Limbic System in Detail

Introduction

Oscar Wilde, a widely known Irish poet and playwright, says about emotion, “I don't want to be at the mercy of my emotions. I want to use them, to enjoy them, and to dominate them.” What he says is not completely wrong. We may sometimes have the ability to use, enjoy and dominate our emotions, yet this is not the case for our whole life; sometimes, they can use and dominate us, too. When thought, it is kind of hard to think that all of these emotions are arising from biological roots, but it is the inevitable truth. This entry is written for people who wants to dig deep and learn elaborated information regarding the limbic system. While the entry is written, various textbooks, research, and detailed medical websites are used. In this entry, the source of these biological roots of emotions – the limbic system - will be examined elaborately in its structure, function, and cognitive abilities.

Structurally Limbic System

When people, assuming with a little bit of neuroscience knowledge, hear the word emotion, the words that would pop into their mind is “limbic system”. This system is mostly responsible for behaviors, emotions, memory, learning, and social interactions. While having these responsibilities, the system receives a great deal of input from diverse parts of the nervous system interrelatedly and puts them in a harmony to get its job done. In addition, because to achieve its responsibilities limbic system uses many different inputs, when a damage occurs to it, many deficits would be present due to the diverse inputs it takes. In a nutshell, the limbic system – a system with diverse inputs from nervous system and the body - is important in many different responsibilities, beginning with emotions, is structurally divided into two levels: first and second level.

The First Level

This level contains the cortical structures of the hemispheres that are most close to the middle; also, the areas this level contains are more fundamental than the second level. They form up the “limbic lobe”. In this limbic lobe, there are several areas - they have complicated names but knowing them allows people to understand the functions fairly better: subcallosal area (containing the parolfactory and paraterminal gyri), cingulate gyrus, isthmus of the cingulate gyrus, para-hippocampal gyrus, uncus, and hippocampal formation.

The Second Level

This level contains the complementary areas for the limbic lobe; thus, it makes up the “limbic system”. The areas are basically structures of limbic lobe, some subcortical nuclei, and some tracts. The more detailed area names are: septal nuclei, nucleus accumbens, nuclei of hypothalamus (especially the one regarding mammillary body), nuclei of amygdaloid complex, substantia innominate, parts of thalamus such as anterior and dorsomedial nuclei, habenular nuclei, ventral tegmental area, periaqueductal gray and prefrontal cortex

These names of brain structures/areas can be tiring. Therefore, in the upcoming paragraphs, even though the functions of the limbic system will be discussed in detail, the structural components will be tried to be kept as basic as possible. In addition, the main parts that will be elaborated on are cingulate gyrus, para-hippocampal gyrus and hippocampal formation as hippocampus, nucleus accumbens, nuclei of hypothalamus as hypothalamus, anything regarding amygdala, thalamus, ventral tegmental area, periaqueductal grey, and prefrontal cortex.

Functionally and Cognitively Limbic System

The fore-told structures will now be responsible all the functions of limbic system that are explained. While reading this part, it is very essential to know that in all limbic system, not even the single easiest job is carried out by one structure; yet, everything is collaborated among structures. As known, when someone cries, laughs, shouts, inhales deeply or just grinds his/her teeth, all can be linked to emotions. But the important question is “How?”.

Categorization of Emotions

Emotions are sometimes examined in two: valence as they being either pleasant or unpleasant, or intensity as they being either low or high arousal. This examination in two is named the “core effect” of emotions, and it can be seen in many psychology theories. Also, emotions can be grouped as basic – happiness, fear, anger disgust, sadness - and complex – shame, guilt, embarrassment, pride, jealousy- emotions. Like this grouping, but in a more basic way, psychologist Paul Ekman have categorized basic emotions, too: anger, fear, surprise, disgust, joy, and sadness.

Creation of Emotions

As can be seen, there are many different emotions, and when these emotions are felt by brain detecting certain environmental stimuli/situations, naturally, the body is exposed to broad range of physiological responses. For instance, if a person is waiting for his/her exam score to be revealed, he would feel stressed causing his/her stomach to feel bad. Like this physiological response, many of the physiological responses act on both brain and body, and they are automatic, but they can occur according to context. Regarding brain, arousal levels, cognitive functions such as attention, memory processing and decision making can alter. Regarding body, somatic responses such as endocrine, autonomic and musculoskeletal systems can alter.

There are stimuli present in environment that are learned and affecting emotions such as the given exam example. Yet, there are innately learned unconditional stimuli affecting emotions, too; for example, a painful shock or a very filthy taste are examples. However, many of the stimuli affecting emotions are not innate, yet learned by association.

Physiological Processes After Detecting an Emotionally Significant Stimulus

A person is getting dressed for performing in front of a hundred thousand people. After getting dressed, he sees that he is stressed a lot. He starts to sweat and tremble. The person experiences the detection of an emotionally significant stimulus and his/her body acts upon it.

There are three interrelated physiological systems responding to an emotionally significant stimulus: endocrine glands, autonomic motor system and the musculoskeletal system. Endocrine system takes care of the regulation and secretion of hormones into the bloodstream. Autonomic system aids the changes in cardiovascular system, visceral organs and tissues in the body cavity. The skeletal motor system is responsible for directly seen behaviors such as freezing, fight-or-flight, and some facial expressions.

Neural Circuitry of Emotions with A Little History

First contribution to emotions arose from William James – the founder of American psychology. He proposed that fear feeling is a consequence of bodily changes occurring while fleeing. In other words, people feel afraid because they run, not they run because they fear. James’s this first attempt theory is named “peripheral feedback theory”.

Later on, scientist Walter B. Cannon entered to the field. He thought that hypothalamus might be a crucial region for regulating sympathetic nervous system (fight-or-flight) responses, and some other emotions. Then, by research, he showed that hypothalamus was a crucial region for organizing the emotional reactions. Later on, he came up with a theory while working with his student Phillip Bard. This theory’s name is Cannon-Bard theory. According to it, the incoming sensory information is processed in thalamus. Then, it is sent to both hypothalamus and cerebral cortex. Projections on hypothalamus – through connections to brain stem and spinal cord - produce emotional responses whereas projections on cerebral cortex produce conscious feeling.

Next, the scientist James Papez extended the Cannon-Bard theory. According to Papez, Cannon-Bard theory was correct, yet it needed additions. His addition was claiming descending brain stem and spinal cord connections making emotional responses and ascending brain stem and spinal cord connections to cerebral cortex giving rise to feelings. He, also, said that the signals from hypothalamus first go to the anterior thalamus, then to the cingulate cortex. In cingulate cortex, according to Papez, the signals from hypothalamus and sensory cortex converge creating the conscious experience of feeling. There are outputs from cingulate cortex, too, helping body as a feedback mechanism. These cingulate cortex outputs go to hippocampus, and then to hypothalamus, thus completing the loop.

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A note on hypothalamus: This structure is receiving intense interest in emotion studies. In it, specific parts are linked with defensive emotion states, too. Therefore, it can be said that hypothalamus not only organizes behaviors, but also constitutes emotion itself.

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Regarding emotions, there were research done on monkeys by Heinrich Klüver and Paul Bucy. This research involved removing the temporal lobes of monkeys bilaterally and so lesioning all temporal cortex as well as subcortical structures like amygdala and hippocampus. When the monkey behaviors were observed, it was concluded that amygdala has an important role in emotion, and the phenomenon with lesioned amygdala is came to be known as Klüver-Bucy syndrome. Later on, Klüver and Bucy added on all the findings and said that emotion is the product of visceral brain – different cortical areas that are referred as limbic lobe, later named limbic system (explained in the structural part of the entry).

As a different perspective to Papez’s theory, MacLean developed one. He though same as Papez, yet he had one difference: according to him, the convergence of hypothalamus and sensory cortex were not at cingulate cortex, yet at hippocampus. While his research, his final findings were found to be not parallel to what he thought. Yet, they were very important. From his studies’ findings, it is discovered that hippocampus is responsible for converting short-term memory to long-term memory, yet not directly related to emotions.

After all history, it is understood that emotions are processed by many different regions, yet mainly by limbic system. Within the limbic system, the mostly focused component is amygdala; its role is mostly known as “learned fear”.

Amygdala and Learned Fear Conditioning

To understand amygdala’s major job of “learned” fear, it is important to understand what Pavlovian fear conditioning is. Basically, when an association between an unconditioned stimulus (US) – e.g. an electric shock - and a conditioned stimulus (CS) – e.g. a tone - predicting the unconditioned stimulus (US) is built. To exemplify, if a person is shocked with electricity for 5 times, and at the end of all 5 times’ electric shock is given with a person clapping, the person would pair the clapping and electric shock – the clapping is conditioned stimulus, and the electric shock is unconditioned stimulus. In addition, when the body is presented, a conditioned stimulus showing signs of a punishing unconditioned response, it elicits its defensive freezing and regulates its autonomic system and endocrine activity accordingly.

In many different studies, it is seen that amygdala is a needed structure for the Pavlovian classical conditioning’s classical fear conditional learning. Within the amygdala, there are 12 nuclei: only the lateral and central nuclei are crucially important for fear conditioning. This is because the lateral nucleus receives the most sensory input from the body regarding the conditioned stimulus from thalamus and cortex. Also, it is a site of memory storage regarding fear conditioning. On the other hand, the neurons in the central nucleus organize outputs to the brain stem and control defensive behaviors associated with autonomic responses. If a damage occurs to amygdala, only when these two parts are affected the fear conditioning effects will fade away.

More on lateral nucleus, this structure is thought to be the site of synaptic change regarding fear conditioning. When the conditioned stimulus and unconditioned stimulus link to each other, they link on the lateral nucleus neurons. When they are linked, the conditioned response eliciting action potentials in brain and, in return, creating the emotional changes is enhanced.

Amygdala and Its Neighbors Working Together

Not only amygdala evaluates the emotional charge of the stimulus, but also does the prefrontal cortex. Basically, the prefrontal cortex orchestrates the expression of behavioral and physiological responses via its connections to central amygdala, hypothalamus, and brain stem.

Also, amygdala’s basal and accessory basal nuclei sends many projections to different parts of cerebral cortex including prefrontal, rhinal, and sensory cortices. Thus, amygdala has its ability to influence cognitive functioning. As examples for cognitive functioning, amygdala can modulate attention, perception, memory, and decision making.

When these varied connections and functional effects are seen, it is understood that amygdala is implementing the key feature of emotion: its coordinated, complex, and multi-complemental responses.

Amygdala and Innate Fear

The only fear conditioning is not the one that is learned by association via classical conditioning. There is also an “innate” fear conditioning. For instance, fearing from a possible predator/killer can be an innate fear.

In people, the sensory signals of unconditional stimuli creating innate fear such as predator or conspecific odors are transmitted from olfactory system to the medial amygdala. The unconditional stimuli’s fear responses depend on inputs from hypothalamus, too.

Not only related to innate fear, but also to the general fear, amygdala is altered in many psychiatric disorders in humans, especially the ones regarding problems of fear and anxiety.

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As a wrap up for what is explained, amygdala’s importance in limbic system: It is a major structure of limbic system responsible for many different jobs in emotions. Yet, the most important one of the emotions related to amygdala is fear. Also, whatever the limbic system, and/or the amygdala does, they do not act individually. Contrarily, the whole limbic system works interdependently and interrelatedly.

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Learning and Memory of Amygdala

It may come too much to put on amygdala, but it is not because it is all related to emotion. Amygdala is related to emotional learning and memory, too. When a person learns emotionally or has an emotional memory, it is mostly implicit learning and memory – unconscious form of learning and memory. Yet, in the situations of danger, hippocampus and other parts of medial temporal lobe system participate in explicit learning and memory – the conscious form of learning and memory. Thus, the learned indication of danger has the chance to be recalled consciously, mostly for protection and security.

Declarative memory – consisting of facts and events with conscious effort - is also a part of amygdala’s job. Yet, as can be guessed, amygdala is not alone in this job. It works with hippocampus. In addition, learning that a previously dangerous situation being not dangerous is amygdala’s job, too. Yet, again as can be guessed, amygdala is not alone; it works with prefrontal cortex to understand the situation being not dangerous anymore. This paragraph is told in more detail later on.

Amygdala and Positive Emotions

As explained in previous parts, it can be seen and inferred that amygdala deals with many negative and scary emotions. However, it can be a part of positive emotions, too. Basically, amygdala is involved in positive emotions, too, such as associating neutral stimuli with rewards. Even though it is known that ventral tegmental area and the nucleus accumbens are related with rewards and reward system, amygdala plays a role in it, too. In amygdala, there are distinct populations of neurons that are responsible for encoding rewards and punishments.

Cognitive Processes and Emotions

As can be seen in all above explained, emotions are a major part of cognitive functioning such as memory, decision making, and attention. They can influence declarative memory, too, as told in the upper paragraphs. Via the linkages between hippocampus and amygdala, the learning and its encoding and consolidation into declarative memory is achieved. For instance, people remembering funerals, weddings, or last days of school more vividly are due to the influence of emotion-related amygdala helping hippocampus to create memories.

Decision making process is another cognitive process. In brain, it is mostly carried out by prefrontal cortex, yet it includes amygdala, too. For instance, different choices with same objective risks can cause to different behavioral decisions. For instance, when there is a sure gain of 5 dollars to 0.5 chance of winning 10 dollars, people mostly prefer sure gain. Yet, when there is a 0.5 chance of gaining 10 dollars to a sure loss of 5 dollars, people mostly choose 0.5 chance of gaining 10 dollars. Thus, it can be seen that there is a greater amygdala activation in the case of “winning”.

Neighbors of Amygdala

As said more than enough to understand, amygdala is not alone in limbic system. There are many other parts such as hypothalamus, brainstem, periaqueductal gray region in brain stem, and so on.

Some of these neighbors are responsible for various aspects of emotional processing: cingulate cortex, insular cortex, periaqueductal gray and prefrontal cortex.

Insular cortex is basically helping people to experience disgust. Disgust is a strong negative reaction to unpleasant odor, for instance. Thus, it can be said that insular cortex is aiding in survival and protecting people from ingesting poison or spoiled food. Also, insular cortex is related with feeling and anticipating pain. It does this by having inputs from the homeostatic information in the body. In addition, it also gathers limbic system-wide inputs and comes out with subjective feelings about social life.

Prefrontal cortex is associated with emotions associated with social interactions such as empathy, pride, embarrassment and guilt. When these emotions create bodily changes and behavioral alterations, they become available to be felt consciously as distinct feelings. This effect is done by prefrontal cortex.

Periaqueductal gray is a structure located near brain stem. It is related to pain perception as well as stress responses including defensive and reproductive behaviors, maternal attachment and anxiety. Also, because it is related to pain perception, receptors for pain-reducing compounds such as morphine and oxycodone are clustered in periaqueductal gray.

Thus it can be understood that individual states of emotions are not the outcome of single structure or specific neurons, but are more flexibly assembled over a distributed population of multifunctional neurons.

Conclusion

Limbic system consists of many different structures that have very complicated names with very complex responsibilities to do. The most important information about this system to know is that its lead singer is amygdala, but the system, as an orchestra, makes the amygdala to be heard beautifully. In other words, the system’s structures, when considered individually, would have very limited abilities. Yet, if the system is taken as a whole, it becomes able to conduct the many of the complex processes in the body.

References:

Squire, Larry R., et al., editors. Fundamental Neuroscience (Fourth Edition). Academic Press, 2013.

Kandel, Eric R., et al. Principles of Neural Science. McGraw-Hill, 2021.

Mihailoff, Gregory A., and Duane E. Haines. Fundamental Neuroscience for Basic and Clinical Applications E-Book. Elsevier, 2018.

“BrainFacts.” BrainFacts.org, www.brainfacts.org/.

Neuroscience: Science of the Brain: an Introduction for Young Students. British Neuroscience Association, 2003.