Connections of the mesencephalic locomotor region (MLR) I. Substantia nigra afferents.

These studies were designed to determine whether or not substantia nigra (SN) neurons project to the mesencephalic locomotor region (MLR). An attempt was made to activate SN neurons antidromically from the same site which induced locomotion on a treadmill following a precollicular-postmammillary transection in the same animal. Less than 10% of posterior SN neurons were activated antidromically from the physiologically-identified MLR. These results support previous anatomical findings describing a sparse projection from the SN to the MLR [5]. Locomotion on a treadmill was elicited at low current strengths (20-50 muA) from an area around the cuneiform nucleus in the posterior mesencephalon. This area included the lateral central gray, mesencephalic trigeminal root, dorsal brachium conjunctivum and nucleus tegmenti pedunculopontinus (NTPP) and perhaps anterodorsal locus coeruleus. Stimulation of the area just described induced a complete stepping cycle with a flexion phase and a three-part extension phase. Stimulation of the posterior SN produced spastic locomotion on a treadmill at higher current strengths (70 muA) in cats with a precollicular-postmammillary transection.