Medial forebrain bundle and stria medullaris thalami in rabbits.

Abstract

Summary 1. At the superior mesencephalic level a small part of the medial forebrain bundle sweeps dorsalward, following the fibrae rectae, to enter the ventral part of the central gray substance. Another small part of the bundle crosses and reaches the opposite region of the medial forebrain bundle. 2. The most massive root of the medial forebrain bundle in rabbits curves steeply dorsalward at the caudal level of the interpeduncular nucleus and, after passing through the medial longitudinal fasciculus, terminates in a confined region of the central gray substance just caudolateral to the trochlear nucleus and around the root of the trochlear nerve. The same region of the contralateral side also receives a lesser amount of degenerating fibers, most of which cross through the regions dorsal and ventral to the medial longitudinal fasciculus. Such a type of termination of the medial forebrain bundle can be observed remarkably in cases with lesions in the olfactory tubercle and lateral preoptic region, less remarkably in the case with a lesion in the anterior part of the amygdaloid body and very few, if any, in the case with a sucked cortex. The medial forebrain bundle in a cat, however, can be traced only to the tegmental region ventrolateral to the medial longitudinal fasciculus. 3. Axons degenerated by lesions in the olfactory tubercle, lateral preoptic area and anterior part of the amygdaloid body terminate via the inferior thalamic peduncle and the lateral part of the thalamic medullary stria in the lateral habe-nular nucleus. A moderate number of degenerating axons in the lateral part of the medullary stria terminate in the dorsomedial part of the nucl. medialis medi-us (Ando) after traversing the habenular nucleus and the original part of Meynert's fasciculus. A small, but definite number of degenerating axons in the inferior thalamic peduncle descend, without joining the medullary stria, through the medial part of the thalamus and enter the ventromedial part of the nucl. medialis medius from the ventromedial aspect of this nucleus. Such a type of termination of degenerating axons in the nucl. medialis medius can be remarkably seen in cases with lesions of the olfactory tubercle and preoptic region.