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.
