EXCITATION, INHIBITION, AND THE CONCEPT OF THE STIMULUS 323 



time 



U-4- 



2 a 



-*■ time 



Fig. 5. Hypothetical responses of a neuron to excitatory (sequence 1) and inhibi- 

 tory (sequence 2) transmitter and to the active or passive removal of the trans- 

 mitter. The first line in each diagram represents the nerve action potentials, the 

 second line the changes in membrane potential and the third line represents 

 appearance and disappearance of the time course of appearance and disappear- 

 ance of transmitter. The arrows point in the direction of increasing membrane 

 potential and increasing amount of transmitter, respectively, a, b and c represent 

 increasing time courses of disappearance of transmitter. Rebound excitation (2) 

 and rebound inhibition (1) diminish with increasing time course. It is assumed 

 that the transmitters are released by presynaptic stimulation. The amounts repre- 

 sented in the diagram are taken as the average concentrations maintained between 

 the individual releases of transmitter quanta, assuming a decay time greater than 

 the interval between presynaptic impulses. 



spikes. As soon as the stimulation which gives rise to the inhibitory trans- 

 mitter, and as soon as the action of this transinitter stops, rebound excitation 

 results and the end of stimulation is signaled along the chain of excitatory 

 neurons in the form of information-carrying conducted spikes. Using the 

 terminology of receptor physiology we can call the first-mentioned response 

 type an "on"-response and the second type an "off"-response. The advantage 

 of the off-response lies in the fact that it is effective regardless of accommoda- 

 tion and adaptation that may have set in in the presynaptic element(s). 

 Immediately obvious examples for the significance of such a system are the 

 photoreceptors which give rise to the shadow reflexes in many animals. There 

 are animals which show no response to an increase in light intensity but 



