Transmission-time of Reflexes in Spinal Cord of Frog 25 



In spite of this fact the records do, however, it seems to me, afford evidence 

 that there is a region near the threshold where strength of stimulus and 

 synapse delay vary inversely, and that they at the same time suggest 

 an explanation for any discrepancy there may appear to be between such 

 observations as those of Wundt (which have been confirmed in mammals 

 b\^ Sherrington, H. Franck, and others) and mine. I have already 

 mentioned (p. 19) that in so many of the responses in this preparation (as 

 well as in certain others) the reflex electrical eflect does not become maximal 

 until some time after its first appearance ; and if we grant, as we shall 

 presently find reason for doing, that only a single set of synapses is con- 

 cerned in the reflex we are here considering, this fact, to my mind, can only 

 mean that there was great want of co-ordinate action amongst the motor 

 cells, due to variations obtaining either in the times taken to pass individual 

 synapses, or in the promptitude with which individual motor cells were 

 reacting. Whichever of these two factors determining what I call synapse 

 delay it may be, the few motor cells which first come into play would 

 control only a certain number of the fibres under the recording spots, not a 

 sufiicient number for the whole central stimulus to produce an electrical 

 eflect there even equal in amount to the small one which may generally be 

 obtained from a normal cord (to a stronger peripheral stimulus), and not a 

 suflicient number, I think we may add, to overcome the resistance of the 

 rest of the muscle, and cause the whole to contract. Only when the other 

 synapses too have been crossed and the other motor cells have reacted, are 

 all the fibres excited together (the prolongation of the discharge so charac- 

 teristic of the action of strychnine having brought it about that the cells 

 belonging to the few synapses which were passed first were still discharging 

 at the time the others began to be aflected later), and then only does the 

 electrical response attain its maximum ; and only when a sufficient number 

 of muscle fibres are brought into play to overcome the resistance of the rest 

 of the muscle would the mechanical response begin. 



Since the strength of the stimulus was the same for all, this great 

 variability in the delay at the individual synapses of the same set can only 

 have been due, it seems to me, to great variability in the thresholds, i.e. in 

 the resistances, of the separate synapses. For a few of them the stimulus 

 possesses that strength which by analogy with what is known for muscle 

 and nerve we may call " maximal " ; for the greater number it possesses 

 submaximal values. We know that in this preparation the thresholds for 

 those synapses or cells which were most readily aflected were changing 

 after the experiment had begun. It is no great assumption to make that 

 others at the end of the experiment had the same threshold which those 

 had had at the beginning, and that these others at the beginning had 

 thresholds lower than those had then. In seeking for the cause of this 

 variability we are struck by the fact that the phenomenon in our records 

 which indicates it (the long delay after the response has begun before it 



