Conduction velocities of corticospinal axons in the rat studied by recording cortical antidromic responses.

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Abstract
The rat corticospinal tract was stimulated at the medullary pyramid and at different levels in the spinal cord (segments C2/3 [cervical], T2 [thoracic], T12) and responses were recorded from the surface of the cerebral cortex and extracellularly from individual cortical neurons. Irrespective of the site stimulated, the earliest surface and single unit responses had frequency-following and other characteristics which indicated they resulted from antidromic invasion of corticospinal neurons. Synaptically mediated discharges with longer latency were also evoked in cortical neurons other than corticospinal neurons. At least in part these discharges probably resulted from stimulus spread to the dorsal column-medial lemniscus pathway. Corticospinal neurons were almost all between 1.0 and 1.5 mm beneath the cortical surface while synaptically excited units were at all depths > 0.4 mm. By stimulating at 2 sites, estimates of conduction velocity were obtained for single corticospinal axons. For those reaching at least as far as T12, velocities caudal to the pyramid ranged from 5-19 m/s (mean 11.4 .+-. 2.9 m/s; SD). Slow axons in the pyramid (antidromic latency > 2.5 ms) could rarely be excited from T12. By stimulating at 3 sites (pyramid, T2, T12) most axons reaching T12 were found to have similar conduction velocities in the cervical (pyramid-T2) and thoracic (T2-T12) cord. In 15% of the axons the thoracic velocity was at least 25% less than the cervial.