A Golgi study of the descending medial forebrain bundle

Abstract

Fibers from several sources in rostral forebrain travel through the lateral preoptic-hypothalamic area into the midbrain tegmentum. Along their course, the fibers give off numerous collaterals to neurons along their path. Furthermore, long collaterals from some of these fibers enter either the stria medullaris thalami or the inferior thalamic peduncle. The neurons in the lateral preoptic-hypothalamic area (termed path neurons) have dendritic trees which stretch perpendicularly across much of the MFB. These dendritic trees are not limited to the lateral preoptic-hypothalamic area but overlap extensively with those of adjoining preoptic and hypothalamic nuclear groups. The axons of the path neurons bifurcate and one axonal division travels caudally with the descending MFB and another division goes rostrally. Both divisions give off collaterals to other path neurons. Additional collaterals go into the stria medullaris, inferior thalamic peduncle, or arch medially over ventromedial hypothalamus into the periventricular fiber system. Although the long MFB fibers and their numerous collaterals converge upon the path neurons and furnish them with a certain common afferent supply, the path neurons are differentiated by (a) the amount of input received from particular forebrain and brain stem areas, and (b) the input received from the medial zone nuclei. It is suggested that the structural features of the MFB furnish the anatomical substrate necessary for the shifting patterns of autonomic activity accompanying the different behavioral states of the animal.