Slowly-conducting pyramidal tract neurons in macaque and rat

Abstract

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) (< ~10 m/s) are reported electrophysiologically. This discrepancy might reflect recording bias towards fast PTNs or prevention of antidromic invasion by recurrent inhibition of slow PTNs from faster axons. We investigated these factors in recordings made with a polyprobe (32 closely-spaced contacts) from motor cortex of anaesthetised rats (n=2) and macaques (n=3), concentrating our search on PTNs with long antidromic latencies. We identified 21 rat PTNs with antidromic latencies > 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 fastconducting 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.

Different subtypes of motor cortex pyramidal tract neurons projects to red and pontine nuclei

“…The mean conduction velocities computed here are slightly less than conduction velocities reported in mice for the pyramidal tract (8.89 ± 1.81 m/s) (Tanaka et al, 2004), as well as for corticospinal tract (13.7 m/s) (Powers et al, 2012). Nevertheless, the discrepancies in conduction velocity estimated from anatomical or electrophysiological data have been explained by a recording bias (Towe and Harding, 1970;Kraskov et al, 2020) or by the precision to measure the distance traveled by the axons. The principal sampling bias rule for electrophysiological recordings is that the size of axon determines the probability of hit, but also neuronal shapes are involved (Towe and Harding, 1970).…”

El Tracto Cortico Espinal: Perspectiva Histórica

“…Visión neurofisiología. Los tractos corticoespinales están conformados por un número promedio de 1026630 fibras nerviosas (Rivara et al), cuya mayoría se encuentran mielinizadas (Nyberg-Hansen & Rinvik), como se ha evidenciado mediante resonancia magnética (Rothwell et al, 1987) incluso en ratas y macacos (Kraskov et al, 2019). La mielinización que presentan estas fibras motoras permite que los impulsos nerviosos se conduzcan a altas velocidades (Nyberg-Hansen & Rinvik), entre 65-80 m/s (Ahmed & Wieraszko, 2012).…”