The amygdala (Latin Corpus amygdaloideum) is the limbic brain structure that is positioned in the part of the brain marked as the temporal lobe. It was named after the Greek word amygdalē, which means almond (1).
Its name comes from its shape. Namely, this part of the brain is an almond-shaped area that belongs to the limbic system. Interestingly, it is a paired structure. This means that the amygdala has parts in both temporal lobes.
However, scientists say that it is possible to function and live without one part of the amygdala. In other words, one of them is enough according to the neuroscientists who had surgically removed one of them from rats, rabbits, and other animals.
This surgery is called selective amygdalohippocampectomy. As a result, those animals did not feel fear after the surgery.
This further reinforces what we know about the functions and roles of the amygdala. Namely, it is very important to regulate different emotions and cognitive functions.
Even though it is usually associated with fear, the amygdala is also important for various other emotional states associated with aggression, eating, sexual functions, etc. Moreover, the amygdala plays a key role in recognizing, conditioning and expressing fear.
Basic anatomy of the amygdala
The amygdala, or the amygdaloid complex, is a group of nuclei. Also, it consists of the so-called cortical regions. It is positioned in the medial temporal lobe. Moreover, it is located rostrally to the hippocampus and tail of the nucleus caudatus. It is considered one of the basal ganglia and is part of the limbic system.
There are two major parts of the amygdala: centrocorticomedial (central, cortical, and medial nuclei) and basolateral (lateral, basal, and accessory basal nuclei). Each part has specific neuroanatomical connections and specific functions.
For example, the lateral nucleus is the main entry area to the amygdala since it receives sensory information (auditory, visual, palatable, sensory, somatosensory, including pain) from various brain regions.
The main exit areas are the central core, associated with the brainstem nuclei, and the basal nucleus connected with the striatum. This is important in understanding the functions of the amygdala.
The amygdala is extensively reciprocally associated with numerous cortical and subcortical structures.
Those include the sensory cortex (audio, visual, olfactory, etc.), the polymodal associative cortex, the hippocampus, and the entorhinal cortex, the prefrontal (regional) cortex, the sensory part of the truncus encephali, and the hypothalamus (including the hormones).
Traditionally, two major neuroanatomical pathways as connections to the rest of the brain structures are recognized. Those are the stria terminalis and the ventral amygdalofugal pathway.
It is important to note that, in reality, there are numerous other unnamed connections, including those with the cortical areas. For example, there are connections between the prefrontal cortex and the amygdala.
The amygdala, Emotions, and Feelings
In order to understand the role of the amygdala, we must first define the emotion. The term emotion is different from the term feeling, although they are often used as synonyms.
Emotion (e.g., fear, anger, sadness) is an unconscious, automatic reaction to the stimuli including somatic (e.g. pulse acceleration, pupil dilatation, muscle activity) and cognitive changes (e.g., changes in our attention and memory).
Feeling, on the other hand, is a conscious representation of those emotions, such as feeling scared. Emotions are the result of the activities of the subcortical structures (the amygdala, hippocampus, striatum, and trunk), and cerebral sensations.
Therefore, we conclude that the amygdala plays a very important role in our emotions. Most noteworthy, it is related to fear. There is a strong connection between the amygdala and fear conditioning.
That being said, fear conditioning is an important function of the amygdala. Scientists commonly use Pavlovian conditioning in the study of ‘learning to fear’. The essence of conditioning (fear) is associating a conditioned stimulus (e.g., an acoustic signal) with a non-conditioned stimulus (e.g., an electric shock). For example, an animal can receive an electric shock after the sound signal is applied.
The non-conditioned stimulus is a biologically potent stimulus that, in itself, triggers an emotional response, such as freezing. The conditioned stimulus is emotionally neutral and does not in itself elicit a response.
After pairing these by repeating several series of beeps followed by an electric shock, the appearance of the neutral stimulus itself triggers a similar emotional response (hence the sound signal that causes the freezing reaction).
Fear conditioning has a great evolutionary value. For example, an animal recognizes the danger by a neutral stimulus (e.g. grass rustling, which is conditioned stimulus, and indicates that the predator is approaching – this is a non-conditioned stimulus.
As a result, this activates the defense mechanisms of the animal (e.g. escape from the predator). Such a system allows the animals to avoid danger more effectively.
Other roles of the amygdala
The amygdala also has an important role in the automatic and endocrine responses associated with emotional states (2). For example, if we take a look at how the oxytocin affects our behavior, we will note that the amygdala again plays a significant role in our hormonal balance and, therefore, feelings, health, and behavior.
Oxytocin plays a role in creating relationships among adult individuals, it is important for bonding and the sense of closeness and loyalty that builds up in relationships.
Following the discovery of the structure and arrangement of amino acids of oxytocin in 1953, scientists have conducted numerous studies that bring new insights into the mechanisms of the oxytocin, but also to influence our behavior.
Today, it is known which receptors bind oxytocin and vasopressin, so it has been established that the highest representation of their receptors is in the uterus, but also in the brain, and especially in the hypothalamus and amygdala, the parts important for the expression of emotions.
Oxytocin is known to play a significant role in enhancing contractions during childbirth as well as releasing milk during breastfeeding, but it may be less well known that this hormone plays a major role in altering maternal behavior after birth.
The effect of oxytocin on behavior was demonstrated in a 1979 experiment in rats in which a rat who had not given birth was given oxytocin injection. It began to show maternal behavior.
The scent of the baby, the touch between the mother and the baby, as well as the mother’s look at the baby contribute to the secretion of oxytocin and increase the attention that the mother directs to the needs of the newborn.
Neuroscientists have found a significantly higher activity of the amygdala, the part of the brain responsible for emotions, which, due to the increased excretion of oxytocin immediately after birth and in the first weeks of motherhood, has the effect of an increased maternal involvement around the baby.
Feeding, providing protection, and meeting other needs establishes a very important sense of closeness, crucial to the healthy emotional development of the child.
Moreover, it is interesting to address the role of the amygdala in the PMS – the premenstrual syndrome. Several studies are examining the ways progesterone affects the brain.
This hormone affects the brain by triggering activity in the amygdala. As we already know, the amygdala is the major site in the brain in which a stimulus-response is generated.
Moreover, it is a structure in which the integration of specific sensory information that gives the adequate emotional importance and context, and electrical stimulation results in experiences of positive or negative emotions, depending on which nuclei are stimulated.
According to studies conducted, progesterone increases the reactivity of the amygdala (3). This explains why women in PMS respond much more violently to some life situations than they did during the times in a month when they were not in PMS.
Therefore, the role of the amygdala was proven once again in our emotions, bindings with other humans, and behaviors.
The importance of studying the amygdala lies in its connection with many diseases in humans, including anxiety disorders, schizophrenia, and autism. For example, PTSD patients have a hyperactive amygdala and such people overreact to memories of the traumatic events.
Moreover, the amygdala could also be partly responsible for impaired social functioning in autism. Even though we now know a lot about the mechanisms of functioning of the amygdala, most research has been done on animal models.
The development of new non-invasive methods will allow for an even more thorough study of the human amygdala.
This is especially important in aspects such as social behavior and human-specific psychiatric illnesses.
1. Paul J. Whalen, Elizabeth A. Phelps (2009) The human amygdala 1st ed., The Guilford Press, New York, London, Found online at: https://epdf.pub/the-human-amygdala.html
2. McNally, GP, Akil H. Role of corticotropin-releasing hormone in the amygdala and bed nucleus of the stria terminalis in the behavioral, pain modulatory, and endocrine consequences of opiate withdrawal. Neuroscience. 2002;112(3):605-17. Found online at: https://www.ncbi.nlm.nih.gov/pubmed/12074902
3. van Wingen, Guido & Broekhoven, F & Verkes, R & Petersson, Karl Magnus & Backstrom, Torbjorn & Buitelaar, Jan & Fernandez, Grisel. (2008). Progesterone selectively increases amygdala reactivity in women. Molecular psychiatry. 13. 325-33. 10.1038/sj.mp.4002030. Found online at: https://www.researchgate.net/publication/6256852_Progesterone_selectively_increases_amygdala_reactivity_in_women