limbic system

A group of interconnected deep brain structures, common to all mammals, and involved in olfaction, emotion, motivation, behavior, and various autonomic functions.

The term ‘limbic system’ (from Latin limbus: edge) was first used by MacLean in 1952 to describe a set of structurally and functionally related structures of the brain bordering the midline, inner surface of each cerebral hemisphere. These structures were considered to be evolutionarily ancient. MacLean called them the ‘visceral brain’ and suggested they mediate behaviourally ‘primitive’ functions inherited from lower mammals, particularly emotion and motivational behaviour. Although such phylogenetic arguments (based on comparison between species) are now commonly rejected, the concept of the limbic system survives and has since grown to be highly influential yet controversial.

First, there is no consensus over exactly which structures comprise the limbic system. Most schemes, however, consider it to consist of various parts of the cerebral cortex forming a set of ‘rings’ on the inner surface of each hemisphere, linked to a central core of structures lying below the cerebral cortex. The cortical areas include the cingulate cortex, hippocampus, parahippocampal cortex, and rhinal cortex. The various subcortical areas included in the limbic system extend down through the core of the brain to the upper part of the brain stem.

Second, there is considerable debate over what the function of the limbic system is. In addition to early ideas relating the limbic system to emotion and motivation, it has also now been implicated in the processing of sensory (especially olfactory) and cognitive information, learning and memory, sexual function (as part of a reward system serving emotional reactions), and motor functions. Most intriguing is the suggestion that the limbic system is concerned with mental integration of all functions related to personal ‘experience’.

As the number of brain areas said to belong to the limbic system has grown, its proposed functions have, not surprisingly, proliferated. It has been argued that such a heterogeneous collection of structures and functions can no longer be defined by a single general criterion and that the concept of the limbic system has become incoherent, even meaningless. An alternative view is that a quantitative approach (a ‘fuzzy limbic system’), in which different brain regions are described as having a certain degree of ‘limbic-ness’, would avoid the problem of having to define precise boundaries.

Despite controversy, the popularity and universal recognition of the term cannot be denied. This may be due partly to the very vagueness of the concept, which has often been used by authors as a convenience to refer to particularly poorly understood areas of the brain.

— Mark J. Buckley

Bibliography

• Kotter, R. and Meyer, N. (1992). The limbic system: a review of its empirical foundation. Behavioural Brain Research, 52, 105-27

See also brain; cerebral cortex; emotion; memory.

A group of structures within the rhinencephalon of the brain that are associated with various emotions and feelings, such as anger, fear, sexual arousal, pleasure, and sadness. Unless the limbic system is modulated by other cortical areas, periodic attacks of uncontrollable rage may occur in some individuals. The function of the system is poorly understood.

Functional brain system mainly associated with the forebrain and concerned with emotional or affective behaviour, learning, and memory. Extensive connections with higher and lower centres of the brain allow the limbic system to respond to a wide range of environmental stimuli.

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Brain: Limbic system
The limbic system within the brain.
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The limbic system is the part of the human brain involved in emotion, motivation, and emotional association with memory. It influences the formation of memory by integrating emotional states with stored memories of physical sensations. (See emotional memory). The term "limbic" comes from Latin limbus, meaning "border" or "edge".

Anatomy

The limbic system includes many structures in the cerebral cortex and sub-cortex of the brain. Different books list different sets of structures. For ease of interpretation, this is a list of all the regions generally considered to be part of the limbic system:

• Amygdala: Involved in signaling the cortex of motivationally significant stimuli such as those related to reward and fear
• Hippocampus: Required for the formation of long-term memories
  • Parahippocampal gyrus: Plays a role in the formation of spatial memory and is part of the hippocampus
• Cingulate gyrus: Autonomic functions regulating heart rate, blood pressure and cognitive and attentional processing
• Fornicate gyrus: Region encompassing the cingulate , hippocampus , and parahippocampal gyrus
• Hypothalamus: Regulates the autonomic nervous system via hormone production and release. Affects and regulates blood pressure, heart rate, hunger, thirst, sexual arousal, and the sleep/wake cycle
• Mammillary body: Important for the formation of memory
• Nucleus accumbens: Involved in reward, pleasure, and addiction
• Orbitofrontal cortex: Required for decision making
• Thalamus: The "relay station" to the cerebral cortex
• Olfactory Bulb: Olfactory sensory input

Function

The limbic system operates by influencing the endocrine system and the autonomic nervous system. It is highly interconnected with the nucleus accumbens, the brain's pleasure center, 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 repeatedly pressed a lever activating this region, and did so in preference to eating and drinking, eventually dying of exhaustion.^[1]

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 who underwent this procedure often became passive and lacked all motivation.

There is circumstantial evidence that the limbic system also provides a custodial function for the maintenance of a healthy conscious state of mind.

Evolution

The limbic system is embryologically older than other parts of the brain. It developed to manage 'fight' or 'flight' chemicals and is an evolutionary necessity for reptiles as well as humans.

Recent studies of the limbic system of tetrapods have challenged some long-held tenets of forebrain evolution. 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.^[2]

History

The French physician Paul Broca first called this part of the brain "le grand lobe limbique" in 1878,^[3] but most of its putative role in emotion was developed only in 1937 when the American physician James Papez described his anatomical model of emotion, the Papez circuit.^[4] Paul D. MacLean expanded these ideas to include additional structures in a more dispersed "limbic system," more on the lines of the system described above.^[5] The concept of the limbic system has since been further expanded and developed by Nauta, Heimer and others.

Human brain: Limbic system
Amygdala · Cingulate gyrus · Fornicate gyrus · Hippocampus · Hypothalamus · Mammillary body · Nucleus accumbens · Olfactory system · Orbitofrontal cortex · Parahippocampal gyrus

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