The amygdala connections

One of the major differences between the amygdala and the other parts of the basal ganglia is that the amygdala is not a homogeneous nuclear structure but is composed of different component parts. These are not usually studied in an introductory course.

The amygdala receives a variety of inputs from other parts of the brain, including the adjacent parahippocampal gyrus (not illustrated). It receives olfactory input directly (via the lateral olfactory stria, see Figure 79) and indirectly from the cortex of the uncal region (as shown on the left side of the diagram).

The amygdaloid nuclei are connected to the hypothalamus, thalamus (mainly the dorsomedial nucleus), and the septal region. The connections, which are reciprocal, travel through two routes:

• A dorsal route, known as the stria terminalis, which follows the ventricular curve and is found on the upper aspect of the thalamus (see previous illustration). The stria terminalis lies adjacent to the body of the caudate nucleus in this location (see Figure 76). This connects the amygdala with the hypothalamus and the septal region.

• A ventral route, known as the ventral pathway or the ventral amygdalofugal pathway. This pathway, which goes through the basal fore-brain region (see Figure 80B), connects the amygdala to the hypothalamus (as shown) and to the thalamus (the fibers are shown "en route"), particularly the dorsomedial nucleus (see Figure 63 and Figure 77B).

The connection with the hypothalamus is likely the basis for the similarity of responses seen in animals with stimulation of the amygdala and the hypothalamus (see previous illustration and Figure 78A). This pathway to the hypothalamus may result in hormonal responses, and the connections with the midbrain and medulla may lead to autonomic responses (see Figure 78A).

Further possible connections of the amygdala with other limbic structures and other parts of the brain can occur via the septal region (see Figure 78B), and via the dorsomedial nucleus of the thalamus to the prefrontal cortex (see Figure 77B).

The anterior commissure conveys connections between the nuclei of the two sides.

Clinical Aspect

Seizure activity in the anterior temporal region may spread to the orbitofrontal region, via a particular group of fibers called the uncinate bundle.

Additional Detail

The association pathway, called the uncinate fasciculus, is a "U-shaped" bundle of fibers between the anterior temporal region and the inferior portion of the frontal lobe. (It is suggested that the learner consult Carpenter — see the Annotated Bibliography — for an illustration of this structure).

Thalamus

Caudate nucleus (body)

Septal nn

Anterior commissure

Thalamus

Caudate nucleus (body)

Septal nn

Hypothalamic-midbrain fibers

Periaqueductal gray

Anterior commissure

Anterior Commissure

Ventral amygdalofugal pathway to thalamus to hypothalamus

Lateral olfactory stria

Hypothalamic-midbrain fibers

Periaqueductal gray

Limbic" midbrain Midbrain autonomic fibers Medulla

Uncal cortex Optic tract

Hypothalamic nn.- pre-optic - medial - lateral

Ventral amygdalofugal pathway to thalamus to hypothalamus

Lateral olfactory stria

FIGURE 75B: Amygdala 2 — Connections

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