Limbic System

Limbic system is a set of brain structures located on both sides of the thalamus, immediately beneath the medial temporal lobe of the cerebrum primarily in the forebrain.

It supports a variety of functions including emotion, behavior, long-term memory, and olfaction. Emotional life is largely housed in the limbic system, and it critically aids the formation of memories.

With a primordial structure, the limbic system is involved in lower order emotional processing of input from sensory systems and consists of the amygdaloid nuclear complex (amygdala), mammillary bodies, stria medullaris, central gray and dorsal and ventral nuclei of Gudden. 

This processed information is often relayed to a collection of structures from the telencephalon, diencephalon, and mesencephalon, including the prefrontal cortex, cingulate gyrus, limbic thalamus, hippocampus including the parahippocampal gyrus and subiculum, nucleus accumbens (limbic striatum), anterior hypothalamus, ventral tegmental area, midbrain raphe nuclei, habenular commissure, entorhinal cortex, and olfactory bulbs.

The structures and interacting areas 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.

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 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.

  • 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.

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