We have all experienced fear or fear of something or someone in particular, the response is involuntary and, on many occasions, we are unable to control our body.
A small structure located in our nervous system is responsible for this, called the amygdala. In humans it is related to emotional learning and social behavior.
It is also in charge of regularizing the response to anxiety, fear or aggressiveness and even our social behavior. Therefore, in many ways, it is the center of emotions.
Joseph E. Ledoux , famous neuroscientist, once stated: “Of all the subcortical structures, the amygdala is the one that has been most consistently associated with emotion, both in animals and in humans.”
The most relevant functions and details of the cerebral amygdala will be described below.
What is the cerebral tonsil?
The cerebral amygdala is a subcortical structure that is part of the limbic system, also known as the amygdala complex or amygdala body, and was discovered in the 19th century by the German physiologist Karl Burdach .
The tonsil is a gray, almond-shaped mass, which is why its name comes from the Greek amygdalo, which means almond. It is in charge of processing emotions.
Structure of the brain tonsil
Anatomically speaking, the human amygdala or human amygdala is a complex structure, made up of numerous nuclei, which in turn have multiple subdivisions of functional importance.
The study of tonsillar morphology has resulted in numerous schemes and interpretations of its anatomical organization.
One of the most popular divisions, introduced by neuropathologist Donald L. Price , divides the amygdala into three large groups. Next, we will briefly explain about them.
The centromedial nuclear complex is composed of the central and medial nuclei, in addition to the tonsillar portion of the stria terminalis. It is subdivided into four regions: capsular, lateral, intermediate, and medial.
The central nuclei have extensive connections with related structures in the autonomic control of our body, that is, the autonomic nervous system.
Some of these are located in the brain stem, which regulates many physiological functions, such as breathing or the heart.
Together with the anterior nuclear group, it forms part of the centromedial complex. It is located on the surface of the amygdala adjacent to the optic tract in the brain. It is subdivided into rostral, central, and caudal regions.
The centromedial complex is responsible for sending neural signals to other regions of the brain, especially those involved with declarative memory .
Examples of these are the perirhinal, entorhinal, parahippocampal cortex, and the hippocampus. The connections between both regions are reciprocal and strong.
The basolateral complex, or deep complex, is composed of three groups of nuclei. The lateral, basal (also called basolateral) nuclei, and accessory (also called basomedial) nuclei.
The lateral nuclei can be further subdivided into three subgroups or subnuclei: dorsal, ventrolateral, and medial.
The dorsal subdivision can be further divided into two groups: the upper and lower dorsal subdivisions.
The upper portion is involved in fear learning and the lower portion is involved in the long-term storage of fear-involved memory.
The basal nucleus is another component of the basolateral nuclei, they correspond to the lateral portion of the amygdala.
It is composed of three subdivisions: magnocellular, caudal, and parvocellular. It forms part of the tonsillar lateral portion.
This last region represents the arrival station of most of the sensory stimuli, be they visual, auditory, olfactory and even related to taste. The basal ganglia, specifically, are seen to be related to anxiety .
Patients suffering from chronic anxiety demonstrate abnormal electrical activity of the amygdalin basal ganglia.
In addition, it has been experimentally shown that inactivation of the basal ganglia favors basal behaviour.
This cell cluster is composed of neurons located around the basolateral complex, whose main function is the modulation of the other neuronal complexes within the amygdala.
The intercalated cells receive stimuli from the basal and lateral nuclei, as well as project information to the aforementioned and the centromedial complex.
It is hypothesized that intercalated cells act as regulators of the action of other nuclei, preventing their excessive action.
It is also involved in the action of “freezing” when observing or hearing a fear-inducing stimulus.
Characteristics of the cerebral tonsil
The amygdala is a small structure, however, its anatomical and physiological features, its function in fear, fear conditioning, and other memory-related reactions have made it an interesting object of study.
Next, we will offer you the most outstanding features.
Located deep in the temporal lobe
The tonsil is a small structure located on the medial surface of the temporal lobe.
The nuclei of the amygdaloid body are located on the anterior end of the temporal horn of the lateral ventricle and the ventral surface of the lenticular nucleus , part of the basal ganglia.
It is made up of different nuclei.
Like many regions in the central nervous system, the amygdala is not a structure, but an agglomeration of different nuclear groups, whose molecular biology, histochemistry, and physiology are similar.
Originally, Burdach described the basolateral complex. Subsequently, with the advancement of technology and greater curiosity about this region, other nuclei were identified, such as the basomedial complex, cortical nuclei, and intercalated cells.
It is connected to the brain
In 1937, Papez proposed the existence of a neural circuit that involves fibers from each of the cerebral lobes , which converge in a series of structures related to fear, learning, and emotions in general.
This series of structures is called the limbic system, and the amygdala is part of it. The amygdala receives information from the thalamus, the brain lobes, the hippocampus, the entorhinal cortex, and even the brain stem.
In turn, it sends information to the prefrontal cortex, basal ganglia , hypothalamus, brain stem, locus coeruleus, basal ganglia, substantia nigra, and other brain regions.
Plays an important role in emotional processing
The amygdala is intrinsically related to the processing of emotions, fear and its acquisition. Fear is not exactly innate, it involves environmental and biological components.
The regulation of fear, sexual and social behavior, the satisfaction of acquiring a reward after having worked hard for it are some of the functions that the amygdala regulates, in one way or another.
It is regulated by other control systems
The prefrontal area of our brain behaves as a regulator of the responses provided by the amygdala and other regions of the limbic system, therefore, it allows the development of more analytical responses to stimuli.
For example, the amygdala develops an impulsive emotional reaction, then the prefrontal area is in charge of developing a more appropriate response.
The connection between the prefrontal area and the amygdala is between the portions of the neocortex located in both.
Their injuries give rise to emotional reaction incapacities
A lesion in the amygdala can produce an alteration in the recognition of emotional facial expressions, giving rise to an inability to react normally emotionally.
Therefore, people with these injuries lose the ability to identify basic human emotions such as fear, anger or disgust. Demonstrating that this structure is necessary for the identification of different emotions.
An example of this is the Klüver-Bucy syndrome , which is characterized by docility, loss of the ability to learn, and excessive exploratory behavior, where the patient uses his mouth more than his hands to explore objects.
Functions of the cerebral amygdala
To understand the principles of neuroscience it is essential to understand how emotions work and how the nervous system processes them. Next, we will provide you with some of the most relevant functions.
Participate in experiencing emotions
The understanding and study of emotions is complex due to their abstract nature and our difficulty, on many occasions, in freely expressing how we feel. However, the role played by the amygdala has been known for a century.
Thus, much of what we know is based on extrapolation and observation from animal studies. The limbic system is related to the processing of emotions, but the amygdala in particular regulates the acquisition of fear.
Not only this, but also in the Pavlovian fear conditioning , the acquired aversion to tastes, smells or even visual stimuli. In summary, many of the functions of the amygdala are related to short- and long-term memory.
Contributes to fixing memories, memory and learning
The brain has two recording systems in which the hippocampus records the event and the amygdala is the one that records the emotional memories that trigger said events, thus acting in the fixation of memories, memory and emotional learning.
For example, the hippocampus is essential in recognizing faces, identifying the person, however it is the amygdala that determines after various experiences, the emotional part being appreciation or dislike for that person.
Processes emotions and responses to fear
Fear is not only an emotional state, it can also manifest itself with physical symptoms. Many of the bodily responses normally associated with emotional states of terror or dread are mediated by amygdalin connections.
When we see, hear or perceive something that incites fear, the amygdala is responsible.
These stimuli reach the basolateral complex, send the information to intercalated neurons, which act as mediators, and finally to the centromedial complex.
These latter nuclei stimulate the central gray matter, the paraventricular nucleus of the hypothalamus, and the lateral hypothalamic nuclei.
This induces physical responses, such as starting or stopping abruptly when we experience panic.
There are also responses by the autonomic nervous system such as profuse perspiration, tachycardia and tachypnea, to name a few examples. It also induces the release of stress-related corticosteroid hormones .
Control sexual stimuli
The amygdala is also relevant in sexual behavior, it contributes to motivation or sexual arousal in both sexes, thus controlling sexual stimuli.
Smell and sexual arousal are closely related, since they can promote or inhibit arousal. Being the amygdala a center of olfactory integration, it is therefore relevant in the expression of sexual behavior.
People with the Klüver-Bucy syndrome manifest hypersexuality, and can even bring with it a sexual interest in inanimate objects. Loss of amygdala function is related to this syndrome.
Regulates aggressive and violent reactions
The amygdala alone does not regulate aggressive behavior, but instead influences cortical inhibitors in the prefrontal cortex.
An individual is able to control the aggressive impulse if it has greater activity in the prefrontal cortex than in the amygdala.
On the contrary, those individuals who are not able to control their aggressive impulses have more activity in their amygdala complex than in the prefrontal cortex. Since the amygdala is linked to aggressiveness.
In cases where there is a malfunction of this cerebral amygdala, aggressive reactions decrease.
On the other hand, when there is tonsillar hyperstimulation, this can lead to very aggressive and violent reactions.
Each experience, good or bad, exerts a conditioning effect by being imprinted in our memories.
Tastes and appetite are no exception. When we try a dish for the first time and completely dislike it, it sticks with us.
In such a way, that the next time the aforementioned dish is in front of us, our appetite will be suppressed.
The amygdala is responsible for long-term memory processing, acquisition, and storage.
At the opposite extreme, a meal that evokes positive emotions is also a product of the regulatory effect that the amygdala has on the acquisition and consolidation of memories.
Participate in eating control
Isolated tonsil lesions are incredibly rare. Any disease that affects the amygdala also affects adjacent regions, such as the hippocampus or even the entire temporal lobe.
Bilateral lesion of both lobes can generate voracious appetite. The regulation of intake by the amygdala is done through a series of neural circuits where the medial nuclei are related.
Within these central nuclei, two types of neurons are located. One of them is activated during ingestion and promotes eating, while the others act as regulators. They inhibit the action of the former and reduce intake.
Generates reactions of satisfaction
The reward system is a neural circuit involved in the feelings of satisfaction we experience when receiving a reward after performing a specific action.
This process is based on conditioning. By doing something and getting a reward based on it, you generate neural activity that is recorded in the limbic system, specifically the central nuclei of the amygdala .
To cite an example, we have domesticated animals. By completing an action, such as sitting down or “playing dead”, they receive a reward (food, for example). Your limbic system then traces an emotional relationship between the action committed and the possibility of a reward.
Interpret other people’s emotions
The amygdala is essential for the correct interpretation of social situations, which profoundly influence our social behavior. Our ability to understand and correctly interpret emotions is vital for the person.
Studies with functional magnetic resonance have shown that the neural activity of the amygdala increases when we seek to interpret the body language of people, especially the face.
Bilateral amygdala lesions can distort a person’s social judgment ability.
In fact, a study published in the journal Nature showed that amygdala lesions negatively impact the interpretation of facial expressions.
communicates to other parts of the brain
The amygdala projects outputs to multiple brain regions such as the hypothalamus, prefrontal cortex, somatosensory cortex, raphe nuclei, central gray matter, substantia nigra, and much more.
Through these strong reciprocal neural connections, the amygdala is able to process emotional events, interpret them, and store them in our long-term memory.
In this sense, a traumatic event with an arachnid during childhood can cause fear reactions in the individual towards them, even in adulthood.