Short-interval intracortical inhibition with incomplete spinal cord injury.

OBJECTIVE

Short-interval intracortical inhibition (SICI) in leg and hand muscles was characterized in individuals with incomplete spinal cord injury (SCI) to understand how such inhibition limits corticospinal drive after spinal insult.

METHODS

We compared SICI during a voluntary contraction in 16 SCI and 14 control subjects, the latter group tested over a larger range of conditioning and test stimulus (CS and TS) intensities to best match the SCI data.

RESULTS

The average peak SICI in the tibialis anterior muscle was typically 3-4 times lower in the SCI subjects compared to controls. When matched for absolute TS intensity, in terms of maximum stimulator output, both U-shaped SICI recruitment curves were produced by similar CS intensities. SICI in the first dorsal interosseous muscle of the hand tended to be larger than in the ankle flexor.

CONCLUSIONS

Incomplete SCI reduces SICI compared to controls, but the absolute CS intensities that produce the U-shaped SICI recruitment curves are unchanged.

SIGNIFICANCE

These findings suggest that although the relative excitability profile of cortical SICI networks is unchanged after SCI, the effective inhibition of corticospinal tract output by these neurons is reduced.