Functional Anatomy of the Limbic System Key Position of the Hypothalamus

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Figure 58-4 shows the anatomical structures of the limbic system, demonstrating that they are an interconnected complex of basal brain elements. Located in the middle of all these is the extremely small hypothalamus, which from a physiologic point of view is one of the central elements of the limbic system. Figure 58-5 illustrates schematically this key position of the hypothalamus in the limbic system and shows surrounding it other subcortical structures of the limbic system, including the septum, the paraolfactory area, the anterior nucleus of the thalamus, portions of the basal ganglia, the hippocampus, and the amygdala.

And surrounding the subcortical limbic areas is the limbic cortex, composed of a ring of cerebral cortex in each side of the brain (1) beginning in the orbitofrontal area on the ventral surface of the frontal lobes, (2) extending upward into the subcallosal gyrus, (3) then over the top of the corpus callosum onto the medial aspect of the cerebral hemisphere in the cingulate gyrus, and finally (4) passing behind the corpus callo-sum and downward onto the ventromedial surface of the temporal lobe to the parahippocampal gyrus and uncus.

Thus, on the medial and ventral surfaces of each cerebral hemisphere is a ring of mostly paleocortex that surrounds a group of deep structures intimately associated with overall behavior and emotions. In turn, this ring of limbic cortex functions as a two-way communication and association linkage between the neo-cortex and the lower limbic structures.

Many of the behavioral functions elicited from the hypothalamus and other limbic structures are also mediated through the reticular nuclei in the brain stem and their associated nuclei. It was pointed out in Chapter 55 as well as earlier in this chapter that stimulation of the excitatory portion of this reticular formation can cause high degrees of cerebral excitability while also increasing the excitability of much of the spinal cord synapses. In Chapter 60, we will see that most of the hypothalamic signals for controlling the autonomic nervous system are also transmitted through synaptic nuclei located in the brain stem.

An important route of communication between the limbic system and the brain stem is the medial

Cingulate gyrus and cingulum

Stria medullaris thalami

Body of fornix

Dorsal fornix

Mamillothalamic tract

Mamillotegmental tract

Isthmus

Gyrus fasciolaris

Fimbria of fornix

Stria terminalis Connecting spinal cord

Cingulate gyrus and cingulum

Stria medullaris thalami

Body of fornix

Dorsal fornix

Mamillothalamic tract

Mamillotegmental tract

Isthmus

Gyrus fasciolaris

Fimbria of fornix

Column of fornix (postcommissural fornix)

Uncus Amygdaloid body

Mamillary body

Hippocampus

Dentate gyrus Parahippocampal gyrus

Indusium griseum and longitudinal striae

Septum pellucidum (supracommissural septum)

Anterior nuclear group of thalamus

Anterior commissure

Subcallosal gyrus

Paraterminal gyrus (precommissural septum)

Orbitofrontal cortex

Prehippocampal rudiment

Paraolfactory area

Olfactory bulb

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Essentials of Human Physiology

Essentials of Human Physiology

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