EXCITATION, INHIBITION, AND THE CONCEPT OF THE STIMULUS 319 



Fig. 1. Intracellular record from slow adapting stretch receptor neuron of 

 crayfish, a. Between first two arrows, the receptor muscles are stretched just 

 enough to set up irregular discharg3s. At second arrow stretch is increased and 

 maintained almost constant: note decline of overall depolarization (accom- 

 modation) and decline in impulse frequency (adaptation), b. Between first arrow 

 and the vertical line the receptor muscle is gradually stretched and then held at 

 constant length until the second arrow. Note increasing depolarization and 

 impulse frequency with increasing stretch. Note silent period and hyperpolariza- 

 tion as receptor muscle is allowed to relax to previous length. (From Eyzaguirre 

 and Kuffler, 1955; with the permission of 7. Gen. Physiol.) 



few conducted spike potentials. An abstraction of the behavior of the 

 receptor neuron is shown in Fig. 2. 



It is of great importance that the same type of response can be elicited by 

 other modes of stimulation: electric current (Turzuelo and Bullock, 1956), 

 temperature and chemicals. Figure 3 gives an example of the changes in the 

 frequency of firing of a crayfish stretch receptor neuron in response to three 

 different stretch stimulations. It can be seen that the times required to reach 

 a steady rate of firing after the onset of stimulation is the same regardless 

 of the amount of tension applied to the muscle and in spite of the fact that the 

 overall frequency increase is larger with larger tensions; it can also be 

 recognized that the duration of the silent period and the time course of 

 the recovery are proportional to the amount of previously applied tension. 

 (For a detailed description of the experiments see Florey, 1956, 1957). 



If a certain concentration of acetylcholine is applied to the neuron and 

 maintained, the frequency of firing instantly changes to a higher value and 

 then declines exponentially to a steady state. Removal of the stimulating drug 

 causes the typical silent period. Again the adaptation times are equal regard- 

 less of the amount of acetylcholine applied but the duration of the silent 

 period is proportional to the amount of acetylcholine previously applied and 

 removed. Furthermore, the adaptation times are of the same magnitude as 

 those observed as a result of mechanical stimulation. Similar observations 

 were made with temperature changes: lowering of the temperature acts as 



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