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The purpose of this study was 1). to characterize the morphology of lumbar commissural neurons (CNs) with reticulospinal inputs and 2). to quantitate their activity during locomotor rhythm generation. Intraaxonal recordings at the L4-7 level of the spinal cord were obtained in 67 neurons in the decerebrate, paralyzed cat. Fourteen of them were subsequently nearly fully visualized following their intraaxonal injection with the tracer neurobiotin. All 14 were CNs with axons projecting across the midline of the spinal cord. Their somata were located mainly in lamina VIII and additionally in laminae VII-VI. Most of the lamina VIII CNs were excited monosynaptically from reticulospinal pathways. They were judged to be interneuronal CNs if they had no, or a short, rostral projection. These CNs commonly gave off multiple axon collaterals in and around their somata's segmental level. They projected mainly to laminae VIII-VII and some additionally to lamina IX. Some laminae VIII and the laminae VII-VI CNs were excited polysynaptically from reticulospinal pathways or were not excited. They were judged to be long propriospinal or ascending tract CNs because they had only an ascending axon. Most lamina VIII CNs discharged rhythmically during fictive locomotion evoked by stimulation of the mesencephalic locomotor region, exhibiting one peak per locomotor cycle. The peak was in phase with neurographic activity of either a left or a right hindlimb extensor nerve. These results suggested that lamina VIII CNs are reciprocally connected bilaterally at each segmental level. Such an arrangement suggests their participation in the generation and coordination of reciprocal and bilateral locomotor activity.

1. The contribution of postural tonus to controlled locomotion elicited by the stimulation of mesencephalic locomotor region (MLR) was studied in the acute precolicular-postmammillary decerebrate (mesencephalic) cat. 2. A microelectrode was placed in the unilateral MLR and another was placed systematically at 1-mm increments throughout the pons (H--4 to H--1O) at level ranging from P2 to P11 dorsoventrally and mediolaterally from 0 to L or R6. Depending on the general condition of the animal, stimuli through this second electrode were delivered preceding, succeeding, or simultaneous with the MLR stimulation. 3. Stimulation of the ventral part of the caudal tegmental field (P3 to P9, H--6 to H--8) increased extensor tonus of the hindlimbs, as assessed by recording muscle activity. Concomitant stimulation of this region converted MLR-elicited hindlimb stepping to coordinated four-legged locomotion and also elicited locomotion even when stimulation of the MLR alone failed to elicit locomotion. Stimulation of this ventral tegmental region alone at a larger stimulus intensity elicited spastic locomotor movements associated with a substantial increase in extensor tonus. 4. Stimulation of the lateral tegmented field surrounding the pontine locomotor region (PLR) also facilitated MLR effects, but had a relatively weaker facilitatory effect on postural tonus then stimulation of the ventrocaudal tegmental field. PLR stimulation alone was also ineffective when postural tonus was not well maintained. 5 Stimulation of the dorsal part of caudal tegmental field (P3 to P9, H--4 to H--6) in its midline dramatically decreased extensor tonus of the hindlimbs. MLR-elicited controlled locomotion was completely suppressed during concomitant stimulation of this inhibitory region. 6. These results indicated clearly that the degree of existing postural tonus greatly affects MLR-elicited locomotor movements and that an increase in postural tonus and an activation of the spinal stepping generator are not separate phenomenia.

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