Reticular neuron activities associated with locomotion in thalamic cats

Unit discharges of the reticular neurons and EMGs of extensors and flexors were recorded in the 4 limbs of thalamic cats during stepping on a motor-driven treadmill to study the functional role of the spino-bulbo-spinal (SBS) reflex in locomotion. (1) Reticular neurons showed alterations of increased or decreased discharge rate depending on the phase of the step cycle; they were divided into input (ascending), output (reticulospinal, R-S) and internuncial neurons underlying the SBS reflex. (2) R-S neurons showed increased spikes from the end o the stance to the beginning of the swing phase; in this period, flexor muscle EMGS were obtained. The spikes decreased or disappeared when extensor EMGs were observed. To analyze the relationship between activity in R-S neurons and flexor muscles, R-S neuron spikes were used for triggering and post-spike EMGs were averaged. Averaged EMGs of the correlated flexor revealed a 12 and 6 ms latency for hind- and forelimbs, respectively. Most R-S neurons elicited EMGs of flexor in muscles of one ipsilateral leg, some elicited flexor EMGs of both the ipsilateral fore-and hindlimbs. (3) Input neurons of the reticular formation from spinal ascending tracts fired spikes upon touching of the skin over the correlated limb. During stepping, they showed two types of response patterns: a burst discharge appeared for the leg which was swinging and touching the belt, the other pattern was of continuous discharges at the end of stance and at the beginning of swing phases. (4) Internuncial neurons which relay impulses between the input and output neurons also showed alternate firings depending on the phase of the step cycle. Spikes could be elicited by touching the skin over the 4 limbs. (5) The relationship among the activities of the 3 groups of reticular neurons was analyzed by the post- or pre-spike averaging technique and appropriate latency responses were obtained. (6) The results indicate that flexor EMGs were excited by cutaneous afferents via the SBS reflex in cats stepping on the treadmill. The role of the SBS reflex in locomotion is discussed.