324 



ERNST FLOREY 



Primary or t^-^ 



receptor 



neuron 



Secondary 

 or inter- 



Tertiary or 

 efferent 



Effector 

 cell 



i 2 



Fig. 6. Hypothetical responses of a chain of neurons to beginning (a) and termina- 

 tion (b) of stimulation of the first neuron. In sequence 1 the first neuron, as well 

 as all the others, is excitatory ; in sequence 2 ; it is inhibitory. The change in somato- 

 dendritic membrane potential and the sequence of resulting action potentials are 

 shown for each neuron, the former as they appear with intracellular recording, 

 the latter as they appear with extracellular recording. Sequence 1 represents the 

 behavior of a typical "on" element, sequence 2, that of a typical "off"' element. 



which, even after hours of exposure to Hght, respond instantly (by flight, 

 withdrawal into a shell or burrow) to a shadow (the approaching enemy). 

 This negative vision is easily understood if we assume that one of the neurons 

 (possibly the receptor neuron) is an inhibitory neuron (see Fig. 6b). Dr. 

 Hartline has given us beautiful examples of the interactions of sensory 

 neurons in the eyes oiLimulus and comparable situations are known from the 

 vertebrate eyes (Kuffler, 1953). Such interactions lead to an exaggeration of 

 the information value of spatial and temporal contrasts of light and dark. 



What applied to patterns of stimulation in eyes may well apply also to 

 patterns of activation (and inhibition) of neurons within the central nervous 

 system. Here too, inhibitory fibers may have decisive functions not only in the 

 suppression of activity in other neurons, but in the production of excitation 

 (information) whenever there is abrupt termination of inhibitory action. And, 

 conversely, we may expect that excitatory neurons can induce temporary 

 inhibition in postsynaptic cells whenever their prolonged activity ceases 

 abruptly. Such inhibition, however, would only assume information-value 

 and functional significance if it lowers the rate of firing of an already active 

 neuron or if it prevents or diminishes its activation from another pathway. 



