Study of propriospinal interneuron system in man. Cutaneous exteroceptive conditioning of stretch reflexes.

Functional characteristics of the propriospinal interneuron system have been studied in 20 patients with clinically complete chronic transverse spinal cord injury by conditioning repetitively elicited Achilles tendon jerks with noxious electrical stimuli applied to the thoracic, lumbar, and sacral dermatomes. Such conditioning stimuli caused an increase of ipsilateral and contralateral Achilles tendon jerks except when they were applied to the ipsilateral plantar surface when they caused suppression. This effect is due to inhibition of the tested extensor motoneurons by the conditioning stimuli applied to the plantar surface. More generally the observed increase of Achilles tendon jerks can be explained by excitation spread from distant ipsilateral and contralateral thoracic, lumbar and sacral segments. It is likely that this effect is mediated by the fibers of the fasciculi proprii of the spinal cord as this system survives complete transection of the spinal cord.