Relation of size and activity of motor cortex pyramidal tract neurons during skilled movements in the monkey

Activity of motor cortex pyramidal tract neurons (PTNs) was recorded in monkeys making large (20 degrees), high velocity and small (1 to 2 degrees), low velocity pronation-supination arm movements in a visual pursuit-tracking paradigm. Antidromic response latencies (ADLs) or PTNs were examined in relation to PTN modulation with the large and small movements to test the hypothesis that PTNs would exhibit a "size principle" analogous to that of spinal cord motoneurons. It was found that smaller PTNs (i.e., those having longer ADLs) discharged just as strongly with small, slow movements as with large, fast movements, while about one-third of the larger PTNs (even those selected for a significant relation to small movement) discharged more intensely with the large movement. Another analysis dealing with PTNs in a selected set of penetrations in an area focal for pronation-supination showed that PTNs with longer ADLs (greater than 1 msec) were more likely to reach maximum frequency with small, slow movement. There was, however, much overlap in the behavior of small and large PTNs, and while there was a statistically significant relation between size and movement-related activity of PTNs, there did not seem to be a "size principle" in the strict sense that this term has been used with reference to spinal cord motoneurons.