The Limbic System and Its Connections with the Hypothalamus

The neural continuum in which the hypothalamus is central is composed of a part of the brain stem and the limbic system. Part of the brain stem, the mesence-phalic reticular formation, which receives inputs from spinoreticular fibers, possesses axons that ascend to the hypothalamus. There are also connections formed by axons directed upward to the hypothalamus from the nucleus of the solitary tract, a cell group in the medulla oblongata. These connections are quite revealing. The nucleus of the solitary tract is the only known case of a circumscribed secondary sensory cell group whose primary sensory input is from the visceral domain.

A second part of the continuum in which the hypothalamus is central lies rostral to it. It is largely interconnected with the phylogenetically primitive cortical tissue that surrounds the upper brain stem. A little more than a century ago, P. Broca observed an almost annular ring of tissue on the medial face of the cerebral hemisphere that represents the free edge of the cerebral cortex. This part of the brain, called le grand lobe limbique (the great limbic lobe) by Broca, has led to the concept of the limbic system. This "lobe" surrounds the diencephalon and the cerebral peduncles. He called it limbique from the Latin limbus because he conceived it as a threshold to the newer pallium. It is sometimes called the rhinencephalon to indicate that these regions of the brain derived during the course of evolution from structures previously associated with the sense of smell. However, the fact that the rhinencephalon is highly developed in animals such as the dolphin and man, in whom the sense of smell is nonexistent or limited, shows that it participates in other activities.

We shall retain only its key-ring structure opening upwards toward the neocortex and downwards toward the brain stem. Like the limbo of Christian mythology, the limbic system is the intermediary between the neomammalian brain heaven (represented by the neocortex) and the reptilian brain hell (including the reticular formation and the striate cortex). The limbic lobe includes the parahippocampal, the cingulate, and the subcallosal gyri. It also includes the underlying cortex of the hippocampal formation, which is composed of the hippocampus, the dentate gyrus, and the subiculum.

In the 1930s, it became evident to J. W. Papez that the limbic lobe formed a neural circuit that provides the anatomical substratum for emotions. He proposed that the hypothalamus is connected with higher cortical centers since it plays a crucial role in the expression of emotion. According to this idea, the neuronal circuit originally proposed by Papez consists of the cortex, which influences the hypothalamus through connections of the cingulate gyrus to the hippocampal formation. Information is then processed by the hippocampal formation and projected to the mammillary bodies of the hypothalamus by way of the fornix. The hypothalamus in turn provides information to the cingulate gyrus through a pathway from the mammillary bodies to the anterior thalamic nuclei and from the anterior thalamic nuclei to the cingulate gyrus (Fig. 1). P. MacLean's resynthesis of Papez's theory of emotions resurrected Broca's concepts and breathed new life into the concept of the all-pervasive limbic system. He included in the limbic system other structures anatomically and functionally related to those described by Papez, including parts of the hypothalamus, the septal area, the nucleus accumbens, neocortical areas such as the orbitofrontal cortex, and the amygdala (Fig. 1).

It is also noteworthy that all of the senses represented in the neocortex—vision, hearing, and the somatic sense—direct part of their information toward either one or both of two cortical districts: the frontal association cortex and the inferior temporal association cortex. The two are interconnected by a massive fiber bundle called the uncinate fasciculus. In turn, the inferior temporal cortex projects to the entorhinal area. The entorhinal area could be

Limbic System Pathways
Figure 1 The neural pathways for emotion. The first circuit proposed by Papez is indicated by thick lines, whereas thin lines illustrate recently described connections.

considered as a cortical gateway for projections to the amygdala. In fact, in primates it gives the amygdala its single most important input. The projection is reciprocated; indeed, the amygdala directs its cortical projections to the inferior temporal cortex and to the frontal cortex (specifically the orbital surface of the frontal cortex). Therefore, the amygdala projects to the parts of the neocortex in which the final stages of the cascade of sensory information occur. Evidently, the amygdala also screens its neocortical input. Therefore, it has been tempting for many scientists to speculate that such a brain region could intervene in ideation and cognition. Ordinarily, one thinks of brain function as working inwards, i.e., sensory information being directed from sensory receptor organs over a sequence of synapses to the sensory cortex and from there (in what Papez called ''the stream of thought'') toward the limbic system. Here, we encounter the opposite: a set of connections directed outward. It is indeed as if the amygdala were participating in the brain's appreciation of the world.

The interoceptive and exteroceptive data reaching the neural continuum in which the hypothalamus is central are clearly distinguishable. The former consist of visceral sensory signals from the spinal cord and the brain stem. These data are unconditional stimuli pertinent to the maintenance of life. On the other hand, what enters the limbic system from the neocor-tex is fundamentally different. One might call it a repeatedly preprocessed, multisensory representation of the organism's environment. In this situation, the perception of the world is only biased by physiological needs.

It is also remarkable that among all the senses, olfaction possesses a particular link with the limbic system that was taken to be the ''nose-brain''. Today, it is clearly established that the primary olfactory cortex projects to the entorhinal area, which in turn contact the hippocampus. Thus, after years of fervent affirmation followed by years of fervent denial, the idea that the hippocampus receives olfactory signals was reintroduced. Indeed, the pathway that links olfaction with the limbic system is privileged. Hence, the path from the olfactory epithelium is more direct than the path from sensory surfaces such as the skin. Moreover, the primary olfactory cortex projects to the amygdala, in large part onto a particular cell group (the lateral nucleus of the amygdala), by bypassing the neocortex (Fig. 1). However, although it is clear that the main olfactory bulb (the first central relay for olfaction) projects to the amygdala in rodents, one wonders whether this connection is still present in humans. Indeed, the existence of a specialized area of the nasal mucosa called the vomeronasal organ, which sends information to a compartment of the accessory olfactory bulb, has been demonstrated in animals such as rats. The vomeronasal organ and the corresponding region of the accessory olfactory bulb are thought to form an apparatus dedicated to the processing of sexually significant odors, but in the fully formed human body none of these structures have been identified. Finally, to emphasize the privileged link between olfaction and the limbic system, it has to be mentioned that the primary olfactory cortex also projects to the hypothalamus.

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