ABSTRACT
This detailed diagram shows one of the major connections of the limbic system via the thalamus� This diagram shows an enlarged view of the thalamus of one side and the internal capsule (see Figure 4�3 and Figure 4�4), the head of the caudate nucleus (see Figure 2�5A), and a small portion of the cingulate gyrus (see Figure 1�7 and Figure 6�13)� Immediately below is the hypothalamus, with only the two mammillary nuclei shown (see Figure 9�3A)�
The fibers of the fornix (carrying information from the hippocampal formation) have been followed to the mammillary nuclei (as the post-commissural fibers; see Figure 9�3B)� A major tract leaves the mammillary nuclei, the mammillo-thalamic tract, and its fibers are headed for a group of association nuclei of the thalamus called the anterior nuclei (see Figure 4�3 and Figure 6�13)�
Note to the Learner: The student is advised to refer to the classification of the thalamic nuclei (see Figure 4�3)�
Axons leave the anterior nuclei of the thalamus and course through the anterior limb of the internal capsule (see Figure 4�4)� These fibers course between the caudate nucleus (head and body) and the lentiform nucleus (which is just visible in the background)� The axons terminate in the cortex of the cingulate gyrus after passing through the corpus callosum (see Figure 1�7 and Figure 6�13)� The continuation of this circuit-the Papez circuit-is discussed below�
James Papez, in 1937, described a pathway involving some limbic and cortical structures and associated pathways� These, he postulated, formed the anatomical substrate for emotional experiences� The pathway forms a series of connections that has since been called the Papez circuit� We have continued to learn about many other pathways and structures involved in processing “emotion,” but this marked the beginning of the unfolding of our understanding�
To review, fibers leave the hippocampal formation and proceed through the fornix, and some of these fibers have been shown to terminate in the mammillary nuclei of the hypothalamus� From here, a new pathway, the mammillothalamic tract, ascends to the anterior group of thalamic nuclei� This group of nuclei projects to the cingulate gyrus (see also Figure 6�13)�
From the cingulate gyrus, there is an association bundle, the cingulum, that connects the cingulate gyrus (reciprocally) with the parahippocampal gyrus as part of the limbic lobe (refer to Figure 9�1A and Figure 9�1B)� The parahippocampal gyrus projects to the hippocampal formation, which processes the information and sends it via the fornix to the mammillary nuclei of the hypothalamus (and the septal region)� Hence the circuit is formed�
We now have a broader view of the limbic system, and the precise functional role of the Papez circuit is not completely understood� Although there is a circuitry that forms a loop, the various structures have connections with other parts of the limbic system and other areas of the brain and thus can influence other neuronal functions (to be discussed in the “synthesis” at the end of this section)�
The thalamus of both sides is shown in this diagram, focusing on the medial nuclear mass of the thalamus, the dorsomedial nucleus, one of the most important of the association nuclei of the thalamus (see Figure 4�3)� Sometimes this is called the mediodorsal nucleus�
Shown below is the amygdala with one of its pathways, the ventral amygdalofugal fibers, projecting to the dorsomedial nucleus (see Figure 9�6A and Figure 9�6B)� This pathway brings “emotional” information to the thalamus� The dorsomedial nucleus collects information from a variety of sources, including other thalamic nuclei, as well as from various hypothalamic nuclei (see Figure 6�13)�
The dorsomedial nucleus projects heavily to the frontal lobe, particularly to the cortical area which has been called the prefrontal cortex (see Figure 1�3 and Figure 6�13)� The projection thus includes the emotional component of the experience� This pathway passes through the anterior limb of the internal capsule, between the head of the caudate nucleus and the lentiform nucleus (see Figure 2�10A and Figure 4�4)� The fibers course in the white matter of the frontal lobes�
Our expanded view of the limbic system now includes its extension to this prefrontal cortex, specifically the orbital and medial portions of the frontal lobe; this has been termed the limbic forebrain� Widespread areas of the limbic system and association cortex of the frontal lobe, particularly the medial and orbital portions, are involved with human reactions to pain, particularly to chronic pain, as well as the human experiences of grief and reactions to the tragedies of life�
The projection of the dorsomedial nucleus to the prefrontal cortex has been implicated as the key pathway that is interrupted in a now banned surgical procedure� Before the era of medication for psychiatric disorders, when up to one half of state institutions were filled with patients with mental illness, a psychosurgical procedure was attempted
to help alleviate the distressing symptoms of these diseases�
The procedure involved the introduction of a blunt instrument into the frontal lobes by inserting an instrument (bilaterally) through the orbital bone above the eye (which is a very thin plate of bone)� This procedure interrupts the fibers projecting through the white matter, presumably including the projection from the dorsomedial nucleus� This operation became known as a frontal lobotomy.
