The effect of posterior column lesions on servo responses from the human long thumb flexor.

Lesions of the posterior column pathways, in which muscle spindle afferents run towards the brain, are associated with loss of servo responses in the long flexor of the thumb, in the absence of motor weakness and with tendon jerks preserved. This evidence is consistent with the hypothesis that the long-latency stretch reflex (on which servo responses are based) uses a supraspinal, possibly a transcortical, reflex arc.