This Neurobiology article on The Limbic System is brought to you by The Healing Center On-Line.
Introduction: The Three Units of the Human Brain
The Limbic System
The Primitive Brain
Throughout its evolution, the human brain has acquired three components that progressively appeared and became superimposed, just like in an archeological site: the oldest, located underneath and to the back; the next one, resting on an intermediate position and the most recent, situated on top and to the front. They are, respectively:
1 - The archipallium or primitive (reptilian) brain, comprising the structures of the brain stem - medulla, pons, cerebellum, mesencephalon, the oldest basal nuclei - the globus pallidus and the olfactory bulbs. It corresponds to the reptile brain, also called "R-complex", by the famous neuroscientist Paul MacLean.
2 - The paleopallium or intermediate (old mammalian) brain, comprising the structures of the limbic system. It corresponds to the brain of the inferior mammals.
3 - The neopallium, also known as the superior or rational (new mammalian) brain, comprises almost the whole of the hemispheres (made up of a more recent type of cortex, called neocortex) and some subcortical neuronal groups. It corresponds to the brain of the superior mammals, thus including the primates and, consequently, the human species.
These three cerebral layers appeared, one after the other, during the development of the embryo and the fetus (ontogenesis), recapitulating, chronologically, the evolution of animal species (phylogenesis), from the lizards up to the homo sapiens. According to Maclean, they are three biological computers which, although interconnected, retained, each one, "their peculiar types of intelligence, subjectivity, sense of time and space, memory, mobility and other less specific functions".
Actually, we have three cerebral units in a single brain. The primitive one is responsible for self preservation. It is there that the mechanisms of aggression and repetitive behavior are developed. It is there that occur the instinctive reactions of the so-called reflex arcs and the commands which allow some involuntary actions and the control of certain visceral functions (cardiac, pulmonary, intestinal, etc), indispensable to the preservation of life. The development of the olfactory bulbs and their connections made possible an accurate analysis of olfactory stimuli and the improvement of answers oriented by odors, such as approach, attack, flight and mating. Throughout evolution, some of these reptilian functions were lost or minimized (in humans, the amygdala and the entorhinal cortex are the only limbic structures that connect with the olfactory system). It is also in the R-complex that started the first manifestations of the phenomena of ritualism, by means of which the animal tries to define its hierarchic position inside the group and to establish its own space in the ecological niche
In 1878, the French neurologist Paul Broca called attention to the fact that, on the medial surface of the mammalian brain, right underneath the cortex, there exits an area containing several nuclei of gray matter (neurons) which he denominated limbic lobe (from the Latin word "limbus" that implies the idea of circle, ring, surrounding, etc) since it forms a kind of border around the brain stem ( in another part of this text we shall write more about these nuclei).
The entirety of these structures, that, years later would receive the name of "limbic system", developed with the emergence of the inferior ( primitive) mammals. This system commands certain behaviors that are necessary for the survival of all mammals. It gives rise and modulates specific functions that allow the animal to distinguish between the agreeable and the disagreeable. Here specific affective functions are developed, such as the one that induces the females to nurse and protect their toddlers, or the one which induces these animals to develop ludic behaviors (playful moods). Emotions and feelings, like wrath, fright, passion, love, hate, joy and sadness, are mammalian inventions, originated in the limbic system. This system is also responsible for some aspects of personal identity and for important functions related to memory. And, when the superior mammals arrived on the Earth, the third cerebral unit was finally developed : the neopallium or rational brain, a highly complex net of neural cells capable of producing a symbolic language, thus enabling man to exercise skillful intellectual tasks such as reading, writing and performing mathematical calculations. The neopallium is the great generator of ideas or, as expressed by Paul MacLean, "it is the mother of invention and the father of abstractive thought".
Theories on the Role of Brain Structures in the Formation of Emotions
Comparison of the James-Lange and Cannon-Bard theories of emotion.
According to the James-Lange theory (red arrows), the
man perceives the frightening animal and reacts with physical (neurovegetative)
manifestations. As a consequence of such unpleasant physical reaction,
he develops fear. In the Cannon-Bard theory
(blue arrows), the frightening stimulus leads, first, to the feeling of
fear which, then, brings about the physical response.
According to the James-Lange theory (red arrows), the man perceives the frightening animal and reacts with physical (neurovegetative) manifestations. As a consequence of such unpleasant physical reaction, he develops fear. In the Cannon-Bard theory (blue arrows), the frightening stimulus leads, first, to the feeling of fear which, then, brings about the physical response.
In 1929, Walter Cannon refuted James's theory and advanced another one, that was soon modified by Phillip Bardand and became known as the Cannon-Bard theory which states that, when a person faces an event that somehow affects him or her, the nervous impulse travels straight to the thalamus where the message divides. One part goes to the cortex to originate subjective experiences like fear, rage, sadness, joy, etc. The other part goes to the hypothalamus to determine the peripheral neurovegetative changes (symptoms). According to this theory physiological reactions and emotional experience occur simultaneously.
Papez believed that the experience of emotion was primarily determined by the cingulate cortex and, secondly, by other cortical areas. Emotional expression was thought to be governed by the hypothalamus. The cingulate gyrus projects to the hippocampus, and the hippocampus projects to the hypothalamus by way of the bundle of axons called fornix. Hypothalamic impulses reach the cortex via relay in the anterior thlamic nuclei.
More recently, Paul McLean, accepting the essential bases of Papez proposal, created the denomination limbic system and added new structures to circuit : the orbitofrontal and medialfrontal cortices (prefrontal area), the parahippocampal gyrus and important subcortical groupings like the amygdala, the medial thalamic nucleus, the septal area, prosencephalic basal nuclei (the most anterior area of the brain) and a few brainstem formations.
The Main Areas Involved with Emotions
It is important to stress that all these structures interconnect intensively and none of them is the sole responsible for any specific emotional state. However, some contribute more than others to this or that kind of emotion. We shall review now, one by one, the best known structures of the limbic system.
Amygdala and Hippocampus
A little almond shaped structure, deep inside the antero-inferior region of the temporal lobe, connects with the hippocampus, the septal nuclei, the prefrontal area and the medial dorsal nucleus of the thalamus. These connections make it possible for the amigdala to play its important role on the mediation and control of major affective activities like friendship, love and affection, on the expression of mood and, mainly, on fear, rage and aggression The amygdala, being the center for identification of danger, is fundamental for self preservation. When triggered, it gives rise to fear and anxiety which lead the animal into a stage of alertness, getting ready to flight or fight. Experimental destruction of both amygdalas (there are two of them, one in each hemisphere) tames the animal, which becomes sexually non-discriminative, deprived of affection and indifferent to danger. The electrical stimulus of these structures elicits crises of violent aggressivity. Humans with marked lesions of the amygdala, loose the affective meaning of the perception of an outside information, like the sight of a well known person. The subject knows, exactly, who the person is, but is not capable to decide whether he likes or dislikes him (or her).
Is particularly involved with memory phenomena, specially with the formation of long-term memory (the one that, sometimes, lasts forever). When both hippocampi (right and left) are destroyed, nothing can be retained in the memory. The subject quickly forgets any recently received message. The intact hippocampus allows the animal to compare the conditions of a present threat with similar past experiences, thus enabling it to choose the best option, in order to guarantee its own survival.
Thalamus and Hypothalamus
Lesion or stimulation of the medial dorsal and anterior nuclei of the thalamus are associated with changes in emotional reactivity. However, the importance of these nuclei on the regulation of emotional behavior, is not due to the thalamus itself, but to the connections of these nuclei with other limbic system structures. The medial dorsal nucleus makes connections with cortical zones of the pre-frontal area and with the hypothalamus. The anterior nuclei connect with the mamillary bodies, and through them, via fornix, with the hippocampus and the cingulate gyrus, thus taking part in the Papez's circuit.Hypothalamus
This structure has ample connections with the other prosencephalic areas and the mesencephalus. Lesions of the hypothalamic nuclei interfere with several vegetative functions and some of the so-called motivated behaviors, like thermal regulation, sexuality, combativeness, hunger and thirst. The hypothalamus is also believed to play a role in emotion. Specifically, its lateral parts seem to be involved with pleasure and rage, while the median part is like to be involved with aversion, displeasure and a tendency to uncontrollable and loud laughing. However, in general terms, the hypothalamus has more to do with the expression (symptomatic manifestations) of emotions than with the genesis of the affective states. When the physical symptoms of emotion appear, the threat they pose returns, via hypothalamus, to the limbic centers and, thence, to the pre-frontal nuclei, increasing anxiety. This negative feed-back mechanism can be so strong as to generate a situation of panic. As it will be seen later on, the knowledge of this phenomenon is very important, for clinical and therapeutic reasons.
Ventral Tegmental Area
Humans display the largest web of connections between the prefrontal area and the traditional limbic structures. Perhapas that is why they present, among all species, the greatest variety of feelings and emotions Although some signs of affection can be perceived in birds, the limbic system only began to evolve, in fact, after the first mammals, being practically non-existent in reptiles, amphibians and all other preceding species.
Paul MacLean uses to say that "it is very difficult to imagine a lonelier and more emotionally empty being than a crocodile". Two behaviors, with affective connotation, that appeared with mammals (birds also display them, but less intensely) deserve to be emphasized because of their peculiarity :
1 - The intense and long-lasting care and nursing of females towards their offsprings.
2 - A playful mood. The more evolved the mammal, the more accentuated are these behaviors.
Ablation of important parts of the limbic system of any animal causes it to loose, totally, both motherly affection and ludic interest.
And the evolution of mammals bring us to mankind. Certainly, our hominid ancestor could already establish differences between the sensations he experienced in distinct occasions, such as being at his cave polishing a stone or a bone, running after a weaker animal, running away from a stronger one, hunting a female of his species etc.
With the development of language, particular names were given to these sensations, allowing their definition and communication to other members of the group. Since there exists an important subjective component, difficult to be communicated, even today there is no uniformity concerning the best terminology to be used, in order to designate, specifically, many of these sensations.
Therefore, the words affect, emotion and feeling are used interchangeably and imprecisely, almost like synonyms. However, we think that each of these words deserves a precise definition, for the sake of their etymology and because of the physical and mental reactions they cause.
Affect (from the Latin affectus meaning to afflict, to shake, to touch) could be defined as "a grouping of physic phenomena manifesting under the form of emotions, feelings or passions, always followed by impressions of pleasure or pain, satisfaction or discontentment , liking or disliking, joy or sorrow".
Curiously, there is a worldwide trend to consider as affect (as well as its derivatives, like affection, affectuous, etc.) only the positive impressions. Thus, when I say : "I feel affection for that girl", I'm expressing love or tenderness, never anger or fear. Counterwise, emotions and feelings can be used to name both positive and negative phenomena: "she has good feelings; I've had painful emotions".
According to Nobre de Melo, affect denominates, generically, events experienced as emotions or feelings. Emotions (from Latin emovere meaning moving, displacing) are, as its etymology suggests, manifest reactions to those affective conditions that, due to their intensity, move us to some kind of action. Confronting the opinion of several authors, we can say that emotions are characterized by a sudden disruption of the affective balance.
Almost always, they are short episodes, with slight or intense, concomitant or subsequent, repercussions upon several organs, that can set up partial or total blocking of logical reasoning. This can provoke, in the affected subject, a high degree of psychic and behavioral loss of control. Conversely, feelings are seen as affective states with a longer duration, causing less intensive experiences, with much fewer repercussions upon organic functions and lesser interference on reasoning and behavior.
Exemplifying: love, fear and hate are feelings; passion, fright and anger (or wrath) are emotions. There also exist two other well characterized conditions that are, in a certain way, inserted in the context of affective life (since, depending on the intensity of the affect, these two conditions may result from the affective state or, sometimes, can be mistakenly considered as an emotional manifestation).
We are talking about mood disturbances (represented by depressions or maniacal euphoria) and the lessening of mental relaxation associated to the alert reaction (represented by anxiety). During centuries, philosophers, physicians and psychologists studied the phenomena of affectivity, questioning their origin, their role upon psychic life, their action favoring or hindering adaptation, their neurophysiological concomitants and their neuroendocrine substratum. Affective manifestations would have as their ultimate cause, the capacity of living matter to answer to stimuli acting upon it. There are two classical and antagonistic theories upon the subject. The first one, supported by Darwin and his followers, state that affective reactions are innate patterns designed to orient behaviour in order to promote the adaptation of a being to its environment, thus guaranteeing its survival and that of its species.
The organic disturbances, that may go together with the process, would only be a consequence of physiological nature. On the contrary, others, like William James, state that, facing a given real or imaginary stimulus, the organism would react with a series of muscular and visceral neurovegetative changes.
The perception of such changes would give origin to the corresponding affective states. More recently, Lehmann proposed a third theory that suggests a compromise between the two classical ones. According to him, affect is a complex phenomenon, initiated by a central process, as result of internal or external causes. It manifests itself as a change in the "I", releasing reflex facial movements and producing manifold organic alterations.
The more the bodily symptoms increase, the more mobilizing becomes the affect, until it evolves into an emotion. This statement finds clinical support in the treatment of patients with performance phobias. When facing situations they fear (speaking before audiences, for instance) these patients present palpitations, sweating, difficult breathing, etc.. Beta blocking agents, that do not cross blood brain barrier (therefore devoid of influence upon brain centers) act only peripherally, blocking the neurovegetative symptoms and, by "emptying" anxiety, facilitate the control of the phobic process.
Divergent are also the opinions about the relationship between affective states and reason. Some philosophical and religious schools consider the affective aspects of personality as inferior, negative or sinful, in need of control and domination by reason. Claparède, in a paper bearing the title "Feelings and emotions", defines emotions as useless, unadaptative and harmful phenomena, true remains of ancestral reactions. Just the opposite of feeling, that would be useful, allowing human beings to estimate the value of things to which they must adapt and differentiating the useful from the noxious.
Quoting the author : "Observation shows how unadaptive are emotional phenomena. Emotions happen precisely when adaptation is hindered by any motive . "The analysis of corporal reactions in emotions shows that the subject does not enact adaptive movements but, on the contrary, reactions that resemble indefinite primitive instincts... "Far from being the psychic aspect of an instinct, emotion represents a confusion of this instinct ". Contrariwise, other authors consider affective reactions as factors favoring adaptation and survival, inducing some behaviours and inhibiting others. In their opinion, even intense emotions, evaluated as disruptive by others, could favour survival.
This is so because their disruptiveness is a selective one : while some actions are abolished, others, more favorable, are allowed to occur. We believe that, within certain limits, the affective participation reinforces the cognitive component, giving more flavour to the day-to-day experiences and facilitating adaptive behaviors. Nevertheless, when above these limits, emotions hamper reasoning and, when below them, as stressed by Damasio in "Descarte's error", affectivity becomes scarce, thus impoverishing the quality of life.
About the Authors:
Júlio Rocha do Amaral, MD -
Jorge Martins de Oliveira, MD, PhD. -
Full Professor and Master of UFRJ (Rio de Janeiro). Associate Professor of UFF. Scientific Coordinator. Coordinador and Director of the Department of Neurosciences of the Institute of Human Being (RJ). Fellow in Research by Saint Vincent Charity Hospital, Cleveland, USA. Full Member of Brazilian Academy of Military Medicine. Member of the Brazilian Academy of Writers Physicians. Graduated by Superior School of War (ESG).