Superposition of antidromic responses in pyramidal tract cell clusters.

Large-seeing-distance microelectrodes were used to record simultaneously the activity of several pyramidal tract neurons in cerebral cortex. When activated antidromically, these neurons responded simultaneously, forming a "stack" of superimposed spikes, rather than responding at different times within the 10-ms time interval during which they might be expected to respond. Using a variety of spike collision tests, we found that these individual spikes arose from separate sources and reflected the activity of individual, albeit neighboring, pyramidal tract neurons. The collaterals of neurons within a stack projected to different structures, further verifying that separate neurons were involved. Such synchrony of antidromic activity among neighboring pyramidal tract neurons is an exceedingly low-probability event, if neighboring fibers conduct independently of one another. Our results imply that fibers from small clusters of neurons in the cortex assemble to form synchronously conducting bundles of fibers within the pyramidal tract.