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.
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 has 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
- 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 and piriform cortex: Receive smell input in 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.
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.
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