Potential wave mechanisms in cat cortex

Abstract

Evidence is presented for two types of potential-wave responses of apical dendrites of cortex. The shorter-latency of these responses decreases when its afferents are stimulated at 6 per sec. or faster, and the longer-latency process increases in amplitude. The incremental sequences include the classical recruiting waves following thalamic stimulation, and similar responses from cortex to stimulation of other regions of cortex, or from stimulating locally at the lead position. Four groups or size ranges of fibers activating cortex can be differentiated. The largest fibers are those of the afferent radiations to sensory primary projection areas. The next smaller activate somewhat similar responses in association area cortex when projection cortex is stimulated. A third group or division activates the decremental short-latency responses of cortex. The fourth group activates incremental or recruiting waves. From its conduction time, the latter group is inferred to consist of unmyelinated fibers. The recruiting property appears to be a function of the excitability cycle of one type of dendritic synapse, which shows two periods of facilitation. After a first stimulus, a second is effective during the 15 msec. duration of response to the first, after which a period of depression or relative refractoriness follows. At one sixth to one tenth sec. this depression passes over into supernormal excitability. That is, the repetition of an initial stimulus causes an increased response during the supernormal phase. Spontaneously arising recruiting spindles in nembutalized or decerebrate preparations show close resemblances to stimulated recruiting trains, and the interaction of these two sequences of waves indicates that they may occur in the same elements. In fact what are apparently waves of the spontaneous spindle type may appear during recruiting responses to repetitive stimulation if the frequency of stimulation is less than the frequency of spontaneous spindle discharge. This phenomenon seems related to that of “doubling” of response to each stimulus during repetitive stimulation that finally passes over into paroxysmal afterdischarge. Certain relations between incremental and decremental sequences of waves, when stimulated at different loci but recorded at the same leads across cortex, indicate that the two types of response may be those initiated at synapses via two afferent paths of different fiber types, impinging on the same dendrites. Inferences that seem reasonable will account for the alpha rhythm and recruiting and spindle responses as processes in the same or similar elements, activated in somewhat different patterns.