Controlled locomotion in the mesencephalic cat: distribution of facilitatory and inhibitory regions within pontine tegmentum.

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.