Pineal gland and its effect on maintaining circadian rhythm

 

"Pineal gland or body is a small glandular structure that projects from the dorsal diencephalon and lies just posterior to the third ventricle. In the past, the pineal body is figured predominantly in philosophic and religious writings, for Devartes, it was the seat of soul. When this idea was discredited, the pineal gland was relegated to the states of a vestigial organ. The identification of melatonin, the pineal hormone followed by recognition of its role in maintaining biologic rhythms and the modulating effects on its secretion by the circadian light dark circle-revived scientific interest in the structure. Even though the hormone was found to have an indirect effect on several other nuro-endocrine systems, neurologist took little interest in the pineal gland because oblation of the gland in humans, with attendant loss of most of its circulating melatonin leads to few if any clinical changes.

            It is the cyclic secretion of melatonin that appears to be the most important activity of the pineal gland. However melatonin secretion is more accurately regarded as a linked manifestation of the circadian rhythms than as its controlling mechanisms. The main cellular element of the gland, the pinealocyte from is thought to be derived from neural photoreceptors in lower vertebrates. In humans, the pineal no longer possesses the ability to transducer light directly. However, it does retain an input from the photic stimuli and influence the circadian light-dark cycle through a pathway that originate in the retina, synapses in the suprachiasmatic nucleus, and passes through descending sympathetic tracts to the intermedio-lateral cell columns and superior cervical ganglia, and then ascends to innervate nor-adrenergic terminals in the pinealocytes. Darkness elicit a release of non-epinephrine from the photoreceptors, stimulating the synthesis and release of melatonin. During day light the retinal photoreceptor cells are hyperpolarized, non-epinephrine release is inhibited, and there is little melatonin production. The concentration of the hormone peaks between 2 A.M -4 A.M. and gradually falls thereafter. An approximate circadian rhythmicity to melatonin release is preserved in continuous darkness and inexpicably; the blind maintain suppression of serotonin. In human, it is difficult to separate the changes that occur in the suprachiasmatic nucleus from those of the pineal gland".

Source:-

Adams and Victor's Principles of Neurology, Tenth Edition, Allan H. Ropper, Martin A. Samuels, Joshua P. Klein.

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