subtract from the totaj 

 delay, interposed between ^the application of the stimulus and the 

 resultant movement, the time taken by the impulse in travelling to 

 and from the central nervous system, as well as the latent period 

 of the muscles themselves. The remainder is known as the ' reduced 

 reflex time.' Wundt found in the frog, when a reflex contraction of 

 the gastrocnemius was excited by a stimulation of a posterior root 

 of the same side, that the reduced reflex time was -008 sec. 

 For a crossed reflex the delay was increased by -004 sec. If 

 we assume that one additional neuron is involved in the crossed 

 reflex, the lost time at a synapse would be -004 sec. ; if two cells 

 are intercalated, the synapse delay would be only -002 sec. Since 

 the uncrossed reflex has a delay of -008 sec., at least two, and 

 possibly four, synapses are involved in the path of this simple reflex. 



The blinking excited by stimulation of the eyelid has a reduced 

 reflex time of -047 sec. 



(3) SUMMATION. When contractile tissues, such as striated or 

 unstriated muscle, are excited by single shocks, a certain minimal 

 strength of stimulus is necessary in order to produce a contraction. 

 Weaker stimuli are spoken of as sub-minimal, and when applied singly 

 have apparently no effect on the muscle. In dealing with the 

 properties of involuntary muscle we saw that a sub-minimal stimulus 

 is not necessarily devoid of effect because it fails to evoke a contrac- 

 tion, since, if repeated at sufficiently frequent intervals, a summation 

 of stimulus occurs, so that at the fifth or sixth application a stimulus, 

 which was previously ineffective, becomes effective and a contraction 

 results. The muscle will now continue to respond to each stimulus, 

 but, if the excitations be discontinued for a time, reapplication of a 

 stimulus of the same strength becomes once more ineffective. This 

 summation of stimulus is a prominent feature in all reflex actions, so 

 much so that it may be often impossible to evoke a reaction to a very 

 strong single induction shock, whereas the application of a tetanising 

 current too weak to be felt on the tongue may produce a marked 

 reaction. We shall have occasion later on to deal with special examples 

 of this summation of stimulus. 



