Long ago, neurologist Paul MacLean, MD, proposed that the human skull functionally holds not one brain, but three, each representing a distinct evolutionary stratum that formed upon the older layer coming before it — somewhat like an archaeological site. He called this the "triune (3-part) brain" and the three components were referred to as the neocortex, the limbic system, and the reptilian brain [see illustration]. Each is intimately connected by nerves to the other two, but each also seems to operate independently and with distinct capacities.
This perspective became an influential paradigm at the time, forcing a reconsideration of how the brain functions. It had previously been assumed that the highest level of the human brain, the neocortex, dominated the other, anatomically lower (and historically older) levels. MacLean showed that this was not the case, and that the limbic system can temporarily hijack higher mental functions whenever it needs to; overwhelming and smothering the “rational mind.” Clinically, this has profound implications for understanding pain and its management. Each component of the triune brain can be further defined: [also see references cited below for more information]
- Reptilian Brain — aka archipallium, primitive or "Basal Brain," hindbrain, or the "R-complex" — is the oldest brain in an evolutionary sense, consisting of structures of the brain stem, the globus pallidus, and the olfactory bulbs. These dominate in lower animals such as reptiles and are the center of archaic behaviors. The reptilian brain is rigid, obsessive, compulsive, ritualistic, aggressive, and paranoid. In the interest of self-preservation it keeps repeating the same behaviors over and over again. It also controls muscles, balance, and autonomic functions, such as breathing and heartbeat, so it is active even in deep sleep. Reflexive responses to acute pain, such as removing one’s hand from a flame, are quickly mediated by this primitive part of the brain (one does not waste time by enlisting higher intellectual centers to think about stopping the pain).
- Limbic System — aka paleopallium, old mammalian brain, or midbrain — is the central portion of the triune brain. It corresponds to the brain of early mammals and is concerned with emotion, motivation, and feeding, fighting, fleeing, and sexual behaviors. Stimuli passing through the limbic brain are valued as being either agreeable or disagreeable, and the repetition of pleasure and the avoidance of pain mediated by this system are essential for survival. The limbic system helps determine valence (eg, whether you feel positive or negative toward something) and salience (eg, what gets your attention). Anatomically, the limbic system has many distinct components and vast interconnections with the neocortex, so that brain functions are rarely purely limbic or purely cortical but a mixture of both. Limbic brain functions play a vital role in pain medicine and many of the medicinal agents used for pain management, both opioid and nonopioid (eg, antidepressants, anxiolytics, anticonvulsants, etc.), affect neurotransmitters and their receptors within limbic structures. The limbic brain also plays a central role in motivating and manifesting substance abuse and addiction, as well as somatization, catastrophizing, and other psychological syndromes that unfortunately hinder effective pain management.
- Neocortex — aka cerebrum, cortex, neopallium, neomammalian brain, or forebrain — comprises almost the whole of the hemispheres and some subcortical neuronal groups, taking up the largest portion of brain mass. It corresponds to the brains of primate mammals and, consequently, humans. However, the neocortex is not the sole domain of higher mammals, as all vertebrates have at least a bit of cortex, just as they all have other components of the triune brain to a degree. Humans have evolved to make the most of their cortex, and the higher cognitive functions of rational thought and reasoning that distinguish humans from other animals are in the neocortex. MacLean referred to the cortex as "the mother of invention and father of abstract thought." As is well known, the left half of the cortex controls the right side of the body and the right side controls the left side. Also, the right brain is more spatial, abstract, musical, and artistic; while the left brain is more linear, rational, and verbal. Certain regions of the neocortex are essential for isolating the location of pain and its intensity, as well as storing memories that help to recognize the type and possible cause of pain; however, it is the limbic system that colors how one feels about the pain and its implications for well-being and survival.
As we have previously discussed [here] and above, the limbic brain is at the heart of central pain processing, signaling alarms, and lending emotional overtones and meaning to the experience of pain. Associated negative perceptions of fear, anxiety, and subsequent suffering are mediated by the limbic system — if human suffering has a "home" it is most likely in the limbic brain. Interestingly, as Maclean once pointed out, many animals cry out in pain one way or another but only human beings actually shed tears when crying. Effective pain management strategies need to take into account unique qualities of the human pain experience; if they are applied in isolation, medications, medical interventions, physical therapies, and the many complementary and alternative medicine strategies may be insufficient and ineffective. However, used as part of a holistic approach — and administered by a compassionate and caring healthcare provider who understands and can tap into primal needs of the limbic brain — they can be a powerful force in relieving suffering if not the pain itself. There is much more to say about this but we will leave it for future entries in this series.
> Joseph R. Chapter 4: The Limbic System. In, Neuropsychiatry, Neuropsychology, Clinical Neuroscience. New York; Academic Press: 2000 [chapter available here].
> Kazlev MA. The Triune Brain [online]. 2003 [available here].
> Maclean PD. Brain Evolution Relating to Family, Play, and the Separation Call. Arch Gen Psychiatry. 1985;42(4):405-417 [abstract here].