Comparison of input-output patterns in the corticospinal system of normal subjects and incomplete spinal cord injured patients

We have examined input-output patterns in the corticospinal system after in complete spinal cord injury. The amplitude of the motor evoked potential (M EP) to transcranial magnetic stimulation (TMS) was used to study the patter ns of recruitment, with increasing stimulus intensity, and facilitation, wi th increasing voluntary contraction, in thenar muscles of 12 patients with incomplete spinal cord injuries and 13 control subjects. The patients had a ll suffered spinal cord injury at a segmental level rostral to C8 and T1, t he segments supplying innervation of thenar muscles. The patients showed a less pronounced increase in MEP amplitude with increasing strength of TMS c ompared with the controls. Specifically, at a stimulus strength of 120% thr eshold and above, the patients showed significantly smaller MEPs relative t o the maximum ulnar nerve M-wave response than the controls. The patients a lso showed a less steep pattern of facilitation with voluntary drive. The M EP continued to increase up to 50% maximum voluntary contraction (MVC) wher eas the controls reached a plateau around 10% MVC. The results indicate tha t the patients show modified corticospinal recruitment and facilitation of the motoneurone pool. We speculate that the function of the adapted cortico spinal system after spinal cord injury might be to regulate and modulate dr ive to motoneurones originating from segmental and other descending inputs. We discuss how such a modified corticospinal system might be of functional benefit to the patients.