Diencephalon

The diencephalon consists of four structures located deep in the cerebral hemispheres just rostral to the midbrain and surrounding the third ventricle: the thalamus, hypothalamus, subthalamus, and epithala-mus. Despite its small size, the diencephalon has major importance in brain function, particularly through the activities of the thalamus and hypothalamus. The subthalamus is a very small region inferior to the thalamus that contains the subthalamic nucleus and zona incerta; these areas have connections to the basal ganglia and cerebral cortex, but their functions are largely unknown. The epithalamus is found superior and caudal to the thalamus and contains the pineal gland and the habenular nuclei. The pineal gland is an unpaired structure that was once thought to harbor the seat of the soul; today, it is known to secrete a hormone called melatonin, hypothesized to have a role in sleep and gonadal function.

The thalamus is an egg-shaped collection of nuclei that makes up about 80% of the diencephalon (Fig. 10). It consists of a collection of nuclei that participate in sensation, movement, cognition, emotion, and arousal. The most characteristic thalamic function is to serve as a sensory relay station for stimuli that will eventually reach the cerebral cortex. All sensory systems with the exception of olfaction pass through the thalamus en route to their respective cortical areas. Thus, somatosensory information from the body and face reach the VPL and ventral posterior medial (VPM) nuclei, respectively, and taste fibers also project to the VPM nucleus. Visual projections from the optic nerve synapse in the lateral geniculate nucleus and auditory fibers in the medial geniculate nucleus. The ventral anterior and ventral lateral nuclei receive fibers from the cerebellum and in turn send projections to the basal ganglia, so that they contribute to the motor system. The dorsal medial nucleus and the pulvinar are the major association nuclei, connected with the frontal and the parietal-temporal-occipital cortices, respectively. The anterior nucleus has a role in the limbic system (see following discussion). Finally, the intralaminar nuclei, the two largest of which are the centromedian and the parafascicular, receive input from the ARAS in the brain stem and then relay this general information rostrally to activate the cerebrum as a whole.

The hypothalamus is considerably smaller than the thalamus, but it exerts powerful effects on autonomic, endocrine, and emotional functions. Located inferior to the thalamus and superior to the pituitary gland, the hypothalamus is a collection of many tiny nuclei that

Figure 10 The thalamus, with its many nuclei depicted. These nuclei act as relay stations for information traveling to and from the cortex. Reprinted with permission from Kandel, E. R., Schwartz, J. H., and Jessell, T. M., Eds., (1991). Principles of Neural Science, 3rd ed., p. 291. Elsevier, New York.

Figure 10 The thalamus, with its many nuclei depicted. These nuclei act as relay stations for information traveling to and from the cortex. Reprinted with permission from Kandel, E. R., Schwartz, J. H., and Jessell, T. M., Eds., (1991). Principles of Neural Science, 3rd ed., p. 291. Elsevier, New York.

contribute to the regulation of the body's homeostasis. The hypothalamus is the control center of the auto-nomic nervous system, an involuntary portion of the nervous system that regulates aspects of body temperature, digestion, circulation, water balance, and sexual function. The autonomic nervous system is divided into the parasympathetic branch, which is generally associated with the anterior regions of the hypothalamus, and the sympathetic branch, which is associated with posterior sites. Endocrine function is also governed by the hypothalamus by virtue of its strong neural and vascular connections with the two lobes ofthe pituitary gland. Finally, the hypothalamus shares the mammillary bodies in common with the limbic system and, in part because of this linkage, plays a role in emotional function. Hypothalamic lesions in experimental animals and in humans, for example, have sometimes been noted to produce rage behaviors.

Was this article helpful?

0 0
Dealing With Impotence Naturally

Dealing With Impotence Naturally

Put an end to the disappointment, frustration and embarrassment of... Erectile Dysfunction. Have Sex Like You are 18 Years Old Again. Have a natural powerfully stiff penis. Eject volumes of semen. Do it again and again night after night. Never make another excuse for not being able to get it up.

Get My Free Ebook


Post a comment