NEURONAL INTEGRATIVE MECHANISMS 7 



attenuation for any given time constant of the membrane than the brief 

 spike potential of the axon can spread into the soma ; thus the soma can 

 excite repetitive discharge of the axon with a single, long-lasting synaptic 

 potential, while the resulting spikes are seen in the soma as tiny, five 

 millivolt electrotonic deflections (Fig. 3). This case also illustrates the in- 

 teraction of several factors in determining when a neuron fires. It is obvious 

 that a fixed threshold voltage across the membrane of the soma does not 

 exist ; something else interacts with voltage so that successive spikes occur 

 at smaller depolarizations. 



A somewhat similar situation may account for the activity of dendrites 

 as analyzed by Clare and Bishop (1955a,b). Dendrites in the vertebrate 

 cortex appear not to conduct all-or-none impulses toward the cell body, 

 as has been classically supposed from the law of dynamic polarity of Cajal 

 (1909). Instead slow potentials generated in dendrites and spread elec- 

 trotonically may influence the spike-initiating region of the soma. One can 

 think of two interacting regions of integration, the dendrites and the soma ; 

 in this way the vertebrate cortex differs from most invertebrate nervous 

 tissue where the soma probably plays little role and cell-free neuropiles are 

 responsible for integration (Bullock, 1952). 



The last property to which we will allude here is spontaneity. This varies 

 not only in the degree to which it is developed but also in the form by which 

 it is manifest, and the question is still unanswered whether these several 

 forms are different in underlying mechanism. Spontaneous subthreshold 

 activity seems to be of two kinds but these are possibly basically the same. 

 In some cases it is quasi-sinusoidal, in others it rises (depolarization) more 

 or less linearly to a point where it reaches a threshold and initiates a spike 

 the repolarization of which carries the membrane back to the high level 

 from which the so-called generator potential can begin again. These two 

 forms of subthreshold change differ at least superficially as a sine wave 

 oscillation from a relaxation oscillation ; that is, one form can continue to 

 go through successive cycles without an all-or-none discharge, the other 

 requires such a discharge to restart the cycle. ^ The generator potential is 

 best known in certain sense organs and in the specialized muscle cells 

 pacing the vertebrate heart, but it is also present in integrative neurons 

 which control other neurons, i.e., pacemaker interneurons in the lobster 

 cardiac ganglion. 



Spontaneous activity may be relatively rhythmic or nonrhythmic. Even 

 in the same unit a continuous spectrum may be shown between very small 

 and very large standard deviations of the intervals between spikes — the 



1 We have recently found, in spontaneous ganglion cells of the lobster cardiac 

 ganglion, that a propagated spike is not necessary to repolarize the soma to a high 

 level and restart the cycle. An active but graded form of soma potential suffices. 



