Brain stem reticular units: some properties of the course and origin of the ascending trajectory.

Experiments were conducted in acutely prepared cats anesthetized with halothane-nitrous oxide. Single units were recorded in the mesencephalic and rostral rhombencephalic reticular core, and their ascending axons were stimulated in the mesencephalon, diencephalon, and telencephalon. The locations of stimulation and recording sites were determined by histological examination of the brains. Antidromic spikes were elicited by electrodes with a stimulus spread of approximately 300 mum and were collided with spontaneous (orthodromic) spikes to confirm antidromicity. Projecting axons were found to be scattered diffusely in the central mesencephalon in both the tegmentum and tectum. At the mesodiencephalic junction, the dorsal extent of the pathway lay in the posterior commissure and the ventral extent in the substantia nigra. More rostrally, a ventral group of axons was found in or near the supraoptic nucleus in the hypothalamus, and near the floor of the forebrain in the basal telencephalon. In agreement with previous reports, a dorsal component was located in the medial thalamus, most often in the nucleus centralis lateralis. The conduction velocity of each axon was determined, and they suggest a wide range of axon size in the ascending pathways. There may be a slight predominance of smaller fibers in the dorsal component and of larger fibers in the ventral component; however, all parts of the pathways were relatively heterogenous. The units from which projecting axons arose were found in the rostral rhombencephalon, and no units sending their axons along the restricted pathways investigated were identified rostral or caudal to this region. However, no conclusion is drawn concerning units within the region extending from 0.0 to 0.6 mm of the midline or other brain stem regions known to be monoamine-rich and to have ascending axons; these cellular populations were not examined.