Thick caliber projections from brainstem to cerebral cortex in the snakes Thamnophis sirtalis and natrix sipedon

Abstract

Unilateral transections of the midbrain in snakes produce degeneration in a thick caliber system of fibers that effects widespread projections to the telencephalon. The anatomy of this diffuse projection system was investigated using Fink-Heimer, horseradish peroxidase, Golgi and electron microscopic preparations. The projection originates, at least in part, from a population of neurons located in the dorsolateral mesencephalic tegmentum. They have fusiform somata and dendrites with isodendritic branching patterns. The axons are of thick caliber and ascend through the forebrain bundles into the caudal, basal telencephalon. Some of the axons cross the midline around the anterior commissure. The major bundle of fibers splits into three fascicles at this level. One fascicle continues into the striatum, the second continues into the anterior dorsal ventricular ridge and the third continues into the septum and cerebral cortex. Fibers in the cortical fascicle are aligned primarily concentric with the ventricle. Some of the thick caliber fibers slant obliquely into the dorsomedial cortex and run concentric with the ventricle in layer 1. Other fibers bear thin caliber collaterals that ascend into layers 2 and 3 of the medial cortex, layers 2 and 1 of the dorsomedial cortex and layers 3 and 2 of the dorsal cortex. Varicosities are present on both the thick, parent fibers and on their thin collaterals. Electron microscopic observations suggest that the varicosities are presynaptic elements. These form synapses with clear, round vesicles and asymmetric active zones, principally on dendritic spines. This study demonstrates the existence in snakes of a thick caliber system of projections that parallels the widespread projections of monoaminergic systems to the cortex.