A small endocrine gland deep in the brain that secretes melatonin and helps regulate the sleep-wake cycle.
Structure
The pineal gland is pinecone-shaped and about the size of a grain of rice. It contains pinealocytes that synthesize melatonin from the neurotransmitter serotonin.
Function
Output rises sharply at night, peaks in the early morning hours, and falls during the day. The rhythm is set by light input that reaches the gland through a circuit starting in the retina and passing through the suprachiasmatic nucleus.
Clinical note
The gland often calcifies with age and can appear as a small white spot on brain imaging.
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