The limbic system

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12 The limbic system

Anatomical components of the limbic system

The exact components of the limbic system remain open for debate. For our purposes in this text the following structures will be considered as components of the limbic system (Figs 12.1 and 12.2):

The olfactory bulb, tract, and cortex

The olfactory bulb and tract develop from ectodermal extensions of the anteromedial part of the primitive cerebral hemisphere. The bulb is radial in structure with six defined layers, which include, from the surface to the central core (Fig. 12.3):

The olfactory nerves (CN I) emerge as the axons of the olfactory cells of the nasal mucosa. These axons are then collected into complex intercrossing bundles, which gather into about 20 nerve axon bundles that traverse the cribriform plate of the ethmoid bone to synapse in the glomeruli of the olfactory bulb (Fig. 12.4). The olfactory epithelium is bathed in lipid substance produced by Bowman’s glands that dissolves particles in the air and makes them accessible to the receptors of the olfactory nerves.

The glomeruli of the olfactory bulb are composed of a complex of axons, dendrites, and neurons including the dendrites of external granule, mitral and tuft cells, and axons from the contralateral olfactory bulb and corticofugal fibres supplying descending modulation. The axons of the mitral and tuft cells course centrally through the olfactory tract to the anterior olfactory nucleus where some axons synapse and others project collaterals but continue with the postganglionic axons to form the olfactory stria. The axons continue posteriorly and divide into the medial and lateral olfactory stria at the junction of the anterior perforated substance, circumventing the olfactory tubercle. The lateral olfactory stria project to the anterior perforated substance, the piriform lobe of the cerebrum, the lateral olfactory gyrus, and the corticomedial group of amygdaloid nuclei. This group of structures is referred to as the primary olfactory cortex (Fig. 12.5). The medial olfactory stria give collaterals to the anterior perforated substance. Some fibres cross the midline via the anterior commissure and synapse in the contralateral anterior olfactory nucleus. These fibres are the only sensory fibres known to reach the cortex without synapsing in one of the thalamic nuclei. The entorhinal area of the parahippocampal gyrus, which forms the caudal area of the piriform lobe, is considered the secondary olfactory cortex. The secondary olfactory cortex mediates emotional and autonomic reflexes associated with smell, along with the hypothalamus. The primary and secondary olfactory cortical areas are responsible for the human perception of smell.

The hippocampal formation

The hippocampal formation is composed of a curved column of phylogenically ancient brain called the archipallium (Fig. 12.6). The hippocampal formation includes the hippocampus proper, the subiculum, and the dentate gyrus. Following the structure of the archipallium from a central point in the dentate gyrus in a radial clockwise fashion, the archipallium can be divided into three zones: the dentate gyrus, the cornu ammonis, and the subiculum (Fig. 12.7). The dentate gyrus and cornu ammonis display the three-layered cortical structure of the ancient cortices; the subiculum demonstrates a gradual shift from four layers to six layers through its length (Fig. 12.8). The hippocampal formation includes the following structures:

The hippocampus consists of the complex interfolding of the dentate gyrus and the cornu ammonis and remains superior to the subiculum and the parahippocampal gyrus throughout its length.

Afferent projections received by the hippocampus include (Fig. 12.9):

The efferent outflow from the hippocampus courses through the fornix, which is mainly composed of the axons from pyramidal neurons in Ammon’s horn and the dentate gyrus. These fibres terminate in the following structures:

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