Definition

The pineal gland is a gland of the endocrine system responsible for releasing melatonin, a hormone which influences our sleep and wake cycles. Light hitting the eyes stimulates nerves, which activate or repress the pineal gland. The gland is also called the “pineal body”.

Overview

Historically, the pineal gland was called the “third eye”, because it was once believed that the pineal gland allowed you to see into different planes of existence. Modern conspiracies claim that fluoride in the water stops pineal gland function. These claims have never been backed by modern science and have largely been shown to be false. However, science has shown that the pineal gland has a major role in influencing our sleep cycle and wakefulness.

The pineal gland has a primary role in controlling sleep cycles by secreting melatonin. This hormone modulates our sleep and waking patterns by affecting different cells within the brain and body. Melatonin was first described by the American physician, Dr. Aaron Lerner in 1958. Research has shown melatonin to play an instrumental role in establishing our circadian rhythm, which is the twenty-four-hour cycle of our bioactivity that matches the solar cycle of the day. The gland can be seen in the image below.

Pineal Gland Diagram
Pineal Gland Diagram

Pineal Gland Function

Melatonin is one of the most ubiquitous and versatile hormones found in animals and some plants. It is highly lipophilic, which allows it to reach our cells in record time. Activation of the pineal gland causes the secretion of melatonin. Activation occurs when the eye stops receiving signals of bright light.

Melatonin derives from the amino acid, tryptophan. This amino acid can be found in humans and other species of mammals, reptiles, birds, and amphibians. Tryptophan-rich foods have been linked to calming effects and reduced anxiety. In humans, specifically, melatonin helps control our daily sleep cycle. The body produces melatonin in response to the light hitting the retina of the eye. Light reception inhibits the release of melatonin. In contrast, the absence of light at nighttime will be read as a signal to produce more melatonin. This helps put you to sleep.

The ability to modulate melatonin levels is lent by the presence of special photoreceptor cells in the human retina that emit a signal to the suprachiasmatic nucleus, or SCN, of the hypothalamus. The hypothalamus, in turn, is a part of the brain that supports the body’s homeostatic functions. The light or dark signals are then sent to the pineal gland, which will begin to modulate melatonin levels.

Pineal Gland Location

The pineal gland is roughly located in the center of the brain, sandwiched between the left and right hemispheres. The gland is approximately the size of a grain of rice. It is the only midline brain element that is not paired. The pineal body is tucked in the groove where the two thalamic bodies meet.

The pineal gland takes on the shape of a pinecone, which explains its naming! Nearly all existing vertebrates contain a pineal gland. The glands are even found in the primitive lamprey. It makes sense that the pineal gland is a primitive organ because it helps set sleep cycles based on the light received in the eyes. However, not all species have conserved the pineal gland.

The exception to the rule is the hagfish, which lacks a perceptible pineal gland. Likewise, a few other more advanced vertebrates have lost theirs sometime in their evolution.

Blood Flow to the Pineal Gland

Unlike the remaining mass of the mammalian brain, the pineal gland is not separated from the body by the blood-brain barrier. Instead, it receives the second most profuse supply of blood in the body, next to the kidney. The pineal gland’s main blood supply comes from the choroidal branches of the posterior cerebral artery. Its sympathetic (or excitable) innervation, on the other hand, comes from the superior cervical ganglion. The otic ganglia will supply the inhibitory, parasympathetic innervation.

Pineal Gland Hormone

The pineal gland is mainly known to produce a single hormone, melatonin. Melatonin has a number of roles within the body, mainly related to inducing sleep and shutting down the conscious brain activities. The pineal gland produces a number of other hormone precursors and chemicals, but their role is not well studied.

Receptors for Melatonin

There are a number of receptors for melatonin that are found scattered in various areas of the body. Most notably, they are found in high concentrations in the SCN and the brain’s pituitary gland. This is the main site of action as melatonin directly plays into the circadian rhythm here. But melatonin receptors are also present in the ovaries. The levels of melatonin affect several facets of the menstrual cycle, such as the timing of its onset, the duration, and the frequency.

In other animal species, it even acts as a mating cue. For instance, greater levels of melatonin in horses are found during the spring, which coincides with the ideal season for mating. This is a direct example of the solar cycle’s impact on the reproductive cycle via pineal gland activity. Other melatonin receptors lie in the blood vessel walls and in our intestinal tracts. In the gut, melatonin protects the mucosal layers from lesions and irritation via its eradication of free radicals. Gut lesions can lead to painful esophagitis, gastritis, and peptic cancer among other illnesses.

Melatonin Production and Release

The largest amount of melatonin is expelled by the pineal gland during the night. At this time, the body will undergo several changes closely tied to the concentrations of melatonin. The body’s internal temperature will drop, as will the breathing rate. These experiences are the ones we most associate with falling asleep. In the daytime, however, our retinas will be exposed to a lot of light that will inhibit melatonin expression. This is essential for making us alert and awake during the daytime.

Melatonin as an Antioxidant

Melatonin also has special antioxidant properties. It is known to neutralize radicals, elements with an unstable electron configuration, that would otherwise cause harmful oxidative damage to tissue. Melatonin can also activate other antioxidant enzymes that will perform restorative functions.

Naturally, melatonin is an anti-aging substance that declines as we get older. The loss of melatonin is thus associated with various age-related illnesses. Melatonin also retains a role in buffering the immune system in light of seasonal adjustments. Its roles are still being studied, but the consensus lies in melatonin acting as a stimulant under suppressive conditions and as an anti-inflammatory agent when the immune system experiences acute inflammation.

Pineal Gland Disorders

Disorders that Affect Sleep and Mood

Since the pineal gland is primarily involved in sleep-wake rhythms, it also takes root in mood disorders. Recent studies have linked chronic stress and poor diet as possible causes of reduced levels of melatonin in the system. This is often found in patients with abnormal circadian cycles of cortisol (or “stress”) hormone. In fact, depression and sexual dysfunction are conditions that are further aggravated by low melatonin output.

Some other reports have shown that the pineal gland can become calcified through its normal operation, which may lead to a variety of symptoms. The pineal gland releases a number of chemicals besides melatonin, which may help cause a number of mental illnesses, such as schizophrenia or depression.

Pineal Cysts

Pineal cysts, or cysts of the pineal gland, are a relatively common occurrence that happens in about ten percent of people undergoing a CT or MRI scan. Most patients with pineal cysts will not display any visible symptoms. Very rarely, patients experience headaches and eye movement abnormalities. In some patients, the cyst can even lead to emotional disturbances, sleep issues, and seizures. Only when the pineal gland cyst is symptomatic will a physician recommend surgical removal.

Pineal Tumors

A pineal tumor, on the other hand, is a more serious complication that represents about one percent of all brain tumors. At least seventeen types of tumors arise in the area of the pineal gland but many are benign. The most common tumors are gliomas, pineal cell tumors, and germ cell tumors.

The pineal gland is located next to a duct called the aqueduct of Sylvius. It acts as a passage through which cerebrospinal fluid (CSF) leaves the center of the brain. Pineal tumors often block this duct, causing a buildup of pressure that expands the ventricles within the skull. This blockage will present complications most often linked to the symptoms of pineal gland tumors:

  • Headaches
  • Seizures
  • Nausea
  • Visual changes
  • Problems with memory recall

Pineal Tumor Treatments

Treatment of the tumor can vary depending on the diagnosis. This diagnosis must be informed with a precise histological analysis from a biopsied sample. A benign, or non-cancerous pineal tumor can be resected surgically at the hands of a skilled surgeon. Radiation therapy and surgery are typically used as a treatment for pineal tumors.

For example, pinealocytomas obtain no benefit from radiation therapy alone. So, they require surgical resection. The most common cancer in this area is germinoma. Germinoma tumors, in contrast, are both very sensitive to chemotherapy and radiation and will be cured in most cases. The same applies to other malignant germ cell tumors near the pineal gland.

Like any other tissue that has undergone intensive cancer therapy, there may be long term effects on the pineal tissue’s ability to perform its endocrine functions. Therefore, the patient will need to work alongside an endocrinologist to address certain hormone deficiencies that may have arisen. Modern medicine can manage most of these issues.

Quiz

1. How will light affect melatonin secretion? Choose the best answer.

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2. How will darkness affect melatonin secretion? Choose the best answer.

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3. Which of the following is indicated for malignant pineal tumors, per the article?

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4. When scientists compared the pineal glands of two birds, one nocturnal and the other diurnal, they noticed that the pineal gland of the nocturnal bird was smaller and less connected. Which hypothesis below supports this finding?

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5. If you suddenly lost your pineal gland, which of the following symptoms would you likely experience?

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