Reflex effects of vibration in patients with spinal cord lesions

The vibration reflex was studied in 49 patients with traumatic spinal cord lesions. It was elicited in all patients, even after presumably complete division of the cord. The vibration relfex consisted of a short-latency, brief outburst of phasic activity of motor units, followed by rapidly decreasing phasic component and a later slowly declining tonic component. When periods of vibration were repeated at short intervals of 2 to 10 seconds, the responses showed an approximately exponential decline, although the beginning of each subsequent response was always larger than the end of the preceding response. A large part of this decline can be characterized as a habituation of the vibration reflex. In comparison with the vibration reflex in normal subjects, the phasic component was increased and the tonic one reduced. The tonic component was especially susceptible to potentiation and dishabituation by voluntary effort to contract the vibrated muscle, even in some patients with no other evidence that the lesion was incomplete. We suggest that the tonic component of the human vibration reflex depends, at least in part, on segmental interneurons and their descending spinal pathways, while the phasic component depends mainly on the excitability level of spinal motoneurons.