The endocrine system includes all of the glands of the body and the hormones produced by those glands. The glands are controlled directly by stimulation from the nervous system as well as by chemical receptors in the blood and hormones produced by other glands. By regulating the functions of organs in the body, these glands help to maintain the body’s homeostasis. Cellular metabolism, reproduction, sexual development, sugar and mineral homeostasis, heart rate, and digestion are among the many processes regulated by the actions of hormones.
Anatomy of the Endocrine System
The hypothalamus is a part of the brain located superior and anterior to the brain stem and inferior to the thalamus. It serves many different functions in the nervous system, and is also responsible for the direct control of the endocrine system through the pituitary gland. The hypothalamus contains special cells called neurosecretory cells—neurons that secrete hormones.
All of the releasing and inhibiting hormones affect the function of the anterior pituitary gland. TRH stimulates the anterior pituitary gland to release thyroid-stimulating hormone. GHRH and GHIH work to regulate the release of growth hormone—GHRH stimulates growth hormone release, GHIH inhibits its release. GnRH stimulates the release of follicle stimulating hormone and luteinizing hormone while CRH stimulates the release of adrenocorticotropic hormone. The last two hormones—oxytocin and antidiuretic hormone—are produced by the hypothalamus and transported to the posterior pituitary, where they are stored and later released.
The pituitary gland, also known as the hypophysis, is a small pea-sized lump of tissue connected to the inferior portion of the hypothalamus of the brain. Many blood vessels surround the pituitary gland to carry the hormones it releases throughout the body. Situated in a small depression in the sphenoid bone called the sella turcica, the pituitary gland is actually made of 2 completely separate structures: the posterior and anterior pituitary glands.
- Posterior Pituitary: The posterior pituitary gland is actually not glandular tissue at all, but nervous tissue instead. The posterior pituitary is a small extension of the hypothalamus through which the axons of some of the neurosecretory cells of the hypothalamus extend. These neurosecretory cells create 2 hormones in the hypothalamus that are stored and released by the posterior pituitary:
- Oxytocin triggers uterine contractions during childbirth and the release of milk during breastfeeding.
- Antidiuretic hormone (ADH) prevents water loss in the body by increasing the re-uptake of water in the kidneys and reducing blood flow to sweat glands.
- Anterior Pituitary: The anterior pituitary gland is the true glandular part of the pituitary gland. The function of the anterior pituitary gland is controlled by the releasing and inhibiting hormones of the hypothalamus. The anterior pituitary produces 6 important hormones:
- Thyroid stimulating hormone (TSH), as its name suggests, is a tropic hormone responsible for the stimulation of the thyroid gland.
- Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex, the outer part of the adrenal gland, to produce its hormones.
- Follicle stimulating hormone (FSH) stimulates the follicle cells of the gonads to produce gametes—ova in females and sperm in males.
- Luteinizing hormone (LH) stimulates the gonads to produce the sex hormones—estrogens in females and testosterone in males.
- Human growth hormone (HGH) affects many target cells throughout the body by stimulating their growth, repair, and reproduction.
- Prolactin (PRL) has many effects on the body, chief of which is that it stimulates the mammary glands of the breast to produce milk.
The pineal gland is a small pinecone-shaped mass of glandular tissue found just posterior to the thalamus of the brain. For years, mystics considered it to be the seat of the mystical third eye, whereas the medical community considered it vestigial and, thus, pretty much non-functioning. Since then, the mystics have not necessarily been refuted, but the medical community has been. The pineal gland is now known to be the major source of melatonin production in the body. It is full size in children, a size it maintains throughout adulthood — although its weight can drop significantly starting with puberty. And it is not unusual for the gland to literally calcify in many adults. The gland most likely plays a significant role in sexual maturation, circadian rhythm and inducing sleep, and in seasonal affective disorder and depression. In animals, it plays a key role in hibernation.
The trigger for production and release of melatonin is total darkness — any light in the room will inhibit this process. Today, however, living in a world with nightlights in the bedroom or streetlights sneaking through the window, we actually have an epidemic of people with insufficient melatonin production, even at a very young age. The problem doesn’t just come from light falling on our eyes while we sleep, but from light falling on any part of the body. Even if you wear an eye-mask, if any light is falling on your arms or chest or feet, that’s enough to slow melatonin production. Without artificial light, we would normally be in total darkness 8–12 hours a night, producing melatonin during all of those hours. Living in a city or suburban area may cut the hours of total darkness to six or less, and in many cases, zero. Melatonin levels also decline significantly as we age. Since its discovery in 1958, melatonin has been studied extensively and shown to be widely beneficial to the body. The benefits of supplementation to compensate for abnormally low production in the body include:
- Better Sleep — Lowered levels of nighttime melatonin reduce the quality of sleep, resulting in the need for more sleep. If your pineal gland does not produce adequate melatonin early enough in the evening, both the quality and quantity of your sleep may suffer. Lack of melatonin may make it difficult for you to fall asleep or may cause you to wake up too soon. Too much melatonin and you will feel exhausted or “drugged” throughout the day. By taking melatonin instead of other sleep aids, rapid eye movement (REM) sleep (dreaming) is not suppressed nor does it induce “hangover” effects when used as directed.
- Enhanced Immune Function — Many people report that supplementation with melatonin has significantly reduced their incidence of colds and infections. The exact way in which melatonin affects the immune system is not known. However, since much of the activity of the immune system takes place at night, some researchers have proposed that melatonin interacts with the immune system during sleep, helping to buffer the adverse effects of stress. It has been proposed by some that the increased incidence of cancer we see today is partially due to the extended time we are exposed to artificial lighting. This is reflected in the fact that melatonin levels in breast cancer and prostate cancer patients are half of normal.
- Powerful Antioxidant Capabilities — Melatonin is one of the most powerful antioxidants produced in the body. In addition, since it is both water and fat-soluble, melatonin can reach almost every cell in the body. However, since it cannot be stored in the body, it must be replenished daily.
- Mood Elevator — Nighttime melatonin levels are low in people with major depressive and panic disorders. Individuals with mood swings or who are melancholic also have lower melatonin levels. Both seasonal affective disorder (SAD) and cyclic depressions are related to the peaks and valleys of melatonin levels.