CHAPTER VIII 



RELATION BETWEEN STIMULUS AND RESPONSE 



Ineffective stimulus becomes effective by repetition — Two types of response in 

 contractile animal tissues, cardiac and skeletal — Response of cardiac muscle 

 on ' all or none ' principle ; parallel case in Biophytum — In skeletal muscle, 

 increasing stimulus causes increasing response, which tends to reach a limit 

 — Parallel^ results in longitudinal and electrical response of plants — Effect of 

 superposition of stimuli — Tetanus. 



The application of stimulus to a tissue initiates a series of 

 events which culminates in the contraction of the excited 

 cells. It is easily seen that a certain minimum intensity 

 of stimulus is necessary in order to bring the excitatory 

 condition of the tissue to the threshold of response. In the 

 case of the electrical stimulation of BiopJiytum, we found this 

 minimum stimulus-intensity to be of a very definite order. 

 In the production of longitudinal response also a certain 

 minimum amount of stimulus, either electrical or thermal, is 

 necessary in order to evoke response. 



Additive effect. — A thermal shock which is singly 

 ineffective, may become effective by repetition. Below is 

 given a record which shows this. A single shock produced 

 by the closure for one second of an electrical, circuit, acted 

 on by six volts, was found to be ineffective in inducing 

 mechanical response. But when the same stimulus was 

 repeated six times in succession, it gave rise to a moderately 

 large response (fig. 44). This additive effect I also find in the 

 electric response of plants (fig. 45). And it is well known in 

 the case of animal tissues. 



In the case of contractile animal tissues, again, we have 

 two distinct types of response. The first is that of cardiac 

 muscle, which is said to be on the ' all or none ' principle. 

 That is to say, on applying a gradually increasing stimulus, 



