Aberrant crossed corticospinal facilitation in muscles distant from a spinal cord injury.

Crossed facilitatory interactions in the corticospinal pathway are impaired in humans with chronic incomplete spinal cord injury (SCI). The extent to which crossed facilitation is affected in muscles above and below the injury remains unknown. To address this question we tested 51 patients with neurological injuries between C2-T12 and 17 age-matched healthy controls. Using transcranial magnetic stimulation we elicited motor evoked potentials (MEPs) in the resting first dorsal interosseous, biceps brachii, and tibialis anterior muscles when the contralateral side remained at rest or performed 70% of maximal voluntary contraction (MVC) into index finger abduction, elbow flexion, and ankle dorsiflexion, respectively. By testing MEPs in muscles with motoneurons located at different spinal cord segments we were able to relate the neurological level of injury to be above, at, or below the location of the motoneurons of the muscle tested. We demonstrate that in patients the size of MEPs was increased to a similar extent as in controls in muscles above the injury during 70% of MVC compared to rest. MEPs remained unchanged in muscles at and within 5 segments below the injury during 70% of MVC compared to rest. However, in muscles beyond 5 segments below the injury the size of MEPs increased similar to controls and was aberrantly high, 2-fold above controls, in muscles distant (>15 segments) from the injury. These aberrantly large MEPs were accompanied by larger F-wave amplitudes compared to controls. Thus, our findings support the view that corticospinal degeneration does not spread rostral to the lesion, and highlights the potential of caudal regions distant from an injury to facilitate residual corticospinal output after SCI

Evidence that the neural pathways involved in backward conditioning are different from those involved in forward conditioning

Effects of forward and backward conditioned-unconditioned stimulus (CS- US) intervals on classical conditioning of the flexion reflex were examined in a spinal cat preparation. A less intense conditioned stimulus (CS) was employed (activation of A-alpha cutaneous fibers) compared to that of a previous study (activation of both A-alpha and A- delta cutaneous fibers). Interstimulus intervals (ISIs) ranging from +3.0 to -3.0 sec were examined in 9 experimental groups, and results contrasted to those of an explicitly unpaired control group. The ISI of -0.25 sec produced optimal backward excitatory conditioning, paralleling the previous results. However, in contrast to the previous study, no conditioning was observed in any of the forward ISI groups. These observations support the hypothesis that backward and forward conditioning may be fundamentally different phenomena, induced by CS activation of different spinal reflex pathways: backward conditioning involves spinal reflex pathways activated by A-alpha cutaneous fibers of the CS, while forward conditioning involves spinal reflex pathways activated by A-delta cutaneous fibers of the CS.</jats:p