Discharge patterns of reticulospinal neurons corresponding with quadrupedal leg movements in thalamic cats

To analyze the discharge patterns of the reticulospinal (R-S) neurons associated with four-limb movement, we recorded the unit spikes of 108 R-S neurons in 18 thalamic cats. (1) Unit spikes of R-S neurons exhibited alternating firings during leg movements, not only stepping on the treadmill but also upon passive flexion and extension movement by the experimenter's hand. (2) R-S neurons manifested firing patterns associated with diagonal, reciprocal and quadrupedal leg movements. About half of the neurons showed reciprocal patterns upon bilateral forelimb movements; spikes were increased when the ipsilateral forelimb was in a backward position; they were decreased when that leg was in a forward position. In contrast, the spikes were increased when the contralateral forelimb was placed forward and decreased when it was backward. About 15% of the R-S neurons showed discharge patterns correlated with quadrupedal leg movements. Firing increased when the left forelimb and right hindlimb were placed backward and the left hindlimb and right forelimb were forward. In contrast, when the position of all 4 limbs was reversed, firing rates decreased. (3) When brief touch stimulation was applied to the skin around the leg, bursting spikes were obtained; these were suppressed upon touching the skin of the contralateral limb. Even after transection of the muscle nerves, alternating firings were observed. (4) Local anesthesia to the shoulder joint resulted in a marked reduction of spontaneous discharges and alternating firings. (5) Our results indicate that afferents of joints and of cutaneous origins in individual limbs ascend to the brainstem reticular formation, that integrative action is organized as pattern generation in that region, and that this patterned information is sent to the spinal cord via the reticulospinal tracts.