The limbic system (or paleomammalian brain) is a complex set of brain structures located on both sides of the thalamus, right under the cerebrum. It is not a separate system but a collection of structures from the telencephalon, diencephalon, and mesencephalon. It includes the olfactory bulbs, hippocampus, amygdala, anterior thalamic nuclei, fornix, columns of fornix, mammillary body, septum pellucidum, habenular commissure, cingulate gyrus, parahippocampal gyrus, limbic cortex, and limbic midbrain areas.
The limbic system supports a variety of functions including epinephrine flow, emotion, behavior, motivation, long-term memory, and olfaction. Emotional life is largely housed in the limbic system, and it has a great deal to do with the formation of memories.
Although the term only originated in the 1940s, some neuroscientists, including Joseph LeDoux, have suggested that the concept of a functionally unified limbic system should be abandoned as obsolete because it is grounded mainly in historical concepts of brain anatomy that are no longer accepted as accurate.
The limbic system was originally defined by Paul Broca as a series of cortical structures surrounding the limit between thecerebral hemispheres and the brainstem: the border, or limbus, of the brain. These structures were known together as thelimbic lobe. Further studies began to associate these areas with emotional and motivational processes and linked them to subcortical components that were grouped into the limbic system. The existence of such a system as an isolated entity responsible for the neurological regulation of emotion has gone into disuse and currently it is considered as one of the many parts of the brain that regulate visceral, autonomic processes.
Therefore, the definition of anatomical structures considered part of the limbic system is a controversial subject. The following structures are, or have been considered, part of the limbic system:
- Cortical areas:
- Limbic lobe
- Orbitofrontal cortex, a region in the frontal lobe involved in the process of decision-making.
- Piriform cortex, part of the olfactory system.
- Entorhinal cortex, related with memory and associative components.
- Hippocampus and associated structures, which play a central role in the consolidation of new memories.
- Fornix, a white matter structure connecting the hippocampus with other brain structures, particularly the mammillary bodiesand septal nuclei
- Subcortical areas:
- Diencephalic structures:
- Hypothalamus: a center for the limbic system, connected with the frontal lobes, septal nuclei and the brain stem reticular formation via the medial forebrain bundle, with the hippocampus via the fornix, and with the thalamus via the mammillothalamic fasciculus. It regulates a great number of autonomic processes.
- Mammillary bodies, part of the hypothalamus that receives signals from the hippocampus via the fornix and projects them to the thalamus.
- Anterior nuclei of thalamus receive input from the mammillary bodies. Involved in memory processing.