There are many other interrelationships among various parts of the limbic system, only a few of which can be indicated here. The best known of these is Papez circuit. It leads from the hippocampus, via the fornix, to the mammillary body and septal and preoptic regions (Fig. 24-1). The mammillothalamic tract (bundle of Vicq d’Azyr) connects the mammillary nuclei with the anterior nuclei of the thalamus, which, in turn project to the cingulate gyrus and then, via the cingulum back to the hippocampus. The cingulum runs concentric to the curvature of the corpus callosum; it connects various parts of the limbic lobe to one another and projects to the striatum and to certain brainstem nuclei. Also, the cingulum receives fibers from the inferior parietal lobule and temporal lobe, which are multimodal association centers for the integration of visual, auditory, and tactile perceptions. It is connected to the opposite cingulum through the anterior corpus callosum.
Physiology of the Limbic System
The functional properties of the limbic structures first became known during the third and fourth decades of the twentieth century. From ablation and stimulation studies, Cannon, Bard, and others established the fact that the hypothalamus contains the suprasegmental integrations of the sympathetic and parasympathetic autonomic nervous systems. Soon after, anatomists found efferent pathways from the hypothalamus to the neural structures subserving parasympathetic and sympathetic reflexes. One such segmental reflex, involving the sympathetic innervation of the adrenal gland, served as the basis of Cannon’s theory of sympathoadrenal action, which for many years dominated thinking about the neurophysiology of acute emotion.
Following Cannon, Bard incorrectly localized the central regulatory apparatus for respiration, wakefulness, and sexual activity in the hypothalamus. Only later, the hypothalamus was found to contain neurosecretory cells, which control the secretion of the pituitary hormones; also within it are special sensory receptors for the regulation of hunger, thirst, body temperature, and levels of circulating electrolytes. Gradually the idea emerged of a hypothalamic–pituitary–autonomic system that is essential to both the basic homeostatic and emergency (“fight-or-flight”) reactions of the organism. The functional anatomy of these autonomic and neuroendocrine systems is discussed in Chaps. 26 and 27.
The impression of the leading psychologists of the nineteenth century that autonomic reactions were the essential motor component of instinctual feeling has been partially partially corroborated. It was proposed that emotional experience was merely the self-awareness of these visceral activities (the James-Lange theory of emotion alluded to in Chap. 23). The limitations of this theory became evident when it was demonstrated by Cannon that the capacity to manifest emotional changes remained after all visceral afferent fibers had been interrupted. Nonetheless, it remains true that perception of visceral activities can greatly alter the emotional state. An example is the perception of a rapid heartbeat, leading to heightened anxiety, which results in further acceleration in the heart rate.