Cells of origin of medullary projections in central gray of rat mesencephalon.

1. Seventy-four neurons in the central gray of the mesencephalon were antidromically activated by electrical stimulation of the gigantocellular and magnocellular nuclei region of the ipsilateral medullary reticular formation in urethan-anesthetized female rats. An additional eight cells were located contralaterally. With control of the lordosis reflex (facilitated from central gray) in mind, each cell was characterized by the properties of its antidromic response and by its response to pressure stimulation of the hindquarters of the hindquarters of the animal. 2. The antidromically activated spikes had constant latencies between 1.0 and 25.5 ms, with a mean of 10.7 +/- 0.7 ms. 3. Cells of origin of the descending projections were located in all parts of the central gray and adjacent mesencephalic reticular field except within the inner ring of the central gray that surrounds the cerebral aqueduct. A particularly dense group of cells was noted in the ventrolateral aspect of the central gray. Fewer were found in the dorsal central gray over the cerebral aqueduct. 4. These cells had very low rates of spontaneous discharge or none at all. No specific changes in activity could be elicited by lordosis-relevant somatosensory stimuli. Therefore, these cells appear not to be involved in stimulus-bound reflexive control of lordosis behavior. Instead, they may be engaged in mediating influences descending from hypothalamus, known to control lordosis (see Ref. 48). In turn, they could modulate activity in reflex arcs completed at lower levels of the neuraxis.