The mesencephalic locomotor region. II. Projections to reticulospinal neurons.

Single neurons were recorded extracellularly in an area of the medioventral medulla known to receive descending mesencephalic locomotor region (MLR) input. A large number (47%) of these cells were found to receive short-latency orthodromic input following stimulation of the physiologically identified MLR. Of the medioventral medulla neurons studied, 34% were found to project to the spinal cord (determined by antidromic activation following stimulation of the ventrolateral funiculus). Approximately one-half (17% of the total population) of these reticulospinal cells were found to receive short-latency orthodromic input from the MLR. The regional distribution of this group of reticulospinal cells corresponded with the area found to receive descending projections from the MLR in previous anatomical studies. In addition, electrical and chemical activation of the same region was found to elicit locomotion on a treadmill in a companion study. The present findings demonstrate that descending MLR projections influence a large number of medioventral medulla cells, some of which have direct spinal projections, and suggest that this area is a primary relay in the manifestation of MLR function.