Reticulospinal projections to spinal motor nuclei.

As described by Brodal (5), the medial pontomedullary reticular formation is a complex brainstem region with a bewildering array of intrinsic intercon­ nections, long ascending projections to diencephalic levels and beyond, and long reticulospinal projections reaching all levels of the spinal cord. Accord­ ing to our present understanding, this region is involved in such functions as descending control of motor activity or of sensory relays, ascending control of cortical arousal, or control of autonomic activity. Each of these functions may be mediated either by fast, myelinated reticular efferent fibers or by the slowly conducting, unmyelinated aminergic systems that originate from nuclei classified (in the past, at least) as belonging to the reticular formation. Because of the complexity of reticular efferent systems, most investiga­ tions dealing with the reticular formation have focused upon a single reticu­ lar function and a restricted subset of reticular efferent pathways. This review, too, focuses on a single reticular efferent system-the fast, myeli­ nated reticulospinal projection to spinal motor nuclei-in an attempt to evaluate our current understanding of this system and to point out questions that need to be answered as we move toward a more integrative under­ standing of reticular function .