The word “limbic” means “border.” Originally, the term “limbic” was used to describe the border structures around the basal regions of the cerebrum, but as we have learned more about the functions of the limbic system, the term limbic system has been expanded to mean the entire neuronal circuitry that controls emotional behavior and motivational drives.
A major part of the limbic system is the hypothalamus, with its related structures. In addition to their roles in behavioral control, these areas control many internal conditions of the body, such as body temperature, osmolality of the body fluids, and the drives to eat and drink and to control body weight. These internal functions are collectively called vegetative functions of the brain, and their control is closely related to behavior.
FUNCTIONAL ANATOMY OF THE LIMBIC SYSTEM; KEY POSITION OF THE HYPOTHALAMUS
Figure 1 shows the anatomical structures of the limbic system, demonstrating that they are an interconnected complex of basal brain elements. Located in the middle of all these structures is the extremely small hypo thalamus, which from a physiological point of view is one of the central elements of the limbic system. Figure 2 illustrates schematically this key position of the hypo thalamus in the limbic system and shows surrounding it other subcortical structures of the limbic system, including the septum, paraolfactory area, anterior nucleus of the thalamus, portions of the basal ganglia, hippocampus, and amygdala.
Fig1. Anatomy of the limbic system, shown in the dark pink area. (Modified from Warwick R, Williams PL: Gray’s Anatomy, 35th ed. London: Longman Group Ltd, 1973.)
Fig2. The limbic system, showing the key position of the hypothalamus.
Surrounding the subcortical limbic areas is the limbic cortex, composed of a ring of cerebral cortex on each side of the brain (1) beginning in the orbitofrontal area on the ventral surface of the frontal lobes, (2) extending upward into the subcallosal gyrus, (3) then over the top of the corpus callosum onto the medial aspect of the cerebral hemisphere in the cingulate gyrus, and finally (4) passing behind the corpus callosum and downward onto the ventromedial surface of the temporal lobe to the parahip-pocampal gyrus and uncus.
Thus, on the medial and ventral surfaces of each cerebral hemisphere is a ring of mostly paleocortex that sur rounds a group of deep structures intimately associated with overall behavior and emotions. In turn, this ring of limbic cortex functions as a two-way communication and association linkage between the neocortex and the lower limbic structures.
Many of the behavioral functions elicited from the hypothalamus and other limbic structures are also mediated through the reticular nuclei in the brain stem and their associated nuclei. We pointed out in Chapter 56, as well as earlier in this chapter, that stimulation of the excitatory portion of this reticular formation can cause high degrees of cerebral excitability while also increasing the excitability of much of the spinal cord synapses. In Chapter 61, we see that most of the hypothalamic signals for con trolling the autonomic nervous system are also transmit ted through synaptic nuclei located in the brain stem.
An important route of communication between the limbic system and the brain stem is the medial forebrain bundle, which extends from the septal and orbitofrontal regions of the cerebral cortex downward through the middle of the hypothalamus to the brain stem reticular formation. This bundle carries fibers in both directions, forming a trunk line communication system. A second route of communication is through short pathways among the reticular formation of the brain stem, thalamus, hypothalamus, and most other contiguous areas of the basal brain.
