The septal region is located rostral to the anterior commissure along the medial aspect of the cerebral hemispheres (eFig. 9.99). This region appears to play a role in pleasurable behaviors. Conversely, lesion studies indicate that damage to this area evokes behaviors of extreme displeasure or rage. Afferent axons to the septal area arise from the amygdala, hippocampus, olfactory tract, and monoaminergic nuclei in the brainstem (eFigs. 9.100 and 9.101). The septal area also connects to a collection of cholinergic neurons along the wall and roof of the third ventricle known as the habenular nuclei. Axons from the habenular nuclei project to the interpeduncular nucleus of the reticular formation, which is believed to play a role in the sleep–wake cycle (eFig. 9.100).
The hippocampal formation is located in the medial ventral temporal lobe (eFig. 9.102). It consists of the hippocampus, dentate gyrus, and subiculum (eFig. 9.103A and B). The hippocampal formation plays a role in memory processes such as episodic memory, short-term memory, working memory, and consolidation of memories. Input to the hippocampal formation is primarily received by the entorhinal cortex from association cortices. Because of this, it is believed that the “storage” of memories is in the association and primary cortices, not in the medial temporal lobe. Neurons from the entorhinal cortex project to the hippocampal formation by two pathways: the perforant pathway and alvear pathway. The perforant courses directly through the hippocampal sulcus to reach the dentate gyrus (eFig. 9.104B). As the hippocampus resembles the appearance of a ram’s horn, early on it was named cornu ammonis (horn of Ammon). Based on its cytoarchitecture, it was subdivided into four regions termed the cornu ammonis 1 to 4 (eFig. 9.104A). From the dentate gyrus, axons project to CA3 of the hippocampus. Axons from the hippocampus leave via the fornix or as Shaffer collaterals to reach CA1. Axons from CA1 may enter the fornix or project to the subiculum. Finally, axons from the subiculum enter the fornix or go back to the entorhinal cortex.
A second afferent pathway from the entorhinal cortex to the hippocampal formation is through the alvear pathway. Axons in the alvear pathway project directly on to CA1 and CA3 of the hippocampus (eFig. 9.104B). Similar to the perforant pathway, axons leaving the alvear pathway primarily originate from CA1 and CA3, which then project to the subiculum.
Efferent axons leaving the hippocampal formation primarily exit from the subiculum and form the fornix (Latin for “arch”), a white matter structure that arches over the ventricular system (eFig. 9.95). The fornix begins with axons exiting the hippocampus to form the alveus along the ventricular surface of the hippocampus. As the axons come together medially, they form a bundle referred to as the fimbria of the fornix. The fornix then emerges from the hippocampal formation and curves under the corpus callosum before coursing medially to run adjacent to the midline (eFig. 9.105). At the anterior commissure, the fornix divides into a precommissural fornix and postcommissural fornix to reach the nucleus accumbens, septal nuclei, medial frontal cortex, mammillary nucleus, ventromedial nucleus of the hypothalamus, and anterior nucleus of the dorsal thalamus (eFig. 9.95).