Limbic system
 | This article needs to be updated. (November 2010) |
| Limbic system | |
|---|---|
 The limbic system within the brain. | |
| Details | |
| Identifiers | |
| Latin | Systema limbicum |
| MeSH | D008032 |
| NeuroNames | 2055 |
| FMA | 242000 |
| Anatomical terms of neuroanatomy | |
The limbic system (or Paleomammalian brain) is a set of brain structures, including the hippocampus, amygdala, anterior thalamic nuclei, septum, limbic cortex and fornix, which seemingly support a variety of functions including emotion, behavior, motivation, long term memory, and olfaction.[1]
Etymology
The term "limbic" comes from the Latin limbus, for "border" or "edge".
Anatomy
The limbic system is the set of brain structures that forms the inner border of the cortex. The components of the limbic system located in the cerebral cortex generally have fewer layers than the classical 6-layered neocortex, and are usually classified as allocortex or archicortex.
The limbic system includes many structures in the cerebral pre-cortex and sub-cortex of the brain. The term has been used within psychiatry and neurology, although its exact role and definition have been revised considerably since the term was introduced.[2] The following structures are, or have been considered to be, part of the limbic system:
- Hippocampus and associated structures:
- Hippocampus:[3][4][5] Required for the formation of long-term memories and implicated in maintenance of cognitive maps for navigation.
- Amygdala:[3][4][5] Involved in signaling the cortex of motivationally significant stimuli such as those related to reward and fear in addition to social functions such as mating.
- Fornix:[3][5] carries signals from the hippocampus to the mammillary bodies and septal nuclei.
- Mammillary body:[3] Important for the formation of memory;
- Septal nuclei: Located anterior to the interventricular septum, the septal nuclei provide critical interconnections
- Limbic lobe
- Parahippocampal gyrus:[4] Plays a role in the formation of spatial memory
- Cingulate gyrus:[3][4][5] Autonomic functions regulating heart rate, blood pressure and cognitive and attentional processing
- Dentate gyrus:[4] thought to contribute to new memories
In addition, these structures are sometimes also considered to be part of the limbic system:
- Entorhinal cortex: Important memory and associative components.
- Piriform cortex:[5] The function of which relates to the olfactory system.
- Fornicate gyrus: Region encompassing the cingulate, hippocampus, and parahippocampal gyrus
- Nucleus accumbens: Involved in reward, pleasure, and addiction
- Orbitofrontal cortex: Required for decision making.
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 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.[6]
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.
Evolution
Paul D. MacLean, as part of his triune brain theory, hypothesized that the limbic system is older than other parts of the brain, and that it developed to manage fight or flight circuitry which is an evolutionary necessity for reptiles as well as humans. However, 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.[7]
History
The French physician Paul Broca first called this part of the brain "le grand lobe limbique" in 1878,[8] 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.[9] 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.[10] The term was formally introduced by Paul D. MacLean in 1952. The concept of the limbic system has since been further expanded and developed by Walle Nauta, Lennart Heimer and others.
Academic dispute
Still, there remains much controversy over the use of the term. When it was first coined, it was posited as the emotional center of the brain, with cognition being the business of the neocortex by contrast. However, this almost immediately ran into trouble when damage to the hippocampus, a primary limbic structure, was shown to result in severe cognitive (memory) deficits. And since its inception, the delineating boundaries of the limbic system have been changed again and again by the community. More recently, attempts have been made to salvage the concept through more precise definition, but there are still no generally accepted criteria for defining its parts. As a concept grounded more in tradition than in facts, some scientists have suggested that the concept should be considered obsolete and abandoned.[11]
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.[11]
See also
- Limbic-hypothalamic-pituitary-adrenal axis (LHPA axis)
- Emotional memory
- Comparative Neuroscience at Wikiversity
- Paralimbic cortex
References
- ^ Medline Plus Medical Encyclopedia
- ^ Conn, Michael P. 2003. Neuroscience in Medicine, 370
- ^ a b c d e Normandy
- ^ a b c d e stanford.edu
- ^ a b c d e Biology.about.com
- ^ Olds, J., Milner, P. 1954. Positive reinforcement produced by electrical stimulation of septal area and other regions of rat brain. J.Comp. Physiolo. Psycholo. 47, 419–427
- ^ Bruce LL, Neary TJ (1995). "The limbic system of tetrapods: a comparative analysis of cortical and amygdalar populations". Brain Behav. Evol. 46 (4–5): 224–34. doi:10.1159/000113276. PMID 8564465.
- ^ 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
- ^ P. D. MacLean (1952). "Some psychiatric implications of physiological studies on frontotemporal portion of limbic system (visceral brain)". Electroencephalography and Clinical Neurophysiology. 4 (4): 407–418. doi:10.1016/0013-4694(52)90073-4. PMID 12998590.
- ^ a b Ledoux, J., (2003). Synaptic Self. New York: Penguin Books.
