Long-loop reflexes in small hand muscles studied in normal subjects and in patients with Huntington's disease

Long-latency electromyographic (EMG) responses of the first dorsal interosseus muscle were evoked by short displacements of the index finger in healthy subjects and in patients with Huntington's disease (HD). In all 20 healthy subjects the early spinal response (mean latency 31.5 ms) was followed by a reproducible later reflex response with a mean latency of 56.5 ms. The activity pattern of single motor units of the stretched muscle was similar to that of the surface EMG. Thus all single motor units tested could be active during either the first or second response phase, but never in both in a given trial. Of the 50 patients with HD, the late EMG response was missing completely in all but 7, but the early spinal component was almost identical to that of the control group. Double stretches at an interval of 25 ms evoked two similar EMG responses in these patients, proving that the motoneuron pool is still excitable during the time at which the second response would have appeared in healthy subjects. The reflex responses of the thenar muscles evoked by electrical stimulation of the median nerve were examined during a voluntary opposing contraction of the thumb in both groups of subjects. In normal subjects, two EMG responses could be distinguished with latencies similar to those of the mechanically elicited responses. Patients with HD again lacked the second response, although the first spinal response was always present. The results are discussed with respect to different proposals concerning the origin of long-latency responses in human muscles. At least for distal hand muscles, the results suggest that the long-latency responses are long-loop reflexes.