Terminal patterns of the tegmental afferents in the interpeduncular nucleus: a Golgi study in the mouse

The intranuclear course, distribution and termination of the tegmental afferents in the interpeduncular nucleus (IP) were studied in the mouse by means of the rapid Golgi method. Primarily on the basis of terminal branching patterns and distribution areas, two types of afferents were distinguished. The type 1 fibers distribute mainly within the rostral half in the form of numerous glomerular endings, the size of which corresponds well with that of the tufted terminal dendrites of the IP neurons. On the other hand, the caudal half of the IP has far fewer fibers than the rostral IP and is innervated by the type 2 fibers, which follow a tortuous course, terminating in dense fiber plexus. Thus, the rostral and caudal IP are innervated in a different fashion by different afferents originating from tegmental regions. These results are discussed in relation to the distribution patterns of another conspicuous afferent system of the IP, the fasciculus retroflexus.