The pineal gland is a pine cone shaped structure located in the diencephalon whose main function is the secretion of melatonin, a hormone that is best known for its role in regulating circadian rhythms. The pineal gland secretes melatonin throughout the 24-hour cycle, with secretion being highest in the middle of the night and lowest during daylight hours. In this video, I discuss the pineal gland and melatonin secretion, including 24-hour patterns of melatonin secretion and how the pineal gland uses signals from the retina about how much light is in the environment to determine what the time of day is.
**CORRECTION** On the chart that appears at :34, the last time on the x-axis should be 12 PM, not AM.
Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics in 2 minutes or less. In this installment I will discuss the pineal gland.
The pineal gland was given its name because it has a pine-cone like shape. Unliked most brain structures, the pineal gland is unpaired, meaning there is only one. It sits directly on the midline of the brain. The function most linked to the pineal gland is the secretion of a hormone called melatonin, which is best known for its role in regulating circadian rhythms.
The pineal gland is made up of secretory cells called pinealocytes, which secrete melatonin throughout the 24-hour cycle. Secretion is highest in the middle of the night. It begins to decrease as it gets closer to dawn and is lowest during daylight hours.
This schedule of melatonin secretion is regulated by signals from the retina about light in the environment, which travel to a nucleus in the hypothalamus called the suprachiasmatic nucleus and then via an indirect route to the pineal gland. The main function of the suprachiasmatic nucleus is to control circadian rhythms, and in addition to sending information about ambient lighting to the pineal gland, the suprachiasmatic nucleus also uses levels of melatonin as a signal to provide information about the time of day.
Because melatonin levels are highest during the hours of darkness, melatonin activity can be used as a signal that circadian rhythms should be in their nocturnal stage. If melatonin levels are high and someone is still wide awake, it is an indication circadian rhythms are not in sync. This might happen, for example, after flying across several time zones. In this case, melatonin is used by the suprachiasmatic nucleus as a signal to get circadian rhythms back on track.
Due to its close association with nighttime and circadian rhythms, melatonin has also been investigated as playing a role in promoting sleep, but the true relationship between melatonin and sleep is still unclear.