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Chapter: Medical Physiology: Behavioral and Motivational Mechanisms of the Brain-The Limbic System and the Hypothalamus

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Functions of the Amygdala

The amygdala is a complex of multiple small nuclei located immediately beneath the cerebral cortex of the medial anterior pole of each temporal lobe.

Functions of the Amygdala

The amygdala is a complex of multiple small nuclei located immediately beneath the cerebral cortex of the medial anterior pole of each temporal lobe. It has abun-dant bidirectional connections with the hypothalamus as well as with other areas of the limbic system.

In lower animals, the amygdala is concerned to a great extent with olfactory stimuli and their interrelations with the limbic brain. Indeed, it is pointed out that one of the major divisions of the olfac-tory tract terminates in a portion of the amygdala called the corticomedial nuclei, which lies immediately beneath the cerebral cortex in the olfactory pyriform area of the temporal lobe. In the human being, another portion of the amygdala, the basolateral nuclei, has become much more highly developed than the olfactory portion and plays important roles in many behavioral activities not generally associated with olfactory stimuli.

The amygdala receives neuronal signals from all por-tions of the limbic cortex, as well as from the neocortex of the temporal, parietal, and occipital lobes—especially from the auditory and visual association areas. Because of these multiple connections, the amygdala has been called the “window” through which the limbic system sees the place of the person in the world. In turn, the amygdala transmits signals (1) back into these same cor-tical areas, (2) into the hippocampus, (3) into the septum, (4) into the thalamus, and (5) especially into the hypothalamus.

Effects of Stimulating the Amygdala. In general, stimulationin the amygdala can cause almost all the same effects as those elicited by direct stimulation of the hypothalamus, plus other effects. Effects initiated from the amygdala and then sent through the hypothalamus include (1) increases or decreases in arterial pressure, (2) increases or decreases in heart rate, (3) increases or decreases in gastrointestinal motility and secretion, (4) defecation or micturition, (5) pupillary dilation or, rarely, constriction, (6) piloerection, and (7) secretion of various anterior pituitary hormones, especially the gonadotropins and adrenocorticotropic hormone.

Aside from these effects mediated through the hypo-thalamus, amygdala stimulation can also cause several types of involuntary movement. These include (1) tonic movements, such as raising the head or bending the body; (2) circling movements; (3) occasionally clonic, rhythmical movements; and (4) different types of move-ments associated with olfaction and eating, such as licking, chewing, and swallowing.

In addition, stimulation of certain amygdaloid nuclei can cause a pattern of rage, escape, punishment, severe pain, and fear similar to the rage pattern elicited from the hypothalamus, as described earlier. Stimulation of other amygdaloid nuclei can give reactions of reward and pleasure.

Finally, excitation of still other portions of the amyg-dala can cause sexual activities that include erection, copulatory movements, ejaculation, ovulation, uterine activity, and premature labor.

Effects of Bilateral Ablation of the Amygdala—The Klüver-Bucy Syndrome. When the anterior parts of both temporallobes are destroyed in a monkey, this removes not only portions of temporal cortex but also of the amygdalas that lie inside these parts of the temporal lobes. This causes changes in behavior called the Klüver-Bucy syn-drome, which is demonstrated by an animal that (1) is not afraid of anything, (2) has extreme curiosity about everything, (3) forgets rapidly, (4) has a tendency to place everything in its mouth and sometimes even tries to eat solid objects, and (5) often has a sex drive so strong that it attempts to copulate with immature animals, animals of the wrong sex, or even animals of a different species. Although similar lesions in human beings are rare, afflicted people respond in a manner not too different from that of the monkey.

Overall Function of the Amygdalas. The amygdalas seem tobe behavioral awareness areas that operate at a semi-conscious level. They also seem to project into the limbic system one’s current status in relation to both surroundings and thoughts. On the basis of this infor-mation, the amygdala is believed to make the person’s behavioral response appropriate for each occasion.


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