An attempt has been made to elucidate how direct stimulation of the mesencephalic locomotor region (MLR, with Horsley-Clarke coordinates P2, L4, and H0) is transmitted through the pons to the spinal cord where a stepping generator is presumed to exist. 2. A longitudinal strip, termed the "pontine locomotor region" (PLR), was identified. It extends ventrocaudally throughout the lateral pontine tegmentum (P3-P9, L4 and about 2 mm beneath the floor of the IVth ventricle). 3. Stimulation of this locomotor strip at P4-5 and P8-9 levels generated hindlimb stepping or four-legged locomotion on a treadmill similar to that elicited by MLR stimulation. However, "PLR stepping" was more often accompanied by spasticity of the hindlimbs. Stimulation of the pontine strip at the P6-7 level produced stepping accompanied by an opening of the mouth. 4. Subthreshold MLR stimulation together with subthreshold PLR stimulation generated locomotion. Ipsilateral and contralateral MLR-PLR stimulations were of equal effectiveness for the generation of locomotion. 5. Stimulation of rostral (P3-6), but not caudal (P6-9), parts of the PLR evoked field potentials in the MLR with two negative components. The points at which these potentials were evoked with minimum current were usually coincident with the best points for eliciting locomotion. Short-latency monophasic negative potentials were evoked in the rostral part of the PLR by MLR stimulation. 6. Locomotion elicited by stimulation of either the MLR or the PLR was suppressed by stimulation within a midpontine region, 1.5-2.0 mm beneath the floor of the IVth ventricle (P6-7, L0-0.5, H-5 to -6). Stimulation applied to the close vicinity of this "inhibitory" region did not evoke field potentials in the MLR. 7. In some animals stimulation between the inhibitory region and the underlying PLR could facilitate locomotion elicited by MLR stimulation, although no stepping was produced by such stimulation alone.
