320 



ERNST FLOREY 



t 



.<:3 



I 



I 

 I 



^ 



Ac 



V 



Fig. 2. Abstraction of behavior of slow-adapting neuron of crayfish stretch 



receptor neuron in response to sudden changes in tension appUed to the muscle 



element of the receptor organ. All scales are linear, the units are arbitrary. 



excitatory stimulation. Responses opposite to those obtained with acetyl- 

 choUne are caused by appUcation and removal of Substance I, the inhibitory 

 substance isolated from crustacean inhibitory neurons. If the receptor neuron 

 is "set" to fire at a moderate rate, application of the inhibitory substance 

 causes an immediate cessation of the discharge. After a certain time the 

 neuron starts to fire again: the curve of the frequency change is identical 

 with that obtained during recovery after a silent period induced by removal 

 of excitatory stimulation (stretch, acetylchoHne). The duration of the silent 

 period is proportional to the concentration of inhibitory substance apphed, 

 and if enough is given, no recovery takes place. We may assume that accom- 

 modation to the inhibitory action proceeds but that the membrane potential 

 does not reach the firing level. When the inhibitory substance is removed, 

 the neuron responds with momentary excitation and behaves as if it had 

 received an excitatory stimulus. This response is indistinguishable from that 

 induced by instant return to the original muscle tension from a period of 



