Papez circuit: what is it and what brain structures does it include?

The Papez circuit is a set of interconnected parts of the brain. Let's see what it looks like.

The Papez circuit refers to a series of brain structures related to the processing of emotions, memories and learning. related to the processing of emotions, memories and learning.

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

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

What is the Papez circuit?

The Papez circuit defines a set of brain structures located in what is known today as 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 linking the hippocampus connections linking the hippocampus to the hypothalamus, thalamus and cingulate cortex, and these back 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 within the diencephalon.

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

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

MacLean's contributions

In 1949, the American neuroscientist Paul Maclean postulated a new neuroanatomical model of the emotions: the triune braina model more in line with current knowledge. MacLean's model took the ideas of Papez and those of Cannon and Bard's theory, according to which emotional stimuli would be capable of provoking both the feeling of the emotion in the brain and the expression of the emotion in the peripheral nervous system; that is, emotion and reaction would occur simultaneously.

MacLean also integrated into his model the knowledge provided by the studies of Klüver and Bucy, who had shown that bilateral removal of the temporal lobes in monkeys led to a characteristic set of behaviors that included, among other symptoms such as hypersexuality or increased exploratory behaviors, the loss of emotional reactivity. Studies indicated that the temporal lobes played a key role in emotional processing.

MacLean's extended model divided the brain in three partsThe first is the reptilian brain, the oldest in evolutionary terms and home to primitive emotions such as fear or aggression; second, the mammalian or visceral brain, responsible for shaping primitive emotions and elaborating more social emotions, would include many of the components of Papez's circuitry; and third, the new brain or neocortex, which connects emotions to cognition and exerts top-down control over emotional responses driven by other systems.

MacLean's essential idea was that emotional experiences involved the integration of external sensations with information coming from the organismthat is, the events of reality would provoke bodily changes. This integration would be responsible for generating the final emotional experience and it was carried out by the visceral brain, which he later called the limbic system.

Structures of the Papez circuit and their functions

The Papez circuit included cortical and subcortical structures such as the hippocampus, the fornix, the mammillary bodies, the mammillothalamic tract, the anterior nuclei of the thalamus, the cingulate gyrus and the entorhinal cortex.

When the circuit was expanded and reconceptualized as the limbic system, other structures such as the amygdaloid complex or the orbitofrontal cortex were added. Let us see what each of them consists of:

1. Hippocampus

A fundamental structure in the human brain, it is involved in memory consolidation and learning..

2. Fornix

Brain structure composed of white matter that originates in the hippocampus and serves as a connector between different areas of the brain. serves as a connector between different areas of the brainmainly from the hippocampus to the hypothalamus and from one hemisphere to the other.

3. Mammillary bodies

Located at the base of the brain, They connect the amygdala to the hippocampus and are involved in memory processes.and are involved in memory processes.

4. Mammillothalamic tract

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

5. Anterior nucleus of the thalamus

Located in the thalamus, they receive fibers from the mammillary bodies forming the mammillothalamic tract and are involved in processes involved in memory, learning and certain emotional behaviors.

6. Cingulate gyrus

It is a cerebral gyrus with important functions within the limbic systemsuch as the formation of emotions and the processing of information related to behavior, memory and learning.

7. Entorhinal cortex

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

8. Amygdaloid complex

Set of nuclei located in the temporal lobes with functions of processing and storing functions of processing and storage of emotional reactions.. They also seem to have an important role in the modulation of memory and in the response to sexual hormones.

9. Orbitofrontal cortex

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

The role of the amygdala

One of the best ways to understand the functioning of a brain structure is by studying and comparing patients with lesions and healthy subjects. As far as the amygdala is concerned, we now know that lesions in this structure can lead to alterations in the processing of faces and other social cues.. And if the lesion is bilateral and deep, typical signs of Klüver-Bucy syndrome may appear, such as hyperorality, passivity or bizarre eating behaviors, among others.

We know that the amygdala is a structure involved in fear conditioning.. In this regard, one investigation described the case of a man with a lesion in the right amygdala who showed a significantly diminished startle response to a sudden outburst. The subject also appeared to become immune to fear conditioning.

In another similar case, it was observed that one of the patients with bilateral amygdala damage did not respond to conditioning to aversive stimuli. In contrast, another subject with hippocampal lesions did successfully acquire fear response conditioning, although he lacked explicit recall of how he acquired it. The latter would indicate that the amygdala plays a fundamental role in fear processing and conditioning.

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

Literature references:

• Papez, J. W. (1937). A proposed mechanism of emotion. Arch. Neurol. Psychiatry 38, pp. 725 - 743.
• Pessoa, L., & Hof, P. R. (2015). From Paul Broca's great limbic lobe to the limbic system. Journal of Comparative Neurology, 523(17), pp. 2495 - 2500.