Amygdala anatomy

Basolateral (ventrolateral) group

This group includes the lateral, basal and the accessory basal nucleus. It is large and phylogenetically newer. Functionally, it belongs to the limbic system. It has reciprocal connections to the sensory association areas of the cortex, all four of them (the auditory, visual and somatosensory cortex).

It projects to three different places:

• Medial dorsal nucleus of the thalamus
• The basal nucleus (of Meynart)
• The ventral striatum

This nucleus modulates different functions of the thalamus, such as:

• Responses to stress
• Somatic and autonomic reflexes
• Feeding and drinking behaviour

Corticomedial (dorsomedial) group

This group contains the cortical nuclei and nucleus of the lateral olfactory tract. In comparing to the basolateral group, it is smaller, phylogenetically older, and functionally belongs to the olfactory system. It is actually the site of termination of the olfactory fibers that arise from the olfactory bulb. This nucleus projects to the ventromedial nucleus of the hypothalamus and it plays a role in behaviors associated with hunger and eating.

Centromedial group

This group is composed of the medial and central nuclei. It is actually a place where basolateral and corticomedial nuclei both project. This nucleus is reciprocally connected to the visceral sensory and autonomic nuclei in the brainstem that are involved in respiratory and cardiovascular functions.

Besides these groups of nuclei, there is a separate set of nuclei that couldn't be easily classified into any of these three groups and therefore are listed separately. These include the intercalated cell masses and the amygdalohippocampal area.

Afferent projections

The basolateral group of nuclei receives impulses from the dorsal and medial thalamus and many cortical areas, such as insula, prefrontal cortex, parahippocampal cortex and temporal cortex. Also, it receives numerous neural fibers from the brainstem.

In any case, it is clear that all of these fibers convey wide spectrum of somatosensory, visual and optical information to the amygdala. For the reason of receiving numerous afferent fibers, basolateral group of nuclei is often referred as the amygdaloid zone of sensory convergence.
All of these impulses received to the basolateral group are then projected to the centromedial group where they are supposed to be arranged and then sent to different motor centers in order to stimulate a reaction to the received information. For that reason, central group is often referred to as the amygdaloid zone of motor divergence.

Specifically these are the structures that send fibers to the amygdala:

• Olfactory bulb
• Orbitofrontal cortex
• Cingulate gyrus
• Basal forebrain
• Medial thalamus
• Hypothalamus
• Brainstem

Efferent projections

It was mentioned before that the principal role of the amygdala is to modulate the activity of the hypothalamus. It does so via two main pathways: the stria terminalis and the ventral amygdalofugal pathway.

The stria terminalis

This pathway runs from the centromedial nucleus of the amygdala to the ventromedial nucleus of the hypothalamus. The pathway also carries fibers to the septal nuclei and to the thalamic regions of the brain.

Along with this efferent fibers, the stria terminalis carries fibers projecting from these three areas back to the amygdala. Stria terminalis plays a major role in various types of organism’s responses to stress.

When it leaves the centromedial nucleus, most of its fibers lie between the caudate nucleus and thalamus, followed by the vena terminalis. In its entire length, stria terminalis is in relation to the group of neurons called the bed nucleus of the stria terminalis (BNST), whose function will be described later.

The ending fibers of the stria terminalis are spreading in a form of many rays and they are reaching various nuclei of the hypothalamus, such as ventromedial, preoptic, anterior nucleus and lateral hypothalamus. They also form synapses with neurons of the nucleus accumbens, septal nuclei, caudate nucleus and putamen. Since it is connected with the stria medullaris through the septal nuclei, stria terminalis indirectly modulates the brainstem.

The aforementioned BNST is a limbic forebrain structure that receives projections (besides other areas), from the basolateral amygdala, and projects in turn to hypothalamic and brainstem target areas that mediate many of the autonomic and behavioral responses to aversive or threatening stimuli. It is a structure that has different morphology in male comparing to female brain, it is as twice as large in men as in women. For that kind of nuclei, we say they are sexually dimorphic.

Different morphology of the BNST among men and women also define specific differences when it comes to phobias. This is the reason why women are stereotypically thought to be easier to scare in comparing to men. Other functions of the stria terminalis correlate with anxiety in response to threat. It is thought that BNST acts like a relay site within the hypothalamic-pituitary-adrenal axis and modulates its activity in response to acute stress that lasts longer than 10 minutes. It is also thought that it promotes the behavioral inhibition when encountering unfamiliar individuals.

Amygdalofugal pathway

This pathway is the major efferent pathway of the amygdala. It begins from the centromedial and basolateral nuclei of the amygdaloid complex and carries the fibers to the multiple locations within the nervous system.

The axons from the basolateral group are going medially through the innominate substance, and are ending in the hypothalamus and the septal nuclei. It is known that innominate substance projects cholinergic neurons to the cortex of the brain that are important for the modulation of social behavior. Also, the basolateral group projects fibers that are diffusely spreading and synapsing with wide areas of the frontal and prefrontal cortex, the cortex of the cingulate gyrus, the insula and the temporal cortex. On the other hand, fibers from the centromedial nucleus are directed caudally towards the brainstem, in order to end in the raphe nuclei, along with other structures such as periaqueductal gray and the locus coeruleus. These structures also send their fibers back to the amygdala.

The amygdalofugal pathway and the stria terminalis together enable the amygdala to directly control the medial hypothalamus, the lateral hypothalamus and midbrain periaqueductal gray. This pathway is specifically important for associative learning.