Broca's Limbic Lobe, Papez's Circuit, and MacLean's Limbic System
The scientists we discuss in this section were all intrigued with the strangely-shaped assemblage of brain structures lying beneath the neocortex that we humans share with all other mammals. What has historically been called the limbic system, this "animal brain," as Thomas B. Czerner calls it in What Makes You Tick? The Brain in Plain English (2001), "orchestrates your emotive, instinctual behavior. In this head versus heart dilemmas you are forever debating, this is the voice of your heart. The genuine smile originates here. So does the mania and the depression of bipolar disease; the uncontrollable, ritualized behavior and fixed ideas of obsessive compulsive disorder; and the desperate, suicidal impulses of major depression."
Pierre Paul Broca (1824-1880) was a French physician, surgeon, and neuroanatomist who coined the term limbic lobe. Limbic is derived from limbus which means generally an edge, border, or hem. I learned why Broca used the term limbic lobe when reading Andrew Lautin's book, The Limbic Brain (2001). Lautin cites W.J.H. Nauta's book, Fundamental Neuroanatomy, wherein Nauta provides an elegant description of what Broca discovered when he examined the differentiation between deeply recessed cortical tissue and underlying, subcortical nuclei: "…the edge is clearly arrayed in a way suggesting the strategy of a seamstress who finishes off the hem of a garment by folding its material several times and even placing over its end an extra piece of fabric before stitching it all together."
In his book, The Emotional Brain: The Mysterious Underpinnings of Emotional Life (1996), Joseph LeDoux explains that anatomist C. Judson Herrick (1868-1960) pointed out a distinction between the lateral and medial parts of the cortex. LeDoux writes: "Imagine the brain as a hot dog bun, with the two halves of the bun being the two cerebral hemispheres. The brownish, toasted part of the bun that we can see from the outside is like the lateral part of the cerebral cortex." This part of the cortex, which we now refer to in evolutionary terms as the neocortex, includes sensory and motor areas and produces rational thought and language. LeDoux goes on to say: "Now, imagine prying the bun apart down the seam in the middle, pulling the two hemibuns away from each other. The white, untoasted part of the hemibuns down the middle is like the medial part of the cortex. This part, according to Herrick, is evolutionarily older and is involved in more primitive functions than the newer cortex, known as the neocortex …." LeDoux notes that "Herrick's medial cortex had earlier been called le grand lobe limbique by the great French anatomist Paul Pierre Broca."
Because of connections to the olfactory bulbs, early anatomists referred to the medial cortex, or the limbic lobe, region of the brain as the rhinencephalon, a term that means "nose brain." In the Gray's Anatomy image to the right (image links to source), which depicts an upside-down view of the base of the human brain, I have circled one of the olfactory bulbs in red. You can see how it interdigitates with deeply recessed cortex.
Scientists have come to understand that in addition to olfaction, neural circuitry in this region is responsible for, as Jaak Panksepp puts it in Affective Neuroscience: The Foundations of Human and Animal Emotions (1998), "emotional and motivational impulses." Panksepp explains that "animals seek out food with the exquisitely sensitive chemodetectors of their nose, and it seems that exploratory and appetitive motivational tendencies (which allow animals to anticipate positive events in the world) emerge from neural systems that encourage the use of the olfactory system." That evolution has elaborated on the existing olfactory system to create motivational systems is an evolutionary concept called exaptation. In mammals, the olfactory system has also evolved as a means to identify friend from foe. Panksepp notes that "rats get acquainted through mutual investigation, especially of the ano-genital region (as do dogs). If animals already know each other well, they spend considerably less time investigating each other than if they are strangers."
In his video course, Biology and Human Behavior: The Neurological Origins of Individuality, 2nd edition, Robert M. Sapolsky explains that the limbic brain was historically called the rhinencephalon, meaning "nose-brain," because it was studied first in laboratory rats. Scientists realized that what rats smell governs their emotions and behavior. In rats, olfaction and emotion are completely entwined. It is by smell that a rat identifies and attacks an intruder entering the nest. "Find the neurons that are olfactory receptors and you are one synapse away from the limbic system," Sapolsky says.
In 1937, James W. Papez (1883-1958), an American neuroanatomist at Cornell University, published a paper titled "A Proposed Mechanism of Emotion" in Archives of Neurology and Psychiatry. Papez delineated the neural circuitry that he believed might mediate emotions. In Affective Neuroscience, Panksepp explains that even though Papez "did not clearly specify which emotion(s) he was concerned with, anatomically he was quite specific. He based much of his reasoning on early brain ablation experiments and the study of a brain disease that induces rage, namely rabies, which is known to damage the hippocampus."
LeDoux explains that Papez "was a great synthesizer and put together Herrick's idea about the evolutionary distinction between the medial and lateral cortex with two other kinds of findings—observations about the consequences of brain damage in the medial cortex in humans and research on the role of the hypothalamus [a subcortical nucleic structure] in the control of emotional reactions in animals. The outcome was a theory that explained the subjective experience of emotion in terms of the flow of information through a circle of anatomical connections from the hypothalamus to the medial cortex and back to the hypothalamus. This is now known as the Papez circuit."
In The Limbic Brain, Lautin provides a comprehensive and scholarly account of the early anatomists' scientific progress. In reviewing the book for the American Journal of Psychiatry (2003), Johannes Pantel provides a succinct account of Papez's contribution, explaining that Papez "proposed that a circulatory consortium of neural components and their connections participate in concert harmoniously to elaborate emotional awareness." Pantel goes on to say, "Although the main elements of Papez's circuit largely conformed to Broca's great limbic lobe, Broca's early speculations regarding an emotive function for this structure did not contribute in any direct way to the development of Papez's concept. Rather, Papez was influenced by new data derived from brain transection and stimulation experiments, developments in comparative neuroanatomy, clinical observations, and an evolving neuroanatomical philosophy (postulating reciprocating cortical to subcortical circuits as central processes elaborating consciousness)."
LeDoux notes that almost all of the anatomical paths which Papez proposed do exist. Ledoux writes: "Unfortunately, at least for the Papez theory of emotion, this circuit appears to have little involvement in emotion."
The rhinencephalon overall, however, proved to do much more than distinguish smells. So although his circuitry was off the mark, Papez was correct in theorizing that the rhinencephalon was involved in producing emotion. LeDoux explains, "as Papez and Herrick had argued, the evolutionarily older areas of the medial cortex, the so-called rhinencephalon, are intimately connected with the hypothalamus [which regulates the autonomic nervous system including the fight-or-flight response]. That the rhinencephalon was not just a smell brain in higher mammals was evident from the fact that dolphins and porpoises, which have no sense of smell, have highly elaborate rhinencephalic regions, and that in humans the olfactory sense is of comparatively less importance but certain areas of the rhinencephalon (especially the hippocampus and cingulate regions) reach their greatest development."
In a footnote, Panksepp explains that Paul D. MacLean gave new life to Papez's theory when in 1949 MacLean published "Psychosomatic disease and the 'visceral brain': Recent developments bearing on the Papez theory of emotion." The preceding link will initiate a download of MacLean's entire article.
According to Panksepp in Affective Neuroscience, Paul D. MacLean coined the concept of the limbic system in a 1952 publication: "Some psychiatric implications of physiological studies on frontotemporal portion of limbic system (visceral brain)" (Electroenceph. Clin. Neurophysiol. 4:407-418). In addition to those structures that early anatomists referred to as the rhinencephalon, Panksepp explains that MacLean "identified the medial surfaces of the telencephalic hemispheres (including cingulate, frontal and temporal lobe areas—especially the amygdala) and interconnections with septal, hypothalamic, and central-medial brain stem areas as part of the neural landscape that constituted the 'emotional brain.'"
LeDoux notes MacLean's early ideas, from MacLean's 1949 publication, about the emotional brain.
Maclean suggested that, "although, in the ascension to higher forms, the rhinencephalon yields more and more control over the animal's movements to the neocortex, its persistent, strong connections with lower autonomic centers suggests that it continues to dominate in the realm of visceral activity." While the neocortex "holds sway over the body musculature and subserves the functions of the intellect," the visceral brain is the region involved in "ordering the affective behavior of the animal in such basic drives as obtaining and assimilating food, fleeing from or orally disposing of an enemy, reproducing, and so forth."
I should note here that the term limbic system is outdated since, as it turns out, limbic structures do not function as a discrete system. Moreover, there is not one but several emotional systems in the brain. We discuss these systems—mediating seeking, rage, fear, panic/loss, play, mating, and care behaviors—in Part 2 of MyBrainNotes.com. Also, the medial cortex (or limbic lobe) and subcortical limbic structures lying beneath the medial cortex intricately connect to both higher and lower brain structures—affecting both cognitive and emotional processing in the brain.
Emotion-producing neurocircuits involving limbic-brain structures, however, as well as older structures that we mammals share with reptiles, can sometimes gain a controlling influence over our human behavior under certain circumstances. This is, I think, the basic reason that obsessions and compulsions occur. Lower-order influence may also account for some violent impulses.
We will discuss specifics regarding the medial cortex and subcortical brain structures—which I refer to as the limbic brain—later in Part 1 of MybrainNotes.com, beginning on a page titled Subcortical Brain Structures, Stress, Emotions, and Mental Illness. In Part 2 of MyBrainNotes.com, we discuss emotional systems including the RAGE system, which produces violent impulses. We will discuss the role of subcortical brain structures in generating obsessions and compulsions in detail in Part 3 of MyBrainNotes.com.
We can be certain that without limbic-brain structures, our emotions as we know them would cease to exist. These structures not only make us mammalian, they make us human. It is then so much more ironic that, as MacLean told the New York Times, "the greatest language barrier lies between man and his animal brains; the neural machinery does not exist for intercommunication in verbal terms."
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