Also called the pineal body or epiphysis cerebri, the pineal gland is important to this discussion for two reasons. First, it is the center for the production of the hormone melatonin. Melatonin is implicated in a wide range of human activities. It regulates daily body rhythms, most notably the day/night cycle (circadian rhythms). Melatonin is released in the dark, during sleep. The recent melatonin craze sweeping through the health conscious community makes claims that the hormone slows the aging process (a defense against free radicals), prevents jet lag, is implicated in seasonal affective disorder, coordinates fertility, and allows for deep restful sleep patterns.
Melatonin is a very ancient hormone that is found throughout the animal kingdom. In reptiles and birds the pineal is close to the skin and needs no interaction with the eye to register day/night cycles (this is where the notion of the 'third eye" comes from). In these animals, the pineal gland is the master clock. In mammals, however, the pineal gland is subordinate to the eye/SCN system. Light severely curtails the production of melatonin.
Melatonin has been shown to inhibit the growth and metastasis of some tumors in experimental animals, and may therefore play a role in cancer inhibition. Removal of the pineal gland and/or reduction in melatonin output have been implicated in the increased incidence of breast cancer in laboratory animals. Patients who have breast cancer have lower levels of melatonin in the blood. The hormone has also been shown to be protective against genetic damage, and it has a stimulatory effect on the immune system.
The anticancer role of melatonin in humans is not established yet, but in a study involving blind women, melatonin production was found to be higher at all times. This finding was associated with the finding that breast cancer in this study was correspondingly lower for these blind women compared to the general population.
The pineal gland has been implicated in a number of disorders including cancer, sexual dysfunction, hypertension, epilepsy, and Paget's disease. The pineal gland calcifies with age and melatonin production correspondingly decreases. This decline in melatonin has been suggested to be a trigger for the aging process.
Environmental stresses affect pineal function, impacting overall body alertness, temperature levels, and hormone operation. Stresses that affect pineal function include unusual light and dark rhythms, radiation, magnetic fields, nutritional imbalances, temperature swings, high altitude, and overall daily stress patterns.
Melatonin, if any of these claims turn out to be substantial, is a powerful body chemical, with important implications for human functioning, especially (I would suggest) for blind and visually impaired individuals. If light inhibits melatonin production, then blind individuals lack a mechanism for balancing the melatonin levels in the blood.
Melatonin overload may be related to seasonal affective disorder, the depression some people feel during the overcast, short days of the winter months (when there is minimal sunlight). Are blind individuals more susceptible to depression because of a melatonin imbalance?
I've worked with enough blind children to know that their sleep patterns get all messed up. We usually tell parents that the reason is simply a lack of feedback about when day has set and night is here. We establish a routine for the blind students that replaces the natural rhythm. The role of melatonin may be important for these children. We may be able to help them sleep better and in a more balanced rhythm using medications to counter the excess melatonin in their blood streams. This is not an endorsement of drug therapy, only speculation as to whether its use would be helpful. For further discussion of sleep disorders associated with circadian rhythm interruption see the section on the hypothalamus.
Studies do show that partially sighted eyes may still maintain subconscious awareness of light or its absence. For these people, the circadian rhythms established by the pineal gland are intact. For totally blind individuals, with no subconscious feedback to the pineal gland, there may be periods of severe insomnia, or they may fall asleep at unusual times (like during math class).
The second reason for interest in the pineal gland is that it contains magnetic material in birds and other animals. It is a center for navigation. This, of course, is important for blind individuals. If the pineal gland turns out to contain magnetic material in humans (researchers are looking), then it may be involved in navigational processing. Magnetic processing is subtle and may be part of the bodies unconscious navigational system. Mobility specialists are aware of this possibility, but we have yet to use it to train students.
Studies done mostly with birds strongly suggest that the pineal gland is a center for navigation. Scientists believe that the pineal body is a magnetoreceptor, capable of monitoring magnetic fields, and helping to align the body in space. Changing the direction of magnetic fields around the heads of birds alters their ability to orient.
Electromagnetic fields (EMF) suppress the activity of the pineal gland and reduce melatonin production. EMF activity therefore disrupts the bodies circadian rhythms.
In lower vertebrates, the pineal gland has an eye like structure and it functions as a light receptor. The pineal gland may be the evolutionary forerunner of the modern eye. It is located within the third ventricle. It is large in children, but shrinks at puberty. It appears to play a major role in sexual development, hibernation in animals, metabolism, and seasonal breeding. In humans it affects circadian rhythms, sleep patterns (melatonin levels increase at night), and is implicated in seasonal affective disorder. The abundant melatonin levels in children is believed to inhibit sexual development. When puberty arrives, melatonin production is reduced.
There is a pathway from the retinas to the hypothalamus called the retinohypothalamic tract. It brings information about light and dark cycles to a region of the hypothalamus called the suprachiasmatic nucleus (SCN). From the SCN, nerve impulses travel via the pineal nerve (sympathetic nervous system) to the pineal gland. These impulses inhibit the production of melatonin. When these impulses stop (at night, when light no longer stimulates the hypothalamus), pineal inhibition ceases and melatonin is released. The pineal gland is therefore a photosensitive organ and an important timekeeper for the human body.
Retinal research done with hamsters demonstrates another center for melatonin production. Located in the retina, this center implies that the eyes have their own built in circadian timepiece. This retinal system is distinct from the brains body clock in the suprachiasmatic nucleus (SCN). Biologists found that they could throw the retinal rhythms out of sync with other circadian cycles. They also found that they could set and reset the retinal clock even when the SCN was destroyed.
The retinal clock produces (stimulates the production of?) melatonin. Researchers are now looking for the exact location (s) of this clock in the human eye (and expect to find it). No one yet knows what the separate clock is for or how it relates to the SCN. One thing is clear: there is a melatonin craze right now. People are taking regular doses of the hormone. There is a question whether or not regular doses of melatonin may damage the eye over time. Melatonin could affect the timing of hormone peaks in the eye. High retinal melatonin levels combined with exposure to bright light might cause retinal damage. This is speculation (a caution) at this time.
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