It is a pinecone-shaped small gland located in the middle of the human brain in between the two hemispheres in an area called epithalamus. It was once known as “the third eye”. It is the major site for melatonin secretion, which regulates the body’s internal clock (Circadian rhythm).
This gland is rich in calcium levels. The calcium acts as a radiographer to locate the middle of the brain in X-ray images. It was also one of the last glands to discovered.
Anatomy of Pineal Gland
The pineal gland is about 0.3 inches long and weighs 0.1 grams. The adrenergic nerves entering the pineal gland regulate its functions.
The pineal gland is composed of pinealocytes and supporting cells that resemble the astrocytes present in the brain.
Some lower vertebrates have a well-developed eye-like structure that acts as a light receptor.
Hormones Secreted by Pineal Gland
The pineal gland synthesises melatonin and serotonin. The pineal gland also produces neurosteroids.
Melatonin: The Pineal Gland Hormone
Serotonin is the precursor of melatonin. Serotonin is acetylated and methylated to yield melatonin within the pineal gland. The light exposure to the eyes affects the synthesis and secretion of melatonin.
Two melatonin receptors have been found in mammals- Mel1A and Mel1B. These are G-protein coupled cell surface receptors.
Melatonin affects circadian rhythm.
Our circadian rhythm is a 24-hour biological cycle, characterised by the sleep-wake patterns. The daylight and darkness regulate our circadian rhythms. The secretion of melatonin is stopped on exposure to light which in turn controls the circadian rhythm. The secretion of melatonin is high during dark and low during daylight. This influences our reaction to photoperiod.
The secretion of gonadotropins from the anterior pituitary is blocked by melatonin thereby affecting reproduction. These hormones aid in the development of ovaries and testes.
Functions of Pineal Gland
Secretion of melatonin
This gland secretes the hormone melatonin which regulates the circadian rhythm of the body and also certain reproductive hormones. The secretion of this hormone depends upon the amount of light a person is exposed to. This hormone is produced in large amounts when it is dark. The presence of light inhibits the secretion of melatonin which controls our circadian rhythms.
The melatonin secretion has a positive impact on the heart and blood pressure. It may also be used for the treatment of cardiovascular diseases.
Melatonin inhibits the secretion of reproductive hormones from the anterior pituitary, which are responsible for the development and functioning of reproductive organs.
Pineal Gland Disorders
An impaired pineal gland leads to hormonal imbalance. For eg., there is a disruption in sleep patterns if the pineal gland stops functioning. Low melatonin secretion also leads to anxiety, low thyroid hormone production, menopause symptoms, etc.
Too much of melatonin secretion can lead to low blood pressure and improper functioning of the thyroid and adrenal glands.
Seasonal Affective Disorder, a depressive disorder, is caused in winters when melatonin secretion is high due to low sunlight.
If a tumour develops in the pineal gland, it affects several other factors in the body:
Impaired vision and other senses
Research is being carried out to know more about the pineal gland and its undiscovered functions. Its function is still a bit of a mystery.
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