The Limbic System

The meaning of the term “limbic system” has changed since Broca’s time. It is still meant to include structures between the cortex and the hypothalamus and brainstem, but different specialists have included different structures as part of the limbic system. The amygdala and hippocampus are widely included, as is the olfactory cortex. From there, however, opinions diverge as to what is considered part of the limbic system, and what is paralimbic, meaning a structure that interacts closely with the limbic system but is not truly part of it.

The limbic system serves a variety of fundamental cognitive and emotional functions. The hippocampi, which lay on the inside edge of the temporal lobes, is essential to memory formation. The amygdalae sit on top of the front portion of each hippocampus. Each amygdala is thought to be important in processing emotion. The amygdala communicates closely with the hippocampus, which helps explain why we remember things that are more emotionally important. The amygdala also communicates closely with the hypothalamus, the area of the brain that is responsible for regulating temperataure, appetite, and several other basic processes required for life. The hypothalamus itself is sometimes, but not always, included as part of the limbic system. Through the hypothalamus, as well as some key areas in the brainstem, the limbic system communicates with our autonomic nervous system (which regulates things like heartbeat and blood pressure), endocrine system, and the viscera (or “gut”).

Nerve cells in the brain are organized in different fashions depending on location. The cerebral cortex is predominantly neocortical, meaning that cells exist in 6 layers. This is different from the limbic system, where cells are either arranged in fewer layers (e.g. paleocorticoid), or more jumbled (corticoid). This less complex organization of the limbic system, as well as the limbic system’s control of fundamental processes of life, has led doctors to believe that the limbic structure is evolutionarily older than the cerebral cortex.

The paralimbic structures form a complex network with the limbic system. Examples of paralimbic structures include the cingulate gyrus, orbitofrontal cortex, temporal pole, and part of the insula. The basal forebrain, nucleus accumbens, mammillary bodies and parts of the thalamus (the anterior and mediodorsal nuclei) are also often considered paralimbic structures due to their close interaction with the limbic system.

The side of the hypothalamus that is closest to the outside of the brain (the lateral side) has the opposite effect on appetite. Because this area is important in stimulating appetite, lesions in this area can lead to severely decreased body weight. This area is also important in thirst, as lesions of the more frontal part can lead to decreased water intake. Functionality of the hypothalamus is also divided from front to back. For example, anterior parts of the hypothalamus seem more involved with cooling the body off by increasing blood flow to the skin and causing sweat to be produced. The back of the hypothalamus is more involved with keeping the body warm. In addition, the hypothalamus is responsible for regulating our natural cycle of wakefulness and sleep. The suprachiasmatic nucleus at the front of the hypothalamus serves as our internal clock, letting us know when it’s bedtime. This part of the brain is connected with light sensitive regions that adjust our internal clock to daylight.

How Does the Hypothalamus “Talk” to the Body?

The hypothalamus modulates physical responses by communicating with the body through two routes. The first route is through the autonomic nervous system. The second is through the endocrine system, meaning the secretion of hormones into the blood stream. Autonomic fibers primarily come from the paraventricular nucleus of the hypothalamus, but also from the dorsomedial hypothalamic nucleus and from the lateral and posterior hypothalamus. Initially, these autonomic fibers travel in a white matter path called the medial forebrain bundle. They then pass on into the dorsolateral brainstem and periaqueductal gray matter. The fibers synapse on parasympathetic nuclei in the brainstem and intermediate zone of the sacral spinal cord, and on sympathetics in the intermediolateral cell column of the thoracolumbar spinal cord. Many autonomic nuclei in the brainstem receive inputs from hypothalamus, such as the nucleus solitarius, noradrenergic nuclei, raphe nucleus, and pontomedullary reticular formation.

The hypothalamus also works in conjunction with the pituitary gland to control the body’s endocrine system. The pituitary has the ability to secrete hormones directly into the blood stream. This is a rare example of a place where the blood-brain barrier normally designed to keep infections from crossing into the brain is absent from the brain’s architecture. Some hormones, such as oxytocin and vasopressin, are made directly in the hypothalamus (in the paraventricular and supraoptic nuclei, for example), and secreted near the back of the pituitary. The anterior part of the pituitary contains cells that make their own hormones. These hormones are regulated by other neurological secretions which are passed down nerve fibers into a vascular plexus, where they are released by the blood. All of these hormonal secretions are regulated by negative feedback loops, meaning that the brain is able to detect when levels of the hormone are high, and decrease production as a result.

This may seem enormously complicated, and it is. But the ultimate task of homeostasis even in the face of adversity is well worth it!