Unusual late responses in a patient with an Arnold-Chiari malformation.

We studied a young man with spastic right hemiparesis, in whom supramaximal stimulation of the left posterior tibial nerve produced toe movements of the both feet and associated late responses in the flexor hallucis brevis muscle bilaterally. These findings indicate that, in this patient, there are central connections between peripheral afferents and contralateral alpha-motor neurons. It may be that such connections are normally present but that they are too weak in normal subjects to produce firing of the alpha-motor neurons by themselves. If so, the loss of cortical inhibition in our patient may have allowed these connections to produce movement.