Functions of the Limbic System
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In 1878 Paul Broca, the French neurologist famous for so-called Broca's aphasia, coined the term "le grand lobe lymbique." The term "limbus" refers to a margin or rim. Dr. Broca was referring to the structures that surround the innermost part of the brain, at the margin of the brain's center.
Meaning of 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.
What Does the Limbic System Do?
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 temperature, 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.
Paralimbic Structures
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.
Each of these paralimbic structures has been connected with emotion or basic cognitive processes. The anterior cingulate gyrus, for example, has been tied to motivation and drive. The insula is connected with our ability to sense our own internal sensations (or "gut feelings"). The orbitofrontal cortex, nucleus accumbens, and basal forebrain are involved with sensations of pleasure or reward. The mammillary bodies and some thalamic nuclei are important to the formation of new memories.
All of these pathways are intricately connected. The amygdala, for example, communicates to the orbitofrontal pathway through a white matter bundle called the uncinate fasciculus, as does the insula. The amygdala communicates to parts of the hypothalamus and cingulate through the stria terminalis, and to the brainstem and several other structures through the ventral amygdalofugal pathway. The hippocampus largely communicates through a large white matter pathway called the fornix, which curves around the ventricles of the brain towards the mammillary bodies, sending out branches to the mammillary bodies, thalamus, and cingulate along the way.
The limbic system is a heterogeneous group of structures and serves many different functions. Those functions are fundamental to how we think, feel, and respond to the world around us.
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