Figure 3. Disynaptic components of field potentials evoked by group II afferents in the intermediate zone after abolition of monosynaptic components.

The three top traces in A and D are averaged records of 20 successive field potentials evoked by stimulation of Q without and following conditioning stimulation of Q and DP, respectively; stimulation was either subthreshold (2T) or near-maximal (5T) for group II afferents. Bottom traces in A and D, records from the cord dorsum. B and E, records of field potentials evoked by group II afferents (rightmost parts of middle traces in A and D, expanded 2 × vertically and 12 × horizontally) are superimposed on records of potentials evoked by weaker stimuli (upper pair) and on records in which these potentials were depressed by presynaptic inhibition (lower pair). Dotted lines indicate the original latency of monosynaptic components of field potentials evoked by group II afferents and the latency of potentials concluded to be evoked disynaptically. C and F, records of field potentials evoked from Sart are similarly superimposed. The conditioning stimuli were in this case applied to Sart and Saph, as indicated. Note that the group I component of the field potential in B was enhanced and that those in C, E and F were only marginally affected by the conditioning stimuli.