Review of Medical Physiology - William F. Ganong 2002

Functions of the Nervous System
Neural Bases of Instinctive Behavior and Emotion
Anatomical Specification

The term limbic lobe, or limbic system, refers to the part of the Brain that forms the cortical border around the hilus of the cerebral hemisphere, along with its associated deep structures—the amygdala, hippocampus, and septal nuclei (Figs. 15-1, 15-2). This region was formerly known as the rhinencephalon (olfactory brain), although today only a small part of it is associated with the SENSE OF SMELL.

Histological Structure

The limbic cortex is the phylogenetically oldest part of the Cerebral Cortex. Histologically, it is a primitive type of cortical tissue known as allocortex, which generally exhibits a three-layered structure. The allocortex is surrounded by a transitional cortex, the juxtallocortex, which separates the allocortex from the neocortex. The transitional cortex has three to six layers and is located, in particular, in the areas of the cingulate gyrus and the insula.

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Fig. 15-1. Proportions of the limbic (allocortex and juxtallocortex) and new (neocortex) cortex in rats, cats, monkeys, and humans.

Fig. 15.2. Major connections of the limbic system. Top: Hippocampal system. Bottom: Olfactory and amygdaloid connections (reproduced with permission from Waxman SG: Correlative Neuroanatomy, 24th ed. McGraw-Hill, 2000).

The neocortex is the cortical substance of the non-limbic Regions of the hemisphere. It is composed primarily of six layers (see Chapter 11) and represents the most highly developed type of cortex. The surface area of the allocortex and juxtallocortex changes relatively little during the evolutionary development of mammals; however, these regions become largely covered by the neocortex due to its massive expansion, which reaches its peak development in humans (see Fig. 15-1).

Afferent and Efferent Connections

The major connections of the limbic system are illustrated in Fig. 15-2. The fornix connects the hippocampus to the mammillary bodies, which in turn are connected via the mamillothalamic tract to the anterior thalamic nuclei. Axons from the anterior thalamic nuclei project to the cingulate cortex, which is connected back to the hippocampus, thus completing a circuit for the Circulation of emotional arousal. This circuit was first described by Papez and is therefore known as the Papez circuit.

Structure-Function Relationship

One of the distinguishing Features of the limbic system is the paucity of its connections with the neocortex. Nauta noted that "the neocortex rides astride the limbic system like a rider on a horse, without reins." In reality, however, several such connections do exist; from a functional standpoint, neocortical activity modifies emotional behavior, and vice versa. Nevertheless, a defining characteristic of emotions is that they cannot be voluntarily turned on or off at will. Another property of limbic circuits is the prolongation of their activity after the cessation of the initial stimulus. This partially explains why emotional reactions are generally not fleeting and persist long after the stimuli that triggered them have ended.