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The human brain is comprised of four major regions: the brain stem, cerebellum, diencephalon, and the cerebrum. Located near the center of the brain, the diecephalon is comprised of the hypothalamus, the thalamus, and the epithalamus. Of the three sections that make up the diecephalon, the epithalamus is the dorsal, or farthest to the rear. In terms of function, the epithalamus is responsible for connecting the lymbic system to the rest of the brain, as well as regulating hormones secreted by the pineal gland.
Looking at a side view of the human brain, the epithalamus is located just above the medulla oblongata and the pituitary gland. Following the path of the spinal cord into the skull, the cord transitions into the brain via the medulla oblongata, ending in the lower section of the middle brain. Just above and a few centimeters forward, rests the pituitary gland on a horizontal axis with the eyes and ears. Extending over both the medulla oblongata and the pituitary gland is the epithalamus and its associated components.
Structures within the epithalamus include the habenula, the stria medularis and the pineal body or pineal gland. Researchers do not fully understand all the functions of the various structures within the epithalamus, although some functions are evident. For example, researchers know that the pineal gland, located to the rear of the epithalamus, is responsible for regulating circadian rhythms. Melatonin, the hormone responsible for sleepiness, is secreted by the pineal gland in both humans and mammals to induce normal sleep cycles.
Known as the “third eye,” the pineal body also participates in other body functions. No bigger than a single grain of rice, the production of melatonin by the tiny pineal gland also inhibits sexual development in both humans and animals until adolescence. Emotions and certain motor pathways are also influenced, owing to close similarities between melatonin and serotonin.
Next to the pineal gland is the habenula, also known as the habenular nuclei. Scientists believe the habenula is involved in regulating the body's intake of nutrients and water. Additionally, research indicates the habenula contains affarent, or sensory input, fibers connecting the stria medularis with other parts of the brain involved in serotonin, dopamine, and norepinephrine release.
Projecting backward toward the habenular nuclei is the stria medularis of the thalamus. Little is known about the stria medularis, other than it is a bundle of efferent, or sensory output, nerve fibers structured to form a ridge between the thalamus and the epithalamus. Some anatomy databases list the stria medularis as part of the thalamus, while others list it as part of the epithalamus.