Figure 1.
Latency of the interlimb reflex.
When stimuli were applied in 1-s bursts at different frequencies, a reflex response was always seen in the right thenar EMG following the first stimulus. Each panel shows the response to stimulation for 1 s on an expanded panel where the upward-going stimulus artefact can be clearly seen in response to every stimulus, even when there is no motor response. Unrelated EMG responses can be seen, most clearly at 2 Hz where there is only one reflex response. The inset panel below shows the stimulus-triggered average EMG response from all 585 stimuli. The onset latency did not change with the rate of stimulation. The initial peak is the stimulus artefact (slightly distorted due to digitization error).
Figure 2.
Averaged EMG response to 1-s stimulus trains at different frequencies.
EMG has been rectified and smoothed with a moving 1.5-ms window. Dashed vertical lines indicate the duration of the 1-s stimulus train. A: Average of 20 trains of stimulation at 28 Hz lasting for 1 s. There were sporadic “fasciculation” potentials before and after the train but no self-sustained activity in the thenar muscles. B: Stimuli were delivered for 1 s in 21 trains at different frequencies of 1-100 Hz. Sustained activity is evident in wrist extensors but not in the thenar EMG. Data recorded in two different experiments.
Figure 3.
Self-sustained activity in response to 1-s trains of stimuli at different frequencies.
All data were recorded within a 10-min period. Although 28 Hz produced the most consistent and longest responses, the amplitude and duration of the response, even at the same frequency varied from trial to trial. Responses were typically shorter at frequencies above 28 Hz. Data are shown in the order in which they were obtained, but some trains have been omitted.
Figure 4.
A: Stimulus train duration. The greatest responses, both in duration and magnitude, were obtained with trains lasting 1 s. Longer train durations (right) commonly curtailed the self-sustained activity in all muscles studied. Note the consistently later response in the left wrist extensors. Dashed vertical lines indicate the duration of the stimulus trains. B–C: Responses to different stimulus patterns (see refs 14,16). Self-sustained activity was evident in the right wrist extensors with both stimulus patterns, but not in the right thenar muscles. A: 25 Hz for 2 s then 100 Hz for 2 s then 25 Hz for 3 s, C: ramp stimulus, increasing from 1 Hz to 100 Hz over 5 s and then decreasing to 1 Hz over 5 s.
Figure 5.
A: The effect of voluntary effort. The subject could make weak voluntary contractions of both wrist extensors but could not grade the strength of the contractions. Isolated contraction of either the right (second panel; note that voluntary EMG precedes stimulus train) or the left (third panel; note that voluntary EMG just precedes stimulus train) wrist extensors did not greatly alter the response. When the patient attempted to contract the paralysed right thenar muscles (panels 4-6), the voluntary effort did not produce EMG in the paralysed right intrinsic muscles, but the self-sustained activity in the left wrist extensors was completely suppressed. The final three voluntary efforts were recorded in successive trials. B–C: Transcranial magnetic stimulation did not inhibit self-sustained activity in the wrist extensors. At rest (B) a small motor evoked potential was elicited in the paretic right wrist extensors by transcranial magnetic stimulation but no other changes were noted. When delivered during the stimulus train to the lower limb, there was no silent period in the train-induced EMG. The patient then performed a voluntary contraction of the wrist extensors beginning before stimuli were delivered (C). A silent period can be seen when the stimulus preceded the 1-s burst of stimuli to the left leg, but not when it was delivered during the burst. There was a period of reduced activity in the wrist extensors only when the transcranial magnetic stimulation coincided with the onset of peripheral stimulation, and not when it was delivered later during the train. The motor evoked potential is illustrated in the inset panel, preceded by the stimulus artefact.