Limbic and Cortical Inputs to the Hypothalamus
In addition to having
direct sensory inputs, the hypothalamus receives highly processed information
from the cerebral cortex, which is relayed via the limbic system. The limbic
lobe of the brain was first defined by Paul Broca, in 1878, as the cortex surrounding
the medial edge of the cerebral hemisphere, as shown in orange in the upper
figure. Broca’s limbic lobe includes the cingulate gyrus (the infralimbic,
prelimbic, anterior cingulate, and retrosplenial areas), the hippocampal
formation (including the entorhinal area, subiculum, hippocampal CA fields, and
dentate gyrus), and the amygdala. These limbic regions all receive highly
processed sensory information from the association regions of the cerebral
cortex, process that information for its emotional content, and then project
back to the association cortical areas to provide emotional coloring to
cognition.
Each of the limbic areas also sends descending inputs to the hypothalamus. The inputs from the cingulate gyrus mainly originate in the infralimbic and prelimbic regions (around and just beneath the splenium of the corpus callosum). These areas mainly send axons to the lateral hypothalamus, as well as to components of the autonomic system in the brainstem and the spinal cord, and are believed to provide much of the auto- nomic component of emotional response.
Neurons in the
hippocampal formation, particularly the CA1 field and the subiculum, send axons
to the hypothalamus through the fornix. This long looping pathway, shown in
yellow in the figure, curves just under the corpus callosum, and then dives
into the diencephalon at the foramen of Monro. Many axons leave the fornix in
the hypothalamus and provide inputs to the ventromedial nucleus. However, a
dense column of fornix axons reach the mammillary body, where they terminate.
These structures are shown in blue in the upper figure and red in the lower
one. Although the hippocampus appears to be very important in memory
consolidation, isolated damage to the fornix or mam- millary bodies has more limited
and inconsistent effects on memory, so the function of this pathway remains
enigmatic.
The mammillary
nuclei provide another salient bundle of axons to the anterior nucleus of the
thalamus. This mammillothalamic tract is heavily myelinated and easily seen,
but its contribution to memory formation is more subtle, like that of the
mammillary body itself. Lesions of the mammillothalamic tract have been
reported to prevent the generalization of limbic sei- zures, however, and this
pathway has been suggested as a target for deep brain stimulation to prevent
generalization of seizures. The anterior thalamic nucleus projects to the
cingulate gyrus, and, in 1937, James Papez hypothesized that perhaps the
momentum of emotions could be explained by a “reverberating circuit,” completed
by a projection from the cingulate cortex back to the hippocampus, to neurons
that contribute to the fornix. Although there is no credible evidence for this
last link in the “circuit” actually existing or for the proposed circuit actually
playing a role in emotion, the theory has achieved great attention.
The amygdala provides the hypothalamus with inputs via two pathways. Some axons leave the amygdala in parallel to the fornix, running along the lateral edge of the lateral ventricle just below the tail and body of the caudate nucleus in the stria terminalis, shown in blue in the lower figure. Other amygdaloid inputs to the hypothalamus take a much more direct anterior route, running over the optic tract into the lateral hypothala- mus. Many hypothalamic cell groups receive inputs from the amygdala, which are thought to be important for the vis eral components of conditioned emotional responses.
