EXCITATION, INHIBITION, AND THE CONCEPT OF THE STIMULUS 321 



Fig. 3. Cambants virilis. Changes in impulse frequency of slow-adapting stretch 



receptor neuron induced by sudden changes in the amount of tension (stretch) 



applied to the muscle element of the stretch receptor organ. (From Florey, 1957; 



with the permission of 7. Gen. Physiol.) 



mechanical relaxation, and from that caused by application of maintained 

 stretch or acetylchohne. 



From the similarity of the responses to the different modes of stimulation 

 one may conclude that the stimuli affect the same primary receptor structure 

 and that accommodation is the reaction to this alteration. It is impossible to 

 tell from the frequency responses of the neuron which parameter of the en- 

 vironment has been changed since the effects of any one of them simply add 

 or subtract, as the case may be. An example of the results of interaction of 

 chemical and mechanical stimulation is shown in Fig. 4. 



The results of experiments with chemical stimulation are of particular 

 interest because the substances involved are assumed to function as trans- 

 mitter substances. We may thus assume that neurons can accommodate and 

 adapt to the action of transmitters and that responses seen in stretch receptors 

 are common to chemically activated and inactivated neurons, whether they 

 are sensory, motor- or inter-neurons. 



Stimulation of a neuron by repetitive release of excitatory transmitter 

 from presynaptic nerve endings can be expected to give rise to accommoda- 



