NEURONAL INTEGRATIVE MECHANISMS 3 



ingly confide in you my definition of this category. For present purposes 

 invertebrates shall mean any animals which an invertebrate zoologist finds 

 interesting. 



Some Properties of Units Permitting 

 OR Influencing Integration 



At the level of the single neuron we may first list a number of the prop- 

 erties or conditions which classical or recent experimental facts indicate 

 as the probable bases of the ability of the neuron to integrate incoming 

 signals. Obviously all the static and dynamic characteristics of the cell more 

 or less directly permit or determine the activity, but we shall enumerate 

 only some of them, at the same time expressing the hope that extension of 

 intracellular analysis like those of Hodgkin, Huxley, Katz, Cole, Grund- 

 fest, Eccles, and others will isolate further factors and show their degree 

 of lability and variation in junctional membranes. 



The resting potential is often thought of as a fixed character which has 

 only one value which is "normal," its maximum value. The evidence, how- 

 ever, can be construed to suggest that some synaptic regions normally 

 have a membrane potential which is less than its maximum, and can be 

 pushed either way and maintained at new levels. The level of this potential 

 affects not only the spike height but the excitability and the magnitude and 

 sign of after-potentials and of subthreshold responses. 



Spike threshold and its time course after activity, the excitability cycle, 

 require no development here beyond the reminder that we have little infor- 

 mation on these crucial properties in synaptic regions of various prepara- 

 tions and animals. Accommodation, in particular, has not been examined 

 comparatively or in junctional regions; and examination is the more 

 necessary since the recent discovery that the classical rise in threshold 

 with slowly rising stimuli does not in fact obtain in the frog axon free of 

 connective tissue. (Tasaki and Sakaguchi, 1950; Diecke, 1954). There 

 are, however, other efifects of maintained or slowly changing subthreshold 

 depolarization, as on the form and size of the spike. This is important for 

 us because of the possibility that the action of terminals, dendrites, and 

 somata may be similarly influenced (see below under subthreshold lability) . 

 There may be great differences in the minimal slopes, below which thresh- 

 old is never reached, in different types of neurons. 



Based on the distribution of thresholds or of synaptic endings in a group 

 of neurons, there can be curves of various shapes relating input (number 

 of synchronized fibers) to output (number of postunits firing). The 

 variable relation of input to output becomes of greater potential value as 

 an integrative mechanism when the output of one group of neurons is in 



