Discuss the anatomy, neural connections and functional role of the amygdala in health and

disease

Kirsty Earley

4H Anatomy

0902100e

Introduction

The human brain is a complex structure and one of the most important organs of the body. It

is the antero-most part of the central nervous system within the neurocranium of the human

skull, supported by the fibrous layers of meninges. The brain is divided into specific regions;

the left and right hemispheres, forebrain, midbrain and hindbrain. The outer cortex of nervous

tissue is composed of grey matter, mainly consisting of neural bodies and dendrites, with the

inner layer of white matter containing the myelin sheaths insulating the axons.

The general function of the brain is to have complete control of the body, with several

regions of the brain working together via neural connections. However, there are

controversies in the field of neuroscience over the exact definition and function of certain

brain areas, especially those of the Limbic System.

The Limbic System is a group of cortical and subcortical structures situated in the cerebral

hemispheres. Despite there being unclear definitions of the components and functions of this

system (LeDoux, 2000), it is believed to consist of structures such as the hippocampus,

insular cortex, hypothalamus and amygdala. This system is active in emotions,

communication and memory storage.

One of the most complex structures of this system is the Amygdala, an almond-shaped mass

of the forebrain. Since its discovery, there has been dispute over the exact location,

morphology, neural connections and function of the amygdala. With the evolution of more

advanced brain-imaging techniques, there has been extensive insight into these aspects of the

amygdala in health and disease, which will be further discussed.

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Anatomy of the Amygdala

The amygdala was discovered by Burdach as a small group of nuclei in the temporal lobes of

the forebrain (Meyer, 1971). It is situated anteriorly in the medial temporal lobe, with the left

and right amygdalae sitting laterally to the thalami of the midbrain. It is in close relation with

the caudate nucleus, a structure of the basal ganglia, and the hippocampus, a structure

important for the formation of memories (McDonald, 1998). The caudate nucleus is C-

shaped, with the amygdala sitting at the tail of the nucleus.

There are around 13 nuclei of the amygdala, categorised into three subdivisions; the

centromedial, basolateral and cortical nuclear groups. The basolateral group contains three

nuclei; the lateral, basal and accessory nuclei. The cortical nuclear group includes the cortical

nuclei and structures of the olfactory cortex, such as the olfactory tract. The centromedial

nuclear group contains the central and medial nuclei of the amygdala (McDonald, 1998). The

cortical group is superficial, with the nuclei of the basolateral subdivision in the lateral

regions of the amygdalae. The nuclei of the centromedial division are located medially in the

amygdalae (Sah et al., 2003). Terminology for the nuclear groups is not consistent within this

field of research; hence controversies are common.

There are several receptors of the amygdalar neurons including NMDA, opioid, cannabinoid,

glutamate and GABA receptors (Farb & LeDoux, 1997, Foster et al., 2004, Katona et al.,

2001, Walker & Davis, 2002).This means that several molecules can affect the functions of

the amygdala.

The volume of the amygdala is hard to determine- there is no clarity in the anatomical

boundaries of this structure (LeDoux, 2007). The amygdala nuclei fuse with surrounding

structures, making it difficult to distinguish between different areas. Knowing the volume is

useful for research into clinical disorders of this area (Brierley et al., 2002). The amygdalar

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volume varies between each individual, but it is believed that the mean volume is around 1 to

2 cm³ (Brabec et al., 2010), and is larger in men than women (Goldstein et al., 2001). Further

efforts are required to define the exact boundaries of not only the amygdala, but the entire

limbic system.

The amygdala is a heterogeneous structure of the brain because of the groups of nuclei- it is

not a unified mass of grey matter (Mendoza and Foundas, 2008). Not only does this make it

difficult to analyse anatomically, but also functionally (Brabec et al., 2010). To better

comprehend the functions of the amygdala, it is important to determine the connections of the

different nuclei.

Neural Connections of the Amygdala

The amygdala has several sensory and motor connections with different regions of the brain,

giving rise to its many functions (Amunts, et al., 2005).Techniques for visualising neural

connections include tract tracing, where tracer molecules are injected to examine the

pathways (Aggleton et al., 1979). The amygdala receives afferents from areas of the

forebrain, sends efferents to autonomic structures of the brainstem and is involved in a circuit

of forebrain structures that function in behaviour and motivation (Price, 2003).

The amygdala receives sensory inputs from many brain areas including the visual, auditory

and olfactory cortices. The visual and auditory afferents project to the basolateral nuclear

group of the amygdala, as well as the centromedial nuclear group (Bzdok, et al., 2012). Since

the cortical nuclear group contains structures of the olfactory cortex, the cortical nucleus

receives afferents of olfaction (Solano-Castiella et al., 2010, Price, 2003). Hence, this shows

that the amygdala integrates sensory stimuli with emotions.

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To process the afferent information received from different brain areas, the nuclear groups

must intercommunicate (Sah et al., 2003). The lateral nucleus obtains connections from the

basolateral and centromedial complex, and the central nucleus has interconnections with the

majority of the amygdaloid nuclei (Pitkӓnen et al., 2000).

The amygdala has an influence on responses of the autonomic nervous system (ANS), for

example an increased heart rate in anxiety (Tillfors et al., 2002). Efferents project from the

central, medial and accessory nuclei to the hypothalamus via the amygdalofugal pathway and

stria terminalis. They have the ability to terminate in any brainstem area that is linked to the

ANS, using GABA as the neurotransmitter (Price, 2003, Swanson and Petrovich, 1998).

As well as receiving afferents from the forebrain, the amygdalar nuclei send efferents back to

these areas. The basolateral nuclear group sends efferents to the cortical areas like the

prefrontal cortex and the medial temporal lobe (Sah, et al., 2003). These projections are

active when the amygdala links emotions to memories of past experiences (Del Arco &

Mora, 2009), and use glutamate as the neurotransmitter (Swanson and Petrovich, 1998).

Better understanding of neural connections of the amygdala enables researchers to better

comprehend the functions of this structure.

Function of the Amygdala

As with several structures of the brain, the function of the amygdala has been understood

through lesions studies- however human subjects with amydalar lesions are uncommon

(Amaral et al., 2003). Brain-imaging techniques are another way of determining during which

activities the amygdala is active. Functional magnetic resonance imaging is the measure of

activity of a brain area by the proportion of blood flowing to that area. It is an accurate

method of brain-imaging, but only enables researchers to visualise the gross anatomy of the

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amygdala- not the separate neurons (Brierley et al., 2002). As previously mentioned, the

amygdala is part of the limbic system playing a role in emotions (Papez, 1937), facial

expression recognition and memory storage.

The specific role of the amygdala in emotions has been linked to the delivery of emotion

from fear to anger to pleasure (Burt, 1992). The first study to suggest that the amygdala had

neural control in emotions was that of Klüver and Bucy. Rhesus monkeys are commonly used

as models for the analysis of the human brain, having similar anatomical features to humans.

It was found that after inducing bilateral lesions of the amygdala, the monkeys were not

fearful of stimuli that they previously avoided as well as a decreased emotional response in

general (Klüver and Bucy, 1939). This suggests that the amygdalae must be intact to have

normal emotional responses to the environment. The symptoms that the monkeys expressed

are those of Klüver-Bucy syndrome (Marlowe et al., 1975). Humans can have this condition

(Hooshmand et al., 1974), resulting in a decrease in learning, tendency to examine objects

with the mouth and altered sexual orientation.

Evidence has been provided for the importance of the amygdala in sexual behaviour

(Hamann et al., 2004). Where ablation of the amygdala results in irregular sexual behaviour,

an increased amygdalar volume signifies increased sexual activity (Baird et al., 2004). The

amygdala is active during arousal responses to sexual stimuli, with men having larger

activations than women to the same stimulus (Hamann et al., 2004). Sexual orientation can

also be traced back to neural pathways of the amygdala; homosexual men and heterosexual

women have similar neural connections of the amygdala- the same is seen in homosexual

women and heterosexual men (Swaab, 2008).

Fear conditioning is a type of learning where the subject gradually learns to avoid harmful

stimuli in the environment (Maren, 2001). Reactions to a threat can include “freezing”, where

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the organism stays still to cause the predator to move on. Freezing is an action during which

the amygdala is firing (LeDoux, 1996), with an increase in firing of neurons in the central

amygdala (Duvarci et al., 2011). This response is thought to be caused by the activity of the

ANS (Davis et al., 1994) as a “fight or flight” mechanism for survival.

The amygdala is also functional when interpreting dangerous stimuli, especially when trying

to determine facial expressions of other people (Adolphs et al, 1998). The amygdala neurons

increase their firing when processing an angry face compared to a happy face (Morris et al.,

1996). Furthermore, when conversing with someone, each individual has a certain distance of

separation that they are comfortable with (Kennedy et al., 2009). An invasion of one’s

“personal space” can make them uncomfortable, linking fear and social interaction. This

discomfort caused by someone standing in a close proximity is triggered by the amygdala.

This not only provides evidence that the amygdala plays a role in the fear response, but also

in social situations (Adolphs, 2003). One study suggested that larger sizes of amygdala

signified a greater extent of social interaction (Bickart et al., 2011). A lesion of the amygdala

may cause difficulty with communication and/or detection of danger.

When remembering specific events in life, the ones that are more easily recalled are related to

strong emotions experienced in that event (Kensinger, 2004). This recollection of experiences

related to strong emotions is a function of the amygdala. However, the amygdala plays no

role in the storage of memory- this occurs in other brain areas such as the hippocampus (Paré,

2003).

The above descriptions of the amygdala can be found in the average healthy subject, but the

amygdalar structure, function and neural connections are known to differ in certain

neurological disorders and diseases.

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Amygdala in Disease

Due to the many neural connections and functions of the amygdala, several disorders can

result from abnormalities of this structure. Disorders may involve abnormal neuronal

activities, loss of tissue, abnormal development, etc. The amygdala is unlikely to be damaged

by impact to the skull as it is situated deep in the medial temporal lobe- damage is normally

induced surgically.

Recall that the amygdala is active during fear conditioning. Abnormalities of the neurological

pathways of fear conditioning can result in the development of anxiety disorders, such as

Posttraumatic Stress Disorder and phobias (Rauch et al., 2003). The amygdala is activated in

the phobic response, even if the exposure to the stimulus is very short (Alpers et al., 2009).

Amygdalar neurons fire irregularly- rapidly when determining the stimulus, diminishing after

a short time (Larson et al., 2006). Anxiety disorders are hard to diagnose, but early diagnosis

could prevent future mental disorders, such as depression (Milham et al., 2004).

Depression is a mental disorder more common in women than men, with frequent unpleasant

thoughts, loss of interest in activities, and a variety of other symptoms (Hamann, 2005). In

this condition, the amygdala differs from normal composition and activity; amygdala neurons

fire too frequently and the left amygdala is more active- especially in response to threatening

facial expressions (Peluso et al., 2009, Sheline et al., 2001). The amygdala may also be

significantly larger in depressed individuals (Lange & Irle, 2004). Depression has been

linked to hormonal imbalances, thus amygdalar abnormalities cannot be considered the sole

cause.

Autism is a neuropsychological disorder where the individual finds it challenging to interact

with other people and the environment (Baron-Cohen et al., 2000). Considering the evidence

that the amygdala plays a role in social interaction, is the difficulty autistic people have with

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socialising due to an irregularity of the amygdala? Social interaction problems in autism may

not be due to the amygdala, but an amygdala abnormality may cause the anxiety problems in

autism (Amaral & Corbett, 2003). The neurological causes of autism have not been fully

understood, with several brain areas hypothesised to be involved. It is unsure if the amygdala

is damaged at all, possibly due to the problems with defining the boundaries of this region

(Schumann et al., 2004).

Despite the fact that bilateral lesions of the amygdala in humans are rare, they have been

linked to the genetic condition, Urbach-Wiethe Disease. This condition can affect the organs

of the body, (Rallis et al., 2006), and has been known to damage the temporal lobes of the

brain (Siebert et al., 2003). Regions of the temporal lobe become calcified resulting in

bilateral lesions of the amygdala (Appenzeller et al., 2006). Damage to the amygdala

produces the incapability of experiencing fear, despite all other emotions still being intact

(Feinstein et al, 2011). Having no fear response prevents an individual from recognising

dangerous situations. For example, the awareness of a normal “personal bubble” space

diminishes with damage to the amygdala (Kennedy et al., 2009), which could link to the lack

of ability to detect danger. Being an evolutionary survival method for avoiding danger,

having a lack of fear could be life-threatening (Feinstein et al., 2011). The defence

mechanism of fear is lost because the individual lacks the ability to link previous danger to

fear; they have no emotional arousal of memory.

Researchers must address the problems with terminology in this field of neuroscience to

better comprehend the disorders that may arise from amygdalar abnormalities.

Conclusion

Despite the fact that several research studies have been undertaken to investigate the different

aspects of the amygdala and brain-imaging techniques have well advanced, there is still a lot

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of uncertainty in this area of study. This is due to the lack of clarity into definitions of the

different neural groups and the exact boundaries of this structure. It is generally accepted that

the amygdala plays an important role in the control of emotions- especially fear- and the

recognition of facial expressions, helping to avoid dangerous situations. Over- activation of

the amygdala can cause the development of anxiety disorders, whereas damage to the

amygdala can result in the absence of a fear response. For future research, it is suggested that

names of the different nuclear groups are consistently used in papers and further investigation

should be done into finalising the exact boundaries of the amygdala, which may prove

difficult.

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