Terminal patterns of the fasciculus retroflexus in the interpeduncular nucleus of the mouse: a Golgi study.

The courses and terminal patterns of the fasciculus retroflexus (FR) in the interpeduncular nucleus (IP) were studied in the mouse, using the rapid Golgi method. Mainly on the basis of the distribution areas and terminal patterns, the FR fibers are divided into two types. The type 1 FR fibers are coarse in contour and take zigzag courses to distribute throughout the entire rostral half and core region of the caudal IP. In contrast, the type 2 fibers are fine, travel caudally along the lateral boundary of the IP and terminate in the lateral regions of the caudal half, forming a dense fiber plexus. The distribution areas of the type 1 and type 2 fibers are clearly differentiated from each other, from the cytoarchitectural as well as the fibroarchitectural viewpoint. Thus, the type 1 and type 2 FR fibers form different fiber systems in the IP. These results are discussed in the light of the known hodological, histochemical and ultrastructural studies.