MODULE 13
The AMYGDALA
- - - Start by reading the brief sections on the amygdala given in the suggested reading section (Blumenfeld's Pages 787-9 and Figure 18.17) . This nuclear area, about the size of a large olive, is one of the most fascinating structures in the brain. It receives inputs from all the sensory systems (somatic sensory, visual, auditory, etc.) which reach the basolateral part of the amygdala by passing through the white matter of the temporal lobe. It also receives information from the cerebral cortex and the hippocampus, and certainly from other regions, as well. Most authors try to convey this central position of the nucleus in a figure and no two figures completely agree. There are several of ours in this module (adopted from Krieg) but see also: Blumenfeld's Figures 18.1 and Figure 18.17 or Fitzgerald's Figures 34.12, 13, 14 or Haines' Figure 31-6. Somehow, the amygdala integrates all these inputs and adds an emotional coloration to the sensory information, then passes the message on to the hypothalamus. Our task is to do three things: 1) show you where the amygdala is, 2) describe the input pathways, and 3) describe the output pathways.
Where is it?
- - - First of all you will remember that you have heard about the amygdala before. It's one of those nuclei buried within the cerebral hemisphere that are known as the basal ganglia. The next figure (Figure 1-50) relates it to the other basal ganglia nuclei. From a phylogenetic point of view, the amygdala is by far the oldest of these nuclei. Its function is quite different from that of the caudate and putamen - it has nothing to do with the control of movement - and so it hardly belongs to the same club. Look at Figure 13-1, Figure 13-2 and Figure 13-3 to form your own image of where the amygdala is; then look at the next (unlabeled) view (Figure 1-30) and try (in your mind) to place it. It lies just deep to that obvious surface landmark, the uncus, and its position is marked by the circle in Figure 13-6.
The input pathways.
- - - Blumenfeld shows these in his Figures 18.1 and 18.17. We try to show them in Figure 13-6. The cortical input approaches the amygdala from the temporal lobe and is marked "from neocortex". That's the right idea, but, as mentioned above, it should say "from neocortex and all ascending sensory pathways". The precise route by which the visual pathway, for example, projects to the amygdala is not clear but all these sensory pathways seem to enter the basolateral division of the nucleus.
The output pathways.
- - - Again, Blumenfeld shows these in his Figures 18.1 and 18.17. We are going to focus on two specific pathways. One of these, the ventral amygdalofugal pathway, takes a very direct route rather like that of the ansa lenticularis, that lies nearby. The second, the stria terminalis, takes a circular route, always lying just medial to the caudate nucleus. Both pathways project to septal region, the hypothalamus and the thalamus. Before we look at these pathways on slides take a look at the stria terminalis, as depicted in Figure 13-1 and Figure 13-2. The ventral amygdalofugal pathway is shown in Cu Slide 36. It projects rather directly to the hypothalamus and continues, as the inferior thalamic peduncle, to terminate in the nucleus medialis dorsalis of the thalamus. To follow it there, look at Cu Slide 35, Cu Slide 34, Cu Slide 33 and Cu Slide 32. Because the stria terminalis takes such a long, looping course to reach the septal region and the hypothalamus, it's more difficult to follow in just a few slides. Look for it in Cu Slide 30, where it exits from the caudal pole of the amygdala, in Cu Slide 24, where it runs in the roof of the inferior horn of the lateral ventricle, just medial to the tail of the caudate nucleus, and in Cu Slide 21, where it is cut tangentially as it moves dorsally. Click on the slide listing at the end of the module, to follow it as it passes rostrally to its termination at the level of Slide 38 and Slide 39.
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