Fig. 12. A, Diagrams of the hypothesized synaptic organization and recruitment of VLF interneurons during graded stimulation of the VLF. B, Electrical responses recorded from the ventral roots during graded stimulation of the VLF (data from Fig. 9A). Three conditions are illustrated.Left, Low-threshold stimulation evoking only short-latency responses in motoneurons. Middle, An intermediate intensity of stimulation evoking long-latency responses.Right, Suprathreshold stimulation that triggers regenerative recruitment of the whole network and causes an episode to occur. The color code in A illustrates the neurons we hypothesize are recruited under the three stimulus conditions. The VLF is indicated by the parallel linessurrounded by the ellipses. The red lines(and cell bodies) indicate which axons have been activated directly by the stimulus. Notice that the number increases from left to right as the stimulus intensity is increased. Gray lines andcells are not activated directly by the stimulus. At the lowest stimulus intensity, it is assumed that the synaptic responses recorded in the ventral roots (shown in B) are mediated primarily by direct, monosynaptic contacts between the VLF axons and motoneurons. Notice in A, left, that the cell body of one neuron is antidromically activated but that this does not spread to recruit any of the other VLF neurons to which it is connected. At an intermediate intensity of stimulation (middle), several axons are directly activated (red), and these stimulated neurons synaptically activate other neurons (pink). These recurrently connected, synaptically activated neurons underlie the long-latency responses (asterisk). At the highest intensity of stimulation shown (right), three neurons are stimulated directly (red), and this results in regenerative recruitment of the whole network, and an episode occurs. InB, right, two stimulus intensities are illustrated. The lowest intensity (black trace) is just suprathreshold for an episode and evokes a long-latency response (atsmall arrow) that develops into an episode (atlong arrow). When the stimulus intensity is well above threshold (blue trace), an episode is triggered without the intervening recruitment of the long-latency response (see text for details). S, Stimulus artifact.