Recording an identified pyramidal volley evoked by transcranial magnetic stimulation in a conscious macaque monkey

A descending volley in response to non-invasive transcranial magnetic stimulation has been recorded from the pyramidal tract in a conscious monkey and identified by means of a collision test. The short latency of the earliest wave was inconsistent with a trans-synaptically mediated activation of pyramidal tract neurones. Considerable variability in the size of this wave was seen in response to a constant stimulus, and isoflurane anaesthetic was shown to depress it markedly. These results are consistent with direct activation of pyramidal tract neurones at a site close to the cell body.