Figure 6.

Lack of occlusion between responses to cISMS and contralateral corticospinal stimulation indicates that the responses are not produced by collaterals of the corticospinal tract. A, Example of lack of occlusion between cISMS and cPT stimulation. Green trace represents the combined response when both are stimulated together, and the blue trace represents the expected result as a linear sum of the individual responses (red represents cISMS alone; black, cPT alone). B1, Plot of difference between the combined response and sum of individual responses versus amplitude of the EPSP evoked by cISMS. In 16 MNs, no significant occlusion was seen; in one, there was facilitation (red). B2, Same data as in B1 but plotted in order of net conditioning effect as a proportion of the expected effect. B1, B2, The error bars indicate ± 2 SD of net effect distribution (see Materials and Methods). The effect with the largest (but not significant) facilitation had SD values (±350%) that were truncated to allow visualization of the other data points. C, Example responses from a motoneuron with a clear EPSP response after cISMS stimulation (C1), but no response from cPT (C2), or iMLF (C3), and only a small response to cMLF stimulation (C4, onset latency 1.1 ms, indicated by black triangle; gray triangle represents segmental arrival of descending volley). All stimuli are given at 300 μA.