Limbic system

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, long term memory, and olfaction. The term "limbic" comes from the Latin limbus, for "border" or "edge". Some scientists have suggested that the concept of the limbic system should be abandoned as obsolete, as it is grounded more in transient tradition than in facts.

Anatomy
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 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. The following structures are, or have been 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 in addition to social functions such as mating.
 * Hippocampus:  Required for the formation of long-term memories and implicated in maintenance of cognitive maps for navigation.
 * Parahippocampal gyrus: Plays a role in the formation of spatial memory
 * Cingulate gyrus:  Autonomic functions regulating heart rate, blood pressure and cognitive and attentional processing
 * Fornix: carries signals from the hippocampus to the mammillary bodies and septal nuclei.
 * 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
 * Thalamus: The "relay station" to the cerebral cortex.

In addition, these structures are sometimes also considered to be part of the limbic system:
 * Mammillary body: Important for the formation of memory
 * Pituitary gland: secretes hormones regulating homeostasis
 * Dentate gyrus: thought to contribute to new memories and to regulate happiness.
 * Entorhinal cortex: Important memory and associative components.
 * Piriform cortex: 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 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.

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.

History
The French physician Paul Broca first called this part of the brain "le grand lobe limbique" in 1878, 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. 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. 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.

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.