9. Structure and Functions of Pineal Gland
The pineal gland is a small structure located deep between the cerebral hemispheres, where it attaches to the upper portion of the thalamus near the roof of the third ventricle. The pineal gland secretes the hormone melatonin in response to light conditions outside the body. Nerve impulses originating in the retinas of the eyes send this information to the pineal gland. In the dark, nerve impulses from the eyes decrease, and melatonin secretion increases.
The pinealocyte cells that make up the pineal gland are known to produce and secrete the amine hormone melatonin, which is derived from serotonin. Melatonin acts on certain brain regions that function as a “biological clock,” and may thereby help to regulate circadian rhythms, which are patterns of repeated activity associated with the environmental cycles of day and night. The changing levels of melatonin throughout the 24-hour day may enable the body to distinguish day from night. Circadian rhythms include the sleep-wake rhythm and seasonal cycles of fertility in many mammals.