Papez’s circuit: what it is and what brain structures it includes

The Papez circuit hints at a number of brain structures related to the processing of emotions, memories and learning.

What we know today as the limbic system was a theoretical model that developed over the years and the contributions of different scientists in order to lay the foundation for how human emotions work.

In this article, we explain what this circuit consists of and what is its structure, as well as its main authors and contributions.

    What is the Papez circuit?

    Papez’s circuit defines a set of brain structures located in what we today call the limbic system and involved in the management of emotions, memory, and learning. It was proposed in 1937 by the American neurologist James Papez, with the aim of theorizing a neuroscientific model capable of explaining the functioning of human emotions.

    Papez postulated the existence of a set of serial connections connecting the hippocampus to the hypothalamus, thalamus and cingulate cortex, and these return to the hippocampus. Although the model involved what Paul Broca called the “limbic lobe” (which included the olfactory bulb, cingulate gyrus, and hippocampus), it also included other subcortical structures in the diencephalon.

    This model approached emotions as a function of the activity generated in the cingulate cortex. According to him, projections from this region to the hypothalamus and the nipple bodies would allow cortical regulation of emotional responses from top to bottom and the thalamocortical pathway would be responsible for transforming sensations into perceptions, feelings and memories.

    It should be noted that while Papez took virtually all of the credit, his theoretical model was based on previous research by German physician and philosopher Christofredo Jakob, Who developed a theory of the limbic system and central mechanisms of emotional processing many years before the circuit was attributed to Papez (later his contributions were recognized and the circuit was renamed “Jakob-Papez”) .

    MacLean’s contributions

    In 1949, American neuroscientist Paul Maclean postulated a new neuroanatomical model of emotions: the Trinitarian brain, A model more in tune with current knowledge. MacLean’s model took the ideas of Papez and those of Cannon and Bard’s theory that emotional stimuli are able to induce both the feeling of emotion in the brain and its expression in the brain. in other words, the emotion and the reaction would occur simultaneously.

    MacLean also incorporated into his model the knowledge provided by the studies of Klüver and Bucy, which had shown that the bilateral extraction of temporary lobes in monkeys led to a characteristic set of behaviors that included, among other symptoms such as hypersexuality. or increased exploratory behavior, loss of emotional responsiveness. Studies have indicated that the temporal lobes play a key role in emotional processing.

    MacLean’s extended model divides the brain into three parts: First, the reptilian brain, the oldest in evolutionary terms and the one that harbors primitive emotions such as fear or aggression; second, the mammalian or visceral brain, responsible for shaping primitive emotions and creating more social emotions, would include many components of Papez’s circuit; and third, the new brain or neo-procession, which links emotions to cognition and exerts top-down control over emotional responses driven by other systems.

    MacLean’s core idea was that emotional experiences involved integration of external sensations with information from the body; that is, the events of reality would bring about bodily changes. It would be this integration that was responsible for generating and realizing the final emotional experience was the visceral brain, which he later called the limbic system.

      Papez circuit structures and their functions

      Papez’s circuit included cortical and subcortical structures such as the hippocampus, fornix, nipple bodies, mammothothalamic tract, anterior thalamus nuclei, cingulate gyrus, and entorhinal cortex.

      When the circuit was extended and reconceptualized as a limbic system, other structures such as the amygdala complex or the orbitofrontal cortex were added. Let’s see what each of them consists of:

      1. Seahorse

      A fundamental structure in the human brain, participates in memory consolidation and learning.

        2. Fórnix

        Brain structure made up of white matter that comes from the hippocampus and that serves as a connector between various areas of the brain, Mainly from the hippocampus to the hypothalamus and from one hemisphere to another.

        3. Nipple body

        Located at the base of the brain, they make a link between the amygdala and the hippocampus, And participate in memory processes.

        4. Mamylothalamic treatment

        This structure connects the bodies of the nipple with the anterior nuclei of the thalamus.

        5. Anterior nucleus of the thalamus

        Located in the thalamus, they receive fibers from the nipple bodies forming the mammothelamic tract and are involved in processes related to memory, learning, and certain emotional behaviors.

        6. Giro cingulat

        It is a brain convolution with important functions within the limbic system, Such as the formation of emotions and the processing of information relating to behavior, memory and learning.

        7. Entorhinal cortex

        This structure is located in the medial temporal lobe and is involved in learning and guidance functions, With an important role in autobiographical and spatial memory.

        8. Tonsillus complex

        Set of nuclei located in the temporal lobes with functions of processing and storing emotional reactions. They also appear to play an important role in modulating memory and responding to sex hormones.

        9. Orbitofrontal cortex

        It is a brain region located in the frontal lobe and involved in cognitive processing: decision making and setting expectations.

        The role of the amygdala

        One of the best ways to understand how a brain structure works is to study and compare patients with already healthy injuries. As for the amygdala, we now know that injuries in this structure can lead to alterations in facial processing and other social signals. And if the lesion is bilateral and deep, typical signs of Klüver-Bucy syndrome may appear, such as hyperoralism, passivity or strange eating behaviors, among others.

        The amygdala is known to be a structure involved in conditioning fear. In this regard, an investigation described the case of a man with a right tonsil injury who showed a sudden dramatically diminished response to a sudden explosion. The subject also appears to be immune to fear conditioning.

        In another similar case, it was observed that one of the patients with bilateral amygdala damage did not respond to the conditioning of the aversive stimuli. In contrast, another subject with hippocampal injuries managed to acquire the conditioning of the fear response, although he did not have an explicit recollection of how he had acquired it. The latter would indicate that the amygdala plays a key role in the processing and conditioning of fear.

        Finally, with regard to memory consolidation, it has been confirmed that patients with tonsil damage do not show better memory of the emotional aspects of an event or event (compared to the non-emotional aspects). The studies of Positron emission tomography shows that high levels of glucose metabolism in the right amygdala could predict recall of positive or negative emotional stimuli up to several weeks later.

        Bibliographical references:

        • Papez, JW (1937). A proposed emotion mechanism. Bow. Neurol. Psychiatry 38, pages 725 – 743.
        • Pessoa, L. and Hof, PR (2015). From the great limbic lobe of Paul Broca to the limbic system. Journal of Comparative Neurology, 523 (17), pages 2495-2500.

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