limbic system
| Dictionary: limbic system |
n.
A group of interconnected deep brain structures, common to all mammals, and involved in olfaction, emotion, motivation, behavior, and various autonomic functions.
| World of the Body: limbic system |
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.
| Dental Dictionary: limbic system |
n
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.
| Sports Science and Medicine: limbic system |
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.
| World of the Mind: limbic system |
The term 'limbic system' derives from the concept of the 'limbic lobe'. It was first used by the French anatomist Paul Broca in 1878 to describe that part of the brain surrounding the brain stem and lying beneath the neocortex. A generally accepted modern definition of the 'limbic system' has, however, never been given. Some neuroanatomists believe that the term should be abandoned. Nevertheless it is a widely used 'shorthand' term and most authors would include the following structures within its definition: the hippocampal formation, olfactory regions, hypothalamus, and amygdala.
Functionally, the limbic system is generally said to be concerned with visceral processes, particularly those associated with the emotional status of the organism. Both experimental and clinical data indicate that the amygdala is involved in emotional experiences and reactions, particularly those associated with fear and anger, flight and defence. Stimulation of the amygdala in conscious animals can give rise to quite integrated response patterns evolving over time and involving a wide variety of motor and autonomic responses which are integral parts of the overall behaviour pattern. Although the amygdala would therefore appear to be involved in these responses, it may not be the only such area. Thus changes in aggressiveness can be obtained by stimulation or ablation of septum, certain areas of cerebral cortex, and the grey matter of the mesencephalon. This emphasizes the importance of considering the interaction of connected brain areas, rather than the activity within particular 'centres', as effectors of functions.
The hippocampus has been the object of much experimental work, and theories about the nature of its function have multiplied. One possibility which has attracted attention from behaviourists is that it may be involved in memory. However, at present no clearly formulated results can be stated. This is in consequence partly of the anatomical complexity of the region and partly of the semantic difficulties surrounding the words 'memory' and 'learning'. Since the hippocampus is richly connected to many other brain regions, it is, as in the case of the amygdala, probably misleading to think of such a structure as a 'centre' of such a function. Physiologically one of its most striking properties is seen in relation to activity in the cerebral neocortex. When the neocortex is 'desynchronized' (i.e. shows low-voltage rapid potentials) the hippocampus becomes 'synchronized' and shows rhythmic sinusoidal waves of 4–7 per second ('theta waves': see electroencephalography). When the neocortex is synchronized, on the other hand, the hippocampus is desynchronized. The functions of this reciprocal relationship are not understood, but it appears to be related to the activity of the reticular formation and the state of attentiveness of the subject. Recent work strongly suggests that serotonin and noradrenaline (norepinephrine) are closely involved in the switching of mechanisms between these two states.
Another division of the 'limbic system', the entorhinal area, is closely related anatomically to the hippocampus, providing its main cortical source of afferent fibres. Recent anatomical work has shown that it also receives fibres from the frontal, temporal, and cingulate neocortex as well as the olfactory cortex, indicating that it is the final cortical link between the sensory systems of the neo-and transitional cortex, on the one hand, and the hippocampus and dentate gyrus, on the other. It would seem from an anatomical point of view that the sensory information arriving at the entorhinal area is probably highly refined (see dopamine neurons in the brain).
The information derived from the entorhinal area interacts with hippocampus and amygdala. These structures deliver further messages both directly (from amygdala) and via septum and nucleus accumbens to regions of the hypothalamus concerned with motivational and rewarding mechanisms.
Thus interaction of all the structures in the complex, from the entorhinal area to hypothalamus, is probably of great importance in deciding the final actions of an organism in a particular environment, and in the formation of adaptive behaviour patterns. See also emotional brain.
(Published 1987)
Functionally, the limbic system is generally said to be concerned with visceral processes, particularly those associated with the emotional status of the organism. Both experimental and clinical data indicate that the amygdala is involved in emotional experiences and reactions, particularly those associated with fear and anger, flight and defence. Stimulation of the amygdala in conscious animals can give rise to quite integrated response patterns evolving over time and involving a wide variety of motor and autonomic responses which are integral parts of the overall behaviour pattern. Although the amygdala would therefore appear to be involved in these responses, it may not be the only such area. Thus changes in aggressiveness can be obtained by stimulation or ablation of septum, certain areas of cerebral cortex, and the grey matter of the mesencephalon. This emphasizes the importance of considering the interaction of connected brain areas, rather than the activity within particular 'centres', as effectors of functions.
The hippocampus has been the object of much experimental work, and theories about the nature of its function have multiplied. One possibility which has attracted attention from behaviourists is that it may be involved in memory. However, at present no clearly formulated results can be stated. This is in consequence partly of the anatomical complexity of the region and partly of the semantic difficulties surrounding the words 'memory' and 'learning'. Since the hippocampus is richly connected to many other brain regions, it is, as in the case of the amygdala, probably misleading to think of such a structure as a 'centre' of such a function. Physiologically one of its most striking properties is seen in relation to activity in the cerebral neocortex. When the neocortex is 'desynchronized' (i.e. shows low-voltage rapid potentials) the hippocampus becomes 'synchronized' and shows rhythmic sinusoidal waves of 4–7 per second ('theta waves': see electroencephalography). When the neocortex is synchronized, on the other hand, the hippocampus is desynchronized. The functions of this reciprocal relationship are not understood, but it appears to be related to the activity of the reticular formation and the state of attentiveness of the subject. Recent work strongly suggests that serotonin and noradrenaline (norepinephrine) are closely involved in the switching of mechanisms between these two states.
Another division of the 'limbic system', the entorhinal area, is closely related anatomically to the hippocampus, providing its main cortical source of afferent fibres. Recent anatomical work has shown that it also receives fibres from the frontal, temporal, and cingulate neocortex as well as the olfactory cortex, indicating that it is the final cortical link between the sensory systems of the neo-and transitional cortex, on the one hand, and the hippocampus and dentate gyrus, on the other. It would seem from an anatomical point of view that the sensory information arriving at the entorhinal area is probably highly refined (see dopamine neurons in the brain).
The information derived from the entorhinal area interacts with hippocampus and amygdala. These structures deliver further messages both directly (from amygdala) and via septum and nucleus accumbens to regions of the hypothalamus concerned with motivational and rewarding mechanisms.
Thus interaction of all the structures in the complex, from the entorhinal area to hypothalamus, is probably of great importance in deciding the final actions of an organism in a particular environment, and in the formation of adaptive behaviour patterns. See also emotional brain.
(Published 1987)
— O. T. Phillipson
| Wikipedia: Limbic system |
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| Brain: Limbic system | ||
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| The limbic system within the brain. | ||
| NeuroNames | ancil-247 | |
The limbic system (or Paleomammalian brain) is a set of brain structures including the hippocampus, amygdala, anterior thalamic nuclei, and limbic cortex, which support a variety of functions including emotion, behavior, long term memory, and olfaction.[1] The term "limbic" comes from Latin limbus, loosely translating as "border" or "belt".
Contents |
Anatomy
Essentially the limbic system is the set of brain structures that forms the inner border of the cortex. In an abstract topological sense, each cortical hemisphere can be thought of as a sphere of gray matter, with a hole punched through it in the area where nerve fibers connect it to the subcortical structures of the basal forebrain. The hole is surrounded by a ring of cortical and noncortical areas that combine to make up the limbic system. The cortical components generally have fewer layers than the classical 6-layered neocortex, and are often classified as allocortex or archicortex.
The limbic system includes many structures in the cerebral 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:
- 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.
- Hippocampus:[3][4][5] Required for the formation of long-term memories and implicated in maintenance of cognitive maps for navigation.
- 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
- Fornix:[5][3] carries signals from the hippocampus to the mammillary bodies and septal nuclei.
- Hypothalamus:[3][5] 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
- Thalamus:[3][5] The "relay station" to the cerebral cortex.
In addition, these structures are sometimes also considered to be part of the limbic system:
- Mammillary body:[3] Important for the formation of memory
- Pituitary gland:[3] secretes hormones regulating homeostasis
- Dentate gyrus:[4] thought to contribute to new memories and to regulate happiness.
- 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
- Olfactory bulb: Olfactory sensory input
- 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 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
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.[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 MacLean in 1952. The concept of the limbic system has since been further expanded and developed by Nauta, Heimer and others.
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, many scientists have suggested that the concept should be considered obsolete and abandoned.[11]
See also
- Limbic-hypothalamic-pituitary-adrenal axis (LHPA axis)
- Emotional memory
- Comparative Neuroscience at Wikiversity
References
- ^ Medline Plus Medical Encyclopedia [1]
- ^ Conn, Michael P. 2003. Neuroscience in Medicine, 370
- ^ a b c d e f g h Normandy
- ^ a b c d e stanford.edu
- ^ a b c d e f g 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:. 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:. PMID 12998590.
- ^ Ledoux, J., (2003). Synaptic Self. New York: Penguin Books.
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