The limbic system is a set of brain structures located on both sides of the thalamus, immediately beneath the cerebrum. It has also been referred to as the paleomammalian cortex. It is not a separate system but a collection of structures from the telencephalon, diencephalon, and mesencephalon.
The limbic system supports a variety of functions including 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.
Limbic System Structure
The limbic system was originally defined by Paul Broca as a series of cortical structures surrounding the limit between the cerebral hemispheres and the brainstem: the border, or limbus, of the brain. These structures were known together as the limbic 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:
- 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 bodies and septal nuclei
- Septal nuclei, a set of structures that lie in front of the lamina terminalis, considered a pleasure zone.
- Amygdala, located deep within the temporal lobes and related with a number of emotional processes.
- Nucleus accumbens: involved in reward, pleasure, and addiction.
- 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.
The structures of the limbic system are involved in motivation, emotion, learning, and memory. The limbic system is where the subcortical structures meet the cerebral cortex.
The limbic system operates by influencing the endocrine system and the autonomic nervous system. It is highly interconnected with the nucleus accumbens, which plays a role in sexual arousal and the “high” derived from certain recreational drugs.
These responses are heavily modulated by dopaminergic projections from the limbic system. In 1954, Olds and Milner found that rats with metal electrodes implanted into their nucleus accumbens, as well as their septal nuclei, repeatedly pressed a lever activating this region, and did so in preference to eating and drinking, eventually dying of exhaustion.
The limbic system also includes the basal ganglia. The basal ganglia are a set of subcortical structures that direct intentional movements. The basal ganglia are located near the thalamus and hypothalamus. They receive input from the cerebral cortex, which sends outputs to the motor centers in the brain stem. A part of the basal ganglia called the striatum controls posture and movement.
The limbic system is also tightly connected to the prefrontal cortex. Some scientists contend that this connection is related to the pleasure obtained from solving problems. To cure severe emotional disorders, this connection was sometimes surgically severed, a procedure of psychosurgery, called a prefrontal lobotomy (this is actually a misnomer). Patients having undergone this procedure often became passive and lacked all motivation.
The limbic system is often classified as a “cerebral structure”. This structure is closely linked to olfaction, emotions, drives, autonomic regulation, memory, and pathologically to encephalopathy, epilepsy, psychotic symptoms, cognitive defects.
The functional relevance of the limbic system has proven to serve many different functions such as affects/emotions, memory, sensory processing, time perception, attention, consciousness, instincts, autonomic/vegetative control, and actions/motor behavior. Some of the disorders associated with the limbic system are epilepsy and schizophrenia.
Paul D. MacLean, as part of his triune brain theory, hypothesized that the limbic system is older than other parts of the forebrain, and that it developed to manage circuitry attributed to the fight or flight first identified by Hans Selye in his report of the General Adaptation Syndrome in 1936. It may be considered a part of survival adaptation, leading to what describes evolution adaptation throughout the history of species differentiation in reptiles as well as mammals (including humans).
MacLean postulated that the human brain has evolved three components, that evolved successively, with more recent components developing at the top/front. These components are, respectively:
- The archipallium or primitive (“reptilian”) brain, comprising the structures of the brain stem – medulla, pons, cerebellum, mesencephalon, the oldest basal nuclei – the globus pallidus and the olfactory bulbs.
- The paleopallium or intermediate (“old mammalian”) brain, comprising the structures of the limbic system.
- The neopallium, also known as the superior or rational (“new mammalian”) brain, comprises almost the whole of the hemispheres (made up of a more recent type of cortex, called neocortex) and some subcortical neuronal groups. It corresponds to the brain of the superior mammals, thus including the primates and, as a consequence, the human species. Similar development of the neocortex in mammalian species unrelated to humans and primates has also occurred, for example in cetaceans and elephants; thus the designation of “superior mammals” is not an evolutionary one, as it has occurred independently in different species. The evolution of higher degrees of intelligence is an example of convergent evolution, and is also seen in non-mammals such as birds.
According to Maclean, each of the components, although connected with the others, retained “their peculiar types of intelligence, subjectivity, sense of time and space, memory, mobility and other less specific functions”.
However, while the categorization into structures is reasonable, the recent studies of the limbic system of tetrapods, both living and extinct, have challenged several aspects of this hypothesis, notably the accuracy of the terms “reptilian” and “old mammalian”. The common ancestors of reptiles and mammals had a well-developed limbic system in which the basic subdivisions and connections of the amygdalar nuclei were established.
Further, birds, which evolved from the dinosaurs, which in turn evolved separately but around the same time as the mammals, have a well-developed limbic system. While the anatomic structures of the limbic system are different in birds and mammals, there are functional equivalents.
I. Edward Alcamo, John Bergdahl
Anatomy Coloring Workbook, Second Edition
The Princeton Review. pp. 120–. ISBN 978-0-375-76342-7. Retrieved 10 January 2013
Top Image: Cross section of the human brain showing parts of the limbic system from below. Traité d’Anatomie et de Physiologie (1786)