Connections of the mesencephalic locomotor region (MLR) III. Intracellular recordings

Abstract

The responses of neurons in the area of the cat mesencephalic locomotor region (MLR) following stimulation of the entopeduncular nucleus (EN) were recorded intracellularly. At the end of each experiment a precollicular-postmamillary brainstem transection was performed and stimulation of the recording site(s) was employed to induce locomotion on a treadmill. This procedure was assumed to establish that intracellularly studied cells in the vicinity of a locomotion-inducing site were MLR neurons. About 10% of MLR neurons were found to respond to stimulation of the EN at short latencies. Stimulation of MLR efferent pathways was used to identify output neurons by antidromic activation. Very few MLR output neurons were found to receive EN projections (i.e. to respond at short latency following EN stimulation). These experiments support previous results describing a sparse projection from the EN to the MLR. This projection appears to be functionally varied (EPSP, IPSP and EPSP-IPSP responses were observed in MLR neurons following EN stimulation) and to exert its major influence on interneurons, not on output neurons, of the MLR.