Cutaneous reflex activity of the cat forelimb during fictive locomotion

To investigate mechanisms of rhythmic modulation of cutaneous reflex by the central pattern generator for locomotion, we made intracellular recordings from elbow flexor motoneurons, biceps brachii (Bi) and brachialis (Br), and quantitatively analyzed excitatory postsynaptic potentials (EPSPs) evoked by stimulation of superficial radial (SR) nerves during forelimb fictive locomotion, which was evoked in immobilized, decerebrate cats with the low thoracic spinal cord transected. We found that (1) in both Br and Bi, SR stimulation evoked trisynaptic EPSPs (segmental latencies were 2.10 +/- 0.39 ms for Br and 2.07 +/- 0.93 ms for Bi), (2) the SR-EPSPs of Br and Bi were rhythmically modulated with a similar pattern; the maximal and minimal EPSPs appeared in the flexor and extensor phase, respectively, (3) however, there were differences as follows. In the control state where fictive locomotion was absent, the mean amplitude of SR-EPSPs of Br (4.65 +/- 2.76 mV, n = 14) was much larger than that of Bi (1.25 +/- 1.22 mV, n = 10). During fictive locomotion, the maximum amplitude of SR-EPSPs of Br was 5.63 +/- 1.79 mV (n = 18), and the minimum was 3.82 +/- 1.69 mV (n = 18); that is, the maximum during fictive locomotion was larger or smaller than that of the control, while the minimal were always smaller. In contrast, SR-EPSPs of Bi were modulated over the control level; the maximum was 3.97 +/- 1.71 mV (n = 16) and the minimum was 2.24 +/- 1.11 mV (n = 16), both the maximum and the minimum during locomotion being larger than that of the control. These results may suggest that two mechanisms are involved in the rhythmic modulation, first, cyclic facilitation in the flexor phase, and secondly, cyclic inhibition in the extensor phase. The former was involved in both cases of Bi and Br motoneurons, while the latter would be involved only in the case of Br motoneurons.