Slowly-conducting pyramidal tract neurons in macaque and rat

Anatomical studies report a large proportion of fine myelinated fibres in the primate pyramidal tract (PT), while very few pyramidal tract neurons (PTNs) with slow conduction velocities (CV) ( 2.6 ms and estimated CV 3-8 m/s, and 67 macaque PTNs (> 3.9ms, CV 6-12 m/s). Spikes of most slow PTNs were small and present on only some recording contacts, while spikes from simultaneously recorded fast-conducting PTNs were large and appeared on all contacts. Antidromic thresholds were similar for fast and slow PTNS, while spike duration was considerably longer in slow PTNs. Most slow PTNs showed no signs of failure to respond antidromically. A number of tests, including intracortical microinjection of bicuculline (GABAA antagonist), failed to provide any evidence that recurrent inhibition prevented antidromic invasion of slow PTNs. Our results suggest that recording bias is the main reason why previous studies were dominated by fast PTNs.