Abstract
Taking the evolutionary development of the forebrain as a starting point, the authors developed a biological framework for the subcortical regulation of human emotional behaviour which may offer an explanation for the pathogenesis of the principle symptoms of mental disorders. Appetitive-searching (reward-seeking) and distress-avoiding (misery-fleeing) behaviour are essential for all free-moving animals to stay alive and to have offspring. Even the oldest ocean-dwelling animal creatures, living about 560 million years ago and human ancestors, must therefore have been capable of generating these behaviours. Our earliest vertebrate ancestors, with a brain comparable with the modern lamprey, had a sophisticated extrapyramidal system generating and controlling all motions as well as a circuit including the habenula for the evaluation of the benefits of their actions. Almost the complete endbrain of the first land animals with a brain comparable with that of amphibians became assimilated into the human amygdaloid and hippocampal complex, whereas only a small part of the dorsal pallium and striatum developed into the ventral extrapyramidal circuits and the later insular cortex. The entire neocortex covering the hemispheres is of recent evolutionary origin, appearing first in early mammals. During the entire evolution of vertebrates, the habenular system was well conserved and maintained its function in regulating the intensity of reward-seeking (pleasure-related) and misery-fleeing (happiness-related) behaviour. The authors propose that the same is true in humans. Symptomatology of human mental disorders can be considered to result from maladaptation within a similar amygdalo/hippocampal-habenular-mesencephalic-ventral striatal system.
Highlights
The human cerebrum largely consists of two massive hemispheres which are completely covered by an extensively in gyri and sulci folded, laminated, cortical layer [1]
The cortical surface is primarily so expanded by the C-shaped and outward-to-inward curving of the entire hemisphere during embryological development. As this cortical layer is less well developed in all other animals, which are believed to lack many human skills, it is obvious to most psychiatrists and psychologists that the cerebral cortex is the essential structure in the regulation of human behaviour
We will go on to describe how the amygdala, hippocampus, habenula and accumbens nucleus regulate reward-seeking and misery-fleeing behaviour, and we will explain how this system might be involved in the pathogenesis of addiction [8], depression [9,10], mania [11], delusions [12] and specific stress disorders
Summary
Circuits regulating pleasure and happiness: evolution and role in mental disorders*. Loonen AJM, Ivanova SA. Circuits regulating pleasure and happiness: evolution and role in mental disorders. Almost the complete endbrain of the first land animals with a brain comparable with that of amphibians became assimilated into the human amygdaloid and hippocampal complex, whereas only a small part of the dorsal pallium and striatum developed into the ventral extrapyramidal circuits and the later insular cortex. During the entire evolution of vertebrates, the habenular system was well conserved and maintained its function in regulating the intensity of reward-seeking (pleasurerelated) and misery-fleeing (happiness-related) behaviour. ∙ The human laminated cerebral neocortex is of recent evolutionary origin and is not essential for the regulation of reward-seeking and misery-fleeing emotional behaviour.
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