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The rat corticospinal tract was stimulated at the medullary pyramid and at different levels in the spinal cord (segments C2/3, T2, T12) and responses were recorded from the surface of the cerebral cortex and extracellularly from individual cortical neurones. 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 neurones. Synaptically mediated discharges with longer latency were also evoked in cortical neurones other than corticospinal neurones. At least in part these discharges probably resulted from stimulus spread to the dorsal column-medial lemniscus pathway. Corticospinal neurones were almost all between 1.0 and 1.5 mm beneath the cortical surface while synaptically excited units were at all depths greater than 0.4 mm. By stimulating at two 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 to 19 m/s (mean 11.4 +/- 2.9 m/s; S.D.). Slow axons in the pyramid (antidromic latency greater than 2.5 ms) could rarely be excited from T12. By stimulating at three 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. However, in 15% of the axons the 'thoracic' velocity was at least 25% less than the cervical. The results are discussed and related to those from previous investigations.

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