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The Limbic System
Man differs from animals in the size of the neocortex, the thinking brain that overlays the more basic brain responsible for "primitive" emotional responses. While we may pride ourselves on being rational beings, we nonetheless retain many primitive reflexes and reactions. This is especially true of infants who express hunger, rage or pleasure very directly. Without the moderating influence of learned responses of the mature neocortex, the infant demands immediate gratification. In the adult, the neocortex includes the thinking brain and also affects (the private and personal experiences of the individual such as love, pain or guilt) that are later turned into emotions.
The limbic system appears to be the part of the brain responsible for translating ideas and affects into feelings and emotions. Teaching that uses a cognitive approach would target the brain cortex; behaviour modification would target the limbic system. The question arises as to how emotions are stored. Peptides are short chains of amino acids; chemical messengers or agents of information that are recognized by receptors on the surface of every cell in the body. When peptide binds physically to a receptor, the receptor changes initiates other responses. One suggestion is that emotions are the responses generated by peptides striking receptors: the biochemical substrate of emotions. The emotions are therefore stored in the body (in the form of peptide chains & receptors) (Bill Moyers. Healing and the Mind. New York: Doubleday, 1993)
"Limbic" means forming a border around, and in this case it refers to the brain stem. In evolutionary terms, the limbic system forms a very ancient part of the brain structure linked to the olfactory system and concerned with instinctual behaviours essential to survival such as feeding, drinking and defence. It is also concerned with translating sensory data from the neocortex into motivational forces for behaviour (note how strong our emotional responses to olfactory stimuli can be!). Structurally, the limbic system forms a pair of circuits, upper and lower, comprising several components that bridge the neocortex (the thinking brain) with parts of the endocrine system. Hence, it is a crucial bridge between brain and body; it also appears to be the repository of emotions and drives. The upper portion originates in the septum, and includes the anterior thalamus, the fornix and cingulate gyrus. It appears to be involved in the elaboration of feeling states concerned with pleasure, sexual arousal and reproduction. The lower circuit is driven by the amygdala and includes the hippocampus, hypothalamus, and mamillary bodies. It is primarily concerned with emotional states and behaviours that ensure survival: fight or flight, etc. The amygdala involves a balanced internal system that provides a "mood" or "affective" bias to stimuli and controls aggressive or passionate responses; lesions in the amygdala can cause violent rages in otherwise passive subjects. It is also involved in reactions to stimuli such as caution or fear and in the fight or flight reactions. Via links to the hypothalamus, it controls pituitary and adrenal responses. The hypothalamus is involved in emotions and with short-term memory and learning, especially visual learning. Hippocampal lesions prevent the laying-down of new memories perhaps via damaging a consolidation and integration function. It is involved in balancing the possibly competing demands of instinctual drives and neocortical inputs. The hippocampus forms the main organ linking the neural and endocrine control systems; it has both neural and hormonal connections with the pituitary, and is linked to the autonomic nervous system. Each of these components is a complex set of subsystems with many connections to other brain centres, so it may be more apposite to talk of limbic systems in the plural.
Limbic functions include (a) olfaction; (b) arousal, motivation, etc., (c) coding in laying down new memories; (d) emotional responses, learning and higher control over exchanges between the body and the external world through emotions; (e) regulation of homeostasis through the autonomic and endocrine systems. There are also feed-back loops from the endocrine system to the limbic: catecholamines such as noradrenaline and dopamine produce arousal and aggression, presumably through an influence on the lower crescent of the limbic system.
Hence the limbic system is centrally involved in the mediation between a person’s recognition of an event, their perception of it as stressful, and the resulting physiological reaction to it, mediated via the endocrine system. Stimuli are processed conceptually in the cortex, and passed to the limbic system where they are evaluated and a motivational response is formulated. Removal of the neocortex in the cat does not alter the animal’s basic personality (it remains a passive or a hostile cat). But lesions to the limbic system have a profound effect on behavioural responses, giving a "mysteriously distorted appraisal of the world" reminiscent of psychosis. |
