The major glands of the endocrine system are the hypothalamus; the pituitary, thyroid, and pineal gland; the parathyroid gland, and adrenal glands; as well as the glands in the reproductive organs (ovaries and testes).
Some functions of the components of the endocrine system include the secretion of melatonin by the pineal gland, which plays a big and significant role in the body’s internal clock by producing melatonin.
Melatonin is a hormone that is important in the sleep-wake cycle, adjustment of the levels of various substances in the blood, regulation of the metabolism, growth, menstrual cycle, pituitary gland functions, immune functions, the sexual drive, etc.
Location and Function of Pineal Gland
The pineal gland (also: conarium, or epiphysis cerebri) is a tiny, pine-cone shaped endocrine gland located in the center of the human brain. It’s responsible for the secretion of melatonin, a hormone that is inhibited by light and triggered by darkness.
In addition, light turns off the secretion of melatonin in the pineal gland. When there’s no light and the pineal gland is in the appropriate circadian phase, it’s no longer inhibited and starts secreting melatonin. From the pineal gland, melatonin is secreted into the bloodstream and into the third ventricle of the brain. The synthesis of melatonin occurs in the pineal gland, and it is distributed all over the body tissues through the bloodstream.
In this way, the pineal gland plays a vital role in the seasonal and circadian cycles of the human body, i.e. the physical, mental, and behavioral changes that follow a daily cycle.
What Is a Circadian Cycle?
A circadian cycle is a kind of a 24-hour internal clock, running in the background of the human brain. It coordinates the bodily functions during the cycles of sleepiness and alertness at regular intervals by responding to the light changes of our environment.
During our sleep, the body rests, but this is not the case with the brain. Its billions of neurons continue to send sensory signals to the corresponding centers of the brain, such as those for breathing, the regulation of pulse, heartbeat, etc.
Sleep occurs in cycles, which are made up of lengthening phases of REM sleep (rapid eye movement – a phase when the largest portion of dreaming occurs), and four stages of NREM sleep (nonrapid eye movement – a mostly dreamless phase of the cycle).
During the first stage, our sleep is light. People can wake easily, while the brain waves are very active. In the second stage of this cycle, brain waves begin to slow down. During the third stage, fast and slow waves flow at scattered intervals. In the fourth stage, which is the deepest one of all, only slow waves can occur.
As the human body reaches the deeper stages, body temperature, heart rate, breathing rate, and blood pressure become reduced. During the REM sleep phase, these functions increase slightly. Hence, the largest portion of our dreaming occurs at this stage of the sleep cycle.
However, the circadian cycle is not only a cycle of the sleep-wake phase. This biological circadian system also regulates the body-temperature cycle, the monthly menstrual cycle, as well as the cycles in which numbers of other hormones are secreted.
Anatomy and Structure of Pineal Gland
In general, a gland is constructed by the following elements:
- a fibrous envelope (or capsule)
- a lymphoid tissue
- a free supply of blood vessels
- afferent and efferent vessels exchanging information through the lymph paths in the substance of the gland
The pineal gland is located at the posterior of the diencephalon region in the brain, attached to the outside of the brain substance near the entrance of the ‘aqueduct of Sylvius’ canal. It consists of two types of cells:
- Pinealocytes
- Glial cells
Pinealocytes
The main neuron-like cells that are contained in the pineal gland are called pinealocytes. These calls are located behind the third ventricle and between the two hemispheres of the brain and are responsible for the production of the hormone melatonin and its increase which occurs at night in the human body.
In this way, these structural cells of the pineal gland are in charge of the regulation of circadian rhythms, i.e. the sleep and wake cycles.
Glial cells
Glial cells are supporting cells that aid neurotransmission, adult neurogenesis, and immune surveillance by forming myelin, giving protection and support, as well as maintaining equilibrium in the central nervous system.
Also, one of their roles is to guide the developing neurons to their destinations, buffer ions and chemicals that would otherwise harm neurons, as well as provide myelin (protein) sheaths around axons.
There are several types of glial cells with different functions in the central nervous system of a mature individual:
- Astrocyte cell – A star-shaped glial cell that makes contact with both capillaries and neurons in the central nervous system. It provides nutrients and other substances to neurons and regulates the concentrations of ions and chemicals in the extracellular fluid while providing structural support for synapses.
- Oligodendrocyte – А type of neuronal support cell whose main function is to provide an insulating sheath around the axons by producing myelin. This is essential for fast signal conduction of the sensory information in the central nervous system.
- Microglial cell – A type of glial cell that supports the neurons. It acts as the first and main form of active immune defense in the central nervous system. In this way, the microglial cells play an important role in brain infections and inflammation.
- Ependymal cell – A type of neuronal support cell that facilitates the transport of hormones and other substances in the brain. This type of cell forms the epithelial lining of the ventricles in the brain and the central canal of the spinal cord.
- Satellite cell – A type of glial cell that controls the repair of skeletal muscles by being involved in the process of normal growth of muscles, as well as their regeneration following injury or disease.
- Schwann cell – A type of glial cell that forms the myelinated sheath of the neurons. They maintain the physical integrity of the synaptic junction by releasing neuroactive substances that regulate the strength of the synaptic transmission.
What Happens When the Pineal Gland Is Dysfunctional?
An abnormal function of the pineal gland can cause a hormone imbalance, thus affecting other systems in our body and leading to various medical conditions and disorders.
Due to the fact that the production of melatonin is one of the main functions of the pineal gland, a decreased production of this hormone in the body can result in:
- Insomnia – Since the hormone melatonin promotes the sleep processes, a lack of the hormone can cause a sleep disorder characterized by difficulty falling asleep, and prolonged phases of the awake stage.
- Intestinal hyperactivity (Irritable Bowel Syndrome) – Irritable bowel syndrome (IBS) is a common disorder that can be identified by recurrent abdominal pain or discomfort that is the result of a dysfunctional pineal gland. Namely, melatonin plays an important part in gastrointestinal physiology as a regulator of gastrointestinal motility.
- Menopause – The melatonin produced in the pineal gland is directly related to the menstrual cycle as a part of the circadian body cycles, with the change of hormonal levels and melatonin deficiency.
Having an important role in the regulation of this cycle, a dysfunctional pineal gland can result in difficulties initiating and/or maintaining sleep, with frequent disruption of the sleep pattern during the night in postmenopausal women.
- Mental health problems (Anxiety, Depression) – A regular sleep pattern is of crucial importance for good mental health, and so is the access of light. Some core symptoms of depression show disturbance of the circadian rhythm or are closely linked to some circadian system functions, such as sleep-wake cycle alterations.
Also, a long deprivation of daylight can lead to severe changes in the mood of the affected person, further resulting in more serious health disorders.
To Sum Up
The pineal gland is small and pinecone-shaped, located at the posterior of the diencephalon region in the brain. But, even though it’s tiny in size, it plays a big role in the biological cycles of the human body.
At night, when there’s no light, the pineal gland secretes the hormone melatonin. It regulates the body’s sleep patterns in both circadian (daily) and seasonal patterns.
In the morning, the photoreceptors in the retina of the eyes send signals to the pineal gland, thus decreasing the melatonin production and changing the biological cycles. Namely, the pineal gland receives an indirect neural input from the eyes of an adult individual, which is an important part of its role as the internal clock of the human body.
Fun Facts
Did you know?
1 – Glial cells make up at least 80% of cells in the human brain. Without them, neurons and their synapses would fail to function properly.
2 – The pineal gland is also referred to as the ‘third eye’ because it contains a retina and a cornea – light-sensitive nerve endings that are non-visual photoreceptors.
3 – When the hypothalamus receives information about the presence or absence of light, it sends a signal that activates the pineal gland, which starts or inhibits cortisol and melatonin production. Meditation is useful for the activation of the pineal gland, which is popularly labeled as ‘opening of the third eye’.
4 – Melatonin is considered to be a natural sleep hormone, due to the fact that it is produced according to the amount of light a person is exposed to. In the dark, it is produced in a higher amount and makes us fall asleep fast. In that way, it promotes sleep by helping to regulate the body’s biological clock, as well as the sleep-wake cycles.
5 – The pineal gland has a special place in René Descartes’ (1596–1650) philosophy. Namely, he considered it as the principal seat of the soul, and the place in which all our thoughts are formed, due to the notion that the pineal gland was involved in sensation, imagination, as well as memory.
6 – Most of us typically are able to munch on something all day long, regardless of the time of day. However, our biological circadian cycle would prefer us to eat in the time windows when the sun is up, since this is when our body wants us to eat. In ideal circumstances, we shouldn’t eat in the 12-hour period between our last meal of the day and the first of the next day (very likely, this is also the reason why we don’t feel so good after having a late-night dinner).
7 – Due to its antioxidant characteristics, melatonin acts as a natural anti-aging substance. Hence, the loss of this hormone produced by the pineal gland is linked with several age-related illnesses.
8 – The circadian system controls countless physiological and cell biological functions, which sometimes last more than 24 hours. Those circadian cycles are labeled as infradian rhythm in chronobiology and include the menstrual cycle, breeding, seasonal rhythms, etc.
9 – Aging is typically associated with both impairments of the circadian system and a decrease in melatonin secretion.
10 – The circadian rhythms could also be modulated by external stimuli. Namely, they can be affected by temperature (heat-cold), light (light-dark), sound, food supply, time-zone changing travel (jet lag), as well as various social factors (social jet lag).