Septal efferents in the cat

Abstract

Four selected cases were studied following placement of 1–2 mm lesions in the septum of the cat. Degeneration of nerve fibers and terminals was observed in the olfactory peduncle, olfactory tubercle, gyrus rectus and overlying gyrus cinguli. Projections to posterior cortex occurred over the fornix longus and body of the fornix to the gyrus cinguli, subiculum and hippocampal gyrus. Other projections occurred downstream via three pathways. A dorsal route was traced over the stria medullaris to thalamic nuclei and midbrain. A central pathway coursed through the internal medullary lamina and zona incerta into the prerubral field. Ventrally, degeneration traveled over the diagonal band, medial forebrain bundle and ventral tegmental tract into the midbrain. The over-all degeneration preferential was to the anterior thalamic nuclei, lateral reticular nucleus of the thalamus and hippocampus. Differential projections evidenced by terminals occurred in the septum, anterior and other thalamic nuclei, hypothalamus and midbrain reticular nuclei. Degeneration was noted differentially within the columns of the fornix, diagonal band and medial forebrain bundle. These differential topographic projections of the septum are proposed as a possible structural basis for diverse effects reported in the literature concerning septum studies. The septum is considered as part of an accessory thalamus functioning within a larger system referred to as a “dynamic scale”. It is also implied that this functional concept could be true of other cerebral structures.