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Pineal glandImage:Illu pituitary pineal glands.jpgImage:Gray719.png The pineal gland (also called the pineal body or epiphysis) is a small endocrinegland in the brain. It is located near the center of the brain, between the two hemispheres, tucked in a groove where the two rounded thalamic bodiesjoin.
LocationThe pineal gland is a small, pea-sized, (8 mmin humans) reddish-gray body located above the superior colliculusand behind and beneath the stria medullaris, between the laterally positioned thalamic bodies. It is part of the epithalamus. The pineal gland is a midline structure and is often seen in plain skullX-raysas it is often calcified. Structure and compositionThe pineal gland consists mainly of pinealocytes, but four other celltypes have been identified: interstitial cells, perivascular phagocyte, pineal neuronsand peptidergicneuron-like cells. The pineal body has nervoustissue, and consists of follicles lined by epitheliumand enveloped by connective tissue. These follicles contain a variable quantity of gritty material, called brain sand, acervuli, or corpora arenacea, which is composed of calcium phosphate, calcium carbonate, magnesium phosphate, and ammonium phosphate. Pinealocytesin lower vertebrateanimalshave a strong resemblance to the photoreceptorcells of the eye. It is indeed so that the human pineal gland can be evolutionaryretraced to the pineal (or third) eye in reptilesand birds. In some reptiles this pineal eye is an almost fully developed eye with lens, vitreous humor, and retina. In birds the pineal glad is located on the surface of the brain, directly under the skull. In some animals there is even a light-permeable membrane in the scalpand skull so that light may reach the photoreceptor-like pinealocytes. The function of this "third" eye is to allow these animals to use light as prime zeitgeber(light-detector) in their biological clock[{{fullurl:Template:FULLPAGENAME}}#endnote_Moore]. In humans and other mammals this function is taken over by the "regular" eyes. FunctionThe pineal gland is responsible for the production of melatonin, which is regulated in a circadian rhythm. Melatonin is a derivative of the amino acidtryptophan. The production of melatonin by the pineal gland is stimulated by darkness and inhibited by light. The retina detects the light and directly signals and entrains the suprachiasmatic nucleus(SCN). Fibers project from the SCN to the paraventricular nuclei (PVN) which relay the circadian signals to the spinal cordand out via the sympathetic system to superior cervical ganglia(SCG) and from there into the pineal gland. The pineal gland is large in children, but shrinks at puberty. It appears to play a major role in sexual development, hibernationin animals, metabolism, and seasonal breeding. The abundant melatonin levels in children is believed to inhibit sexual development. When puberty arrives, melatonin production is reduced. Ancient amphibianssuch as Ichthyostega, which existed in the Late DevonianPeriod, had an orifice on the top of the skull through which the pineal gland was exposed and received light input. Over the course of time and for unknown reasons, the pineal gland migrated into the skull of later tetrapodsand the skull orifice sealed. Modern birdsand reptileshave been found to express the phototransducingpigmentmelanopsinin the pineal gland. Avian pineal glands are believed to act like the suprachiasmatic nucleusin mammals. Reports in rodents suggest that the pineal gland may influence the actions of drugs of abusesuch as cocaine [1]and antidepressants such as fluoxetine (Prozac)[2]; and contribute to regulation of neuronal vulnerability[3]. Additionally, it has been proposed by Rick Strassman that the pineal gland is responsible for manufacture of endogenous dimethyltryptamine, or DMT, although this is only speculation. There are some who believe DMT has a role in dreaming and possibly near-death experiences and other mystical states and, although not proven, it was hypothesized in 1988 by Jace Callaway that DMT is connected with visual dreaming. MythologyThe pineal gland was the last endocrine gland to have its function discovered. Its location deep in the brain seemed to indicate its importance. This combination led to its being a "mystery" gland with myth, superstition and even metaphysicaltheories surrounding its perceived function. Rene Descartescalled the pineal gland the "seat of the soul" [{{fullurl:Template:FULLPAGENAME}}#endnote_Descartes], believing it was unique in the anatomy of the human brain in being a structure not duplicated on the right and left sides. This observation is not true, however; under a microscope one finds the pineal gland is divided into two fine hemispheres. The pineal gland is occasionally associated with the sixth chakra(also called Ajnaor the third eyechakra in yoga). It is believed by some to be a dormant organ that can be awakened to enable "telepathic" communication. See Discordianismfor the relevance of the pineal gland in this religion. References
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