Limbic system

The limbic system (Latin limbus: “border” or “edge”) includes the putative structures in the human brain involved in emotion, motivation, and emotional association with memory. The limbic system influences the formation of memory by integrating emotional states with stored memories of physical sensations. (see emotional memory).

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 signaling the cortex of motivationally significant stimuli such as those that are reward and fear related;
• Hippocampus: Required for the formation of long-term memories;
• 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;
• 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;
• Mammillary 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 and is part of the hippocampus;
• Thalamus: The “relay station” to the cerebral cortex.
• Olfactory Bulb: Olfactory sensory input

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. In 1954, Olds and Milner found that rats with metal electrodes implanted into their nucleus accumbens would repeatedly press a lever activating this region, and would do so in preference to eating and drinking, eventually dying of exhaustion.^[1]

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.

The limbic system is embryologically an older part of the brain. It developed to manage ‘fight’ or ‘flight’ chemicals and is an evolutionary necessity for reptiles as well as Homo sapiens.

Recent studies of the limbic system of tetrapods have made data available that challenge some of the 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.^[2]

The French physician Paul Broca first called this part of the brain “le grand lobe limbique” in 1878,^[3] but its putative role in emotion was not 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.^[4] Papez’s ideas were, in turn, later expanded on by Paul D. MacLean to include additional structures in a more dispersed “limbic system,” more on the lines of the system described above.^[5] The concept of the limbic system has since been further expanded and developed by Nauta, Heimer and others.

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