Pineal Gland

The pineal gland definition is that of a small pea-sized organ within the brain that plays an important role in the endocrine system. It was the last organ of the body to be discovered and early researchers believed the pineal gland housed the soul. Although the soul and life's secrets are not hidden within the pineal gland, the gland secretes melatonin under the control of the suprachiasmatic nucleus, located in the anterior hypothalamus, to regulate the body's circadian rhythms and normal sleep-wake cycle.

Researchers have studied the various roles that the pineal gland plays in development as well as in melatonin production. Different studies have suggested that the pineal gland can influence bone metabolism, mental health, puberty, and drug addiction. Pineal gland functions will be explored in more detail below.

Pineal Gland Location

The pineal gland's location influenced it being referred to as the "third eye." It is located at the midline of the brain between the two hemispheres (halves) of the brain. The pineal gland is also called the "pineal body" and is about the size of a pea. In Latin, "pinea" means pine cone, and the pineal gland was so named due to its pinecone shape.

The pineal gland weighs only 0.1 grams and is 5-9mm long. It is located between the two hemispheres of the brain at the midline and is composed of pinealocytes, endocrine cells that secrete melatonin. It is also composed of glial cells which are nerve cells that facilitate communication within the brain. Because the pineal gland is rich in calcium, it shows up well on an x-ray. Radiologists will use the pineal gland as a landmark on brain images to pinpoint the midline of the brain. The pineal gland is larger prior to puberty and its function decreases with age.

Pineal Gland Function

What does the pineal gland do? Its main function as an endocrine organ is to secrete the hormone melatonin. The pineal gland also plays a role in other bodily functions. These will be reviewed in detail below.

Hormone Secreted by the Pineal Gland

When asked which gland is responsible for releasing melatonin, the answer is the pineal gland. Melatonin is a hormone that plays a role in regulating circadian rhythms under the control of the hypothalamus. Circadian rhythms manage the body's internal clock to make an individual feel drowsy, alert, or sleepy and to signal when they should wake up. Circadian rhythms are the twenty-four-hour cycle of bodily regulations that influence sleep and activity.

The hormone secreted by the pineal gland, melatonin, is released depending on the amount of light present. When there is more light present, melatonin secretion is inhibited to keep a person awake. When it is dark, more melatonin is secreted to promote drowsiness.

Melatonin's response to light explains why humans might become more sleepy after the sun goes down. If pineal gland sleep regulation is disrupted, supplemental melatonin is recommended to assist with sleep.

What Else Does the Pineal Gland Do?

In addition to melatonin production, the pineal gland plays a role in other regulatory processes in the body, as seen below:

• Bone metabolism - Research suggests that higher levels of melatonin improve bone metabolism. This is being studied to determine if melatonin supplementation can reduce the risk of osteoporosis.
• Aging - Melatonin levels decrease with advancing age. This is evidenced as older people will typically sleep less and require less sleep.
• Sense of direction - Studies suggest that the pineal gland plays a role in spatial navigation. If the pineal gland is absent, the ability to determine direction lessens.
• Pituitary gland function - The pituitary gland is another endocrine gland located in the hypothalamus. Melatonin can block hormone production by the pituitary gland to delay sexual maturity. High melatonin levels can impede the development of the ovaries and testicles to delay the onset of puberty.
• Drug metabolism - Research suggests there is a relationship between the pineal gland and cocaine and stimulant addiction.
• Mental health - Studies linked the relationship between the pineal gland to Seasonal Affective Depressive Disorder (SADD), which presents increased depression symptoms in the dark winter months.
• Circadian rhythms - Melatonin production helps to regulate the body's normal sleep/wake cycle to inform the body when to rest. Increased melatonin production stimulates the drowsiness needed for sleep.
• Reproduction - The pineal gland plays a role in female reproduction.
• Heart health - Higher melatonin levels have a positive impact on blood pressure and heart health.

Abnormalities of the pineal gland can cause depression, sexual dysfunction, stomach ulcers, sleep disruption, and issues with blood pressure. An example of damage to the pineal gland occurs when a tumor invades the organ. In addition to causing insomnia and sleep issues, a tumor can affect vision, disrupt memory, as well as cause seizures and nausea.

Lesson Summary

The pineal gland is a pea-sized organ located between the right and left hemispheres of the brain. The principal function of the pineal gland is to secrete the endocrine hormone melatonin, which plays a role in the body's circadian rhythms under the control of the hypothalamus. Circadian rhythms are the daily cycle of a person's activity and sleep in a twenty-four-hour period. More melatonin is produced in darkness to influence sleep and make a person drowsy. Higher levels of light inhibit melatonin production to keep a person more alert.

The pineal gland plays a role in other endocrine and body systems. It can influence sexual maturation and female reproduction as melatonin blocks certain pituitary hormones. Melatonin affects bone metabolism with the suggestion that more melatonin may have a future role in the treatment of osteoporosis. Studies suggest that drug metabolism is influenced by melatonin with some relation to addiction. As well, if the pineal gland does not function normally after injury or in relation to a tumor it can cause abnormalities in sleep, sexual dysfunction, and memory issues.

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