The limbic system is a convenient way of describing several functionally and anatomically interconnected nuclei and cortical structures that are located in the telencephalon and diencephalon. These nuclei serve several functions, however most have to do with control of functions necessary for self preservation and species preservation. They regulate autonomic and endocrine function, particularly in response to emotional stimuli. They set the level of arousal and are involved in motivation and reinforcing behaviors. Additionally, many of these areas are critical to particular types of memory. Some of these regions are closely connected to the olfactory system, since this system is critical to survival of many species.
Areas that are typically included in the limbic system fall into two categories. Some of these are subcortical structures, while many are portions of the cerebral cortex. Cortical regions that are involved in the limbic system include the hippocampus as well as areas of neocortex including the insular cortex, orbital frontal cortex, subcallosal gyrus, cingulate gyrus and parahippocampal gyrus. This cortex has been termed the "limbic lobe" because it makes a rim surrounding the corpus callosum, following the lateral ventricle. Subcortical portions of the limbic system include the olfactory bulb, hypothalamus, amygdala, septal nuclei and some thalamic nuclei, including the anterior nucleus and possibly the dorsomedial nucleus.
One way in which the limbic system has been conceptualized is as the "feeling and reacting brain" that is interposed between the "thinking brain" and the output mechanisms of the nervous system. In this construct, the limbic system is usually under control of the "thinking brain" but obviously can react on its own. Additionally, the limbic system has its input and processing side (the limbic cortex, amygdala and hippocampus) and an output side (the septal nuclei and hypothalamus). Most of these regions are connected by pathways that are shown in figure 31.
Hypothalmus
The hypothalamus, the primary output node for the limbic system, has many important connections. It is connected with the frontal lobes, septal nuclei and the brain stem reticular formation via the medial forebrain bundle. It also receives inputs from the hippocampus via the fornix and the amygdala via two pathways (ventral amygdalofugal pathway and stria terminalis). The hypothalamus has centers involved in sexual function, endocrine function, behavioral function and autonomic control.
In order to perform its essential functions, the hypothalamus requires several types of inputs. There are inputs from most of the body as well as from olfaction, the viscera and the retina. It also has internal sensors for temperature, osmolarity, glucose and sodium concentration. In addition, there are receptors for various internal signals, particularly hormones. These include steroid hormones, and other hormones as well as internal signals (such as hormones involved in appetite control such as leptin and orexin).
The hypothalamus strongly influences many functions including autonomics, endocrine functions and behaviors. Autonomic functions are controlled via projections to the brain stem and
