ACTIVITY OF MEDULLARY RETICULOSPINAL NEURONS DURING FICTIVE LOCOMOTION

1. The pattern of discharge of medullary reticulospinal neurons, ident ified by antidromic stimulation applied at the L1-L2 segment of the sp inal cord, was studied during fictive locomotion, occurring spontaneou sly, or evoked by stimulation of the mesencephalic locomotor region in high-decerebrate, paralyzed cats. Unitary recordings were made in the medial reticular formation (P5.0-14.0 mm; LO.5-2.0 mm), and the ficti ve locomotor pattern was monitored by recording the electroneurogram ( ENG) of representative flexor and extensor muscle nerves from each of the four limbs. 2. In total, 117 reticulospinal neurons were recorded in 15 cats. Among these, 73.5% (86/117) modified their discharge at th e onset of locomotion. These cells were divided into three subpopulati ons: 34/86 of the cells always maintained a fixed temporal relationshi p with the activity of one of the recorded nerves (ENG-related = 39.6% ); the pattern of discharge of 42/86 cells was related to the locomoto r rhythm [(LR-related-48%)] but was not temporally correlated with any of the recorded nerves; and the remaining 10 cells increased their fi ring frequency at the onset of locomotion but remained tonic (TONIC-11 .6%). 3. Of the ENG-related neurons, 64.8% were temporally correlated to extensor nerve activity, whereas the remaining 35.2% were correlate d to flexor nerves. These neurons were either related to forelimb (55. 9%) or hindlimb (44.1%) nerves lying either ipsilateral (38.2%) or con tralateral (61.8%) to the recording site. A few neurons (n = 3; 8.8%) were related to nerve activity of more than one limb. 4. The pattern o f discharge of the LR-related neurons, although not correlated to the activity of any one recorded nerve, could be preferentially related to the locomotor rhythm in either the forelimbs (12/23) or hindlimbs (11 /23). 5. ENG- and LR-related reticulospinal neurons were intermingled in the medial reticular formation. In both cases, cells related to the forelimbs were located more dorsally than those related to the hindli mbs. It is suggested that both the ENG- and LR-related neurons represe nt a single functional population of reticulospinal neurons that is pa rt of an intrinsically organized reticulospinal system that functions to coordinate the activity of the skeletal musculature. 6. The present results show that the majority of reticular neurons projecting as far as the lumbar spinal cord are phasically modulated during locomotion, even in the absence of phasic peripheral afferent inputs. Moreover, t he complexity of the discharge patterns in paralyzed animals was found to be similar to that observed in the intact cat. It is therefore sug gested that the modulation of these neurons during normal locomotion m ight be largely of central origin. The origin of the signals responsib le for the generation of rhythmic activity in reticulospinal neurons i s discussed.