RHINENCEPHALON AND LIMBIC SYSTEM
The rhinencephalon is a term that describes quite literally the “nose” or “smell” regions of the brain. The limbic system refers to the structures and tracts involved with emotion, including memory formation, as well as autonomic and endocrine response to emotional stimuli. The terms rhinencephalon and limbic system are sometimes used synonymously, but the rhinencephalon refers to olfactory structures and related pathways. Located in the medial and inferior surface of the forebrain, these parts include the olfactory bulb, tract and striae, the anterior perforated substance, the uncus, the hippocampus, the dentate gyrus, the gyrus fasciolaris, the indusium griseum, the habenular trigone, the subcallosal area, the paraterminal gyrus, the fornix, and the amygdaloid body as direct olfactory afferents project to the amygdala. The olfactory pathway is described and illustrated in Plate 5-8.
The limbic forebrain refers to the areas that are functionally and anatomically connected structures that relate to emotion, motivation, and self-preservation. The limbic system is thought to be a major substrate for regulation of emotional responsiveness and behavior, for individualized reactivity to sensory stimuli and internal stimuli, and for integrated memory tasks. The main regions of the limbic forebrain include the hypothalamus, amygdala, hippocampus, and limbic cortex (prefrontal cortex and orbital frontal cortex). The hippocampal formation and amygdala send axonal projections through the forebrain, via the fornix and stria terminalis, respectively, to the hypothalamus and septal region. The amygdala also has a more direct pathway to the hypothalamus via the anterior amygdalofugal pathway. The septal nuclei lie rostral to the hypothalamus, and send axons to the habenular nuclei via the stria medullaris thalami.
Piriform Area. The anterior (rostral) perforated substance, the uncus, the anterior end of the dentate gyrus, and the anterior part of the parahippocampal gyrus medial to the rhinal sulcus are often referred to as the piriform area. These regions function to give perception of smell. The anterior perforated substance is continuous with the paraterminal gyrus and separated from the anterior part of the globus pallidus of the lentiform nucleus by the anterior (rostral) commissure, ansa lenticularis, and ansa peduncularis; posteromedially, it blends into the tuber cinereum.
The indusium griseum is a thin layer of gray matter spread over the upper surface of the corpus callosum. Anteriorly, it curves around the genu and rostrum to merge with the paraterminal gyri; laterally, it becomes continuous with the cortex of the cingulate gyrus; and posteriorly, it passes over the splenium to blend with the dentate and parahippocampal gyri through the narrow gyrus fasciolaris. Two slender strands of white fibers, the medial and lateral longitudinal seriae, are embedded in the indusium griseum.