Appendix B

Neuroanatomy of the Three Chambers

Paul MacLean’s Triune Brain Theory

The neurologist Paul MacLean proposed that our skull holds not one brain but three, each representing a distinct evolutionary stratum that formed upon the older layer before it. He calls it the “triune brain.” MacLean says that the three brains operate like “three interconnected biological computers, [each] with its own special intelligence, its own subjectivity, its own sense of time and space, and its own memory.” He refers to these three brains as the neocortex or neomammalian brain, the limbic or paleomammalian brain, and the reptilian brain (brain stem and cerebellum). Each of the three brains is connected by nerves to the other two, but each seems to operate as its own brain system with distinct capacities. This hypothesis has become a very influential paradigm, which has led to a rethinking of how the brain functions. It had previously been assumed that the highest level of the brain, the neocortex, dominates the other, lower levels. MacLean has shown that this is not the case, and that the physically lower limbic system, which rules emotions, can and does at times hijack the higher mental functions.

The brain stem—reptilian brain. This includes the brain stem, medulla, pons, cerebellum, mesencephalon, and the basal nuclei and olfactory bulbs, and is the oldest brain, in evolutionary terms. This brain controls muscles, balance, and autonomic functions, such as breathing and heartbeat. It has the same type of archaic behavioral programs as snakes, turtles, and lizards. It is rigid, obsessive, compulsive, ritualistic, and territorial. Its behavior is repetitive, with limited learning from past experience. This part of the brain is always active, even in deep sleep.

The limbic system—mammalian brain. MacLean first coined the name “limbic system” for the middle part of the brain, which he also termed the old mammalian brain—it includes the hypothalamus, hippocampus, and amygdala. It corresponds to the brain of most mammals. The limbic system maintains emotion-charged memories, and all new experience is judged as either “like” or “dislike.” For the limbic system, survival depends on avoidance of pain and repetition of pleasure. MacLean’s most important discovery was that since the limbic system is the seat of our feeling value judgments, not the more advanced neocortex, it decides whether our higher brain has a “good” idea or not, whether it “feels” true and right.

The neocortex and cerebrum comprises the two hemispheres and some subcortical neuronal groups. It corresponds to the brain of higher primates including homo sapiens. The cognitive functions that most distinguish humans from animals are in the neocortex. MacLean refers to the cortex as “the mother of invention and father of abstract thought.” In humans the neocortex takes up two-thirds of the total brain mass, whereas in animals it is relatively small. The cortex is divided into left and right hemispheres, the famous left and right brain. The left half of the cortex controls the right side of the body, and the right side of the brain, the left side of the body. The right brain is more spatial, abstract, musical, and artistic, while the left brain is more linear, rational, and verbal.

For additional information on MacLean’s work, see his books The Triune Brain in Evolution (1990) and A Triune Concept of Brain and Behavior (1973); and Gardner and Cory’s The Evolutionary Neuroethology of Paul MacLean: Convergences and Frontiers (2002).

Abdominal Brain or Enteric Nervous System

Michael Gershon’s 1998 book, The Second Brain, describes the new understanding of the role of the enteric nervous system, a network of 100 million neurons lining the inside and outside of the gut. Like the familiar sympathetic and parasympathetic systems, this neural system is autonomic—that is, not subject to control from the brain in the head. Whereas the main chemical neurotransmitter for the sympathetic system is norepinephrine, and for the parasympathetic system acetylcholine, the enteric system functions with serotonin, which is the neurotransmitter probably involved with emotional balance.

These findings give new meaning to the popular expressions of “gut-level knowledge” and “intestinal fortitude.” They also are suggestively congruent with the peculiar association between insight and intestinal purging found with the South American divinatory plant concoction ayahuasca.

Although Gershon does not say this, in an evolutionary sense this nervous system is the oldest, and therefore should more accurately be called the first brain, rather than the second. We might hypothesize that the enteric nervous system in the gut evolved in conjunction with what MacLean calls the reptilian brain stem in the head, which is also particularly rich in serotonergic neurons.