Limbic system

The limbic system (Latin limbus: "arc") includes the structures in the human brain involved in emotion, motivation, and emotional association with memory. More active in extroverts and risk-takers than in introverts and cautious people (Evers, 499)Evers, Christine A., Lisa Starr. Biology:Concepts and Applications. 6th ed. United States:Thomson, 2006. ISBN 0-534-46224-3, the limbic system influences the formation of memory by integrating emotional states with stored memories of physical sensations (see emotional memory).

Anatomy

The limbic system includes many different cortical and subcortical brain structures that differ depending upon which book is referenced. For ease of interpretation, this is a list of all the regions generally considered to be part of the limbic system:

Amygdala: Involved in aggression and fear;
Cingulate gyrus: Autonomic functions regulating heart rate and blood pressure as well as cognitive and attentional processing;
Fornicate gyrus: Region encompassing the cingulate, hippocampus, and parahippocampal gyrus;
Hippocampus: Required for the formation of long-term memories;
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;
Mammilary body: Important for the formation of memory;
Nucleus accumbens: Involved in reward, pleasure, and addiction;
Orbitofrontal cortex: Required for decision making;
Parahippocampal gyrus: Plays a role in the formation of spatial memory.

Function

The limbic system operates by influencing the endocrine system and the autonomic nervous system. The limbic system is highly interconnected with a structure known as the nucleus accumbens, commonly called the brain's pleasure center. The nucleus accumbens 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. Rats with metal electrodes implanted into their nucleus accumbens will repeatedly press a lever which activates this region, and will do so in preference over food and water, eventually dying from exhaustion.

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

In evolutionary terms, the limbic system is among the oldest parts of the brain and is related to the olfactory lobes. It can be found in fish, amphibians, reptiles and mammals.

History

The French physician Paul Broca first called this part of the brain "le grand lobe limbique" in 1878, but its putative role in emotion wasn't largely developed until 1937, when the American physician James Papez first described his anatomical model of emotion, which is still referred to as the Papez circuit. Papez's ideas were then later expanded on by Paul D. MacLean to include additional structures in a more dispersed "limbic system," more similar to the system described above. The concept of the limbic system has since been further expanded and developed by Nauta, Heimer, and others.

Practical application

A person can exploit the function of the limbic system to aid in memory retention and recall. Exposure to certain easily recognizable smells (like coffee, peanut butter, chocolate, sulfur, or crayons) while forming memories will link the memory to the smell. Smelling the same thing will help recall the information later. Bottom line: brew coffee while studying for a test then drink coffee while taking the test and it will be much easier to remember the information.

References

Broca, P. Anatomie comparée des circonvolutions cérébrales: le grand lobe limbique. Rev. Anthropol. 1878;1:385-498.

Papez JW. A proposed mechanism of emotion. 1937. J Neuropsychiatry Clin Neurosci. 1995;7(1):103-12. PMID 7711480
Lautin, Andrew. The Limbic Brain. New York, Kluwer Academic/Plenum Publishers, 2001. See: Psychiatryonline
Maclean, PD. Some psychiatric implications of physiological studies on frontotemporal portion of limbic system (visceral brain). Electroencephalogr Clin Neurophysiol Suppl. 1952;4(4):407-18. PMID 12998590

External links

Interview with Joseph LeDoux from February 1997 suggesting the concept of the limbic system is no longer valid

Cerebrum | Limbic system | Emotion | Memory | Motivation