Location of reticular premotor areas of a motor center innervating craniocervical muscles in the mallard (Anas platyrhynchos L.)

The supraspinal nucleus (SSp) in the mallard, which lies in the rostral spi nal cord and caudal brainstem, is a motor nucleus that forms the rostral co ntinuation of the ventral horn. It contains part of the motoneurons innerva ting the craniocervical muscles. Injections with horseradish peroxidase (HR P) and wheat germ agglutinin conjugated to HRP (WGA) in the SSp were used t o localize the craniocervical premotor neurons in the medullary reticular f ormation. A mixture of WGA and HRP (WGA/HRP) or biotinylated dextran amine (BDA) were injected in the different reticular areas to test the results. S mall numbers of craniocervical premotor neurons were found bilaterally in t he ventromedial part of the parvocellular reticular formation (RPcvm) and i n the caudal extension of RPcvm, the nucleus centralis dorsalis of the medu lla oblongata, and the gigantocellular reticular formation (RGc). In a seco nd series of experiments, WGA/HRP and BDA injections in these reticular are as were used to visualize afferent fibers and terminals in the SSp. The com bination of the two types of experiments shows that RPcvm and RGc contain m odest numbers of craniocervical premotor neurons. Because the reticular for mation also contains jaw and tongue premotor neurons and receives a variety of sensory projections, the present results suggest that the medullary ret icular formation plays a role in the coordination of complex movements (e.g ., feeding). The pattern of afferent and efferent connections of the reticu lar formation is used to redefine its subdivisions in the myelencephalon of the mallard. (C) 1999 Wiley-Liss, Inc.