Serotonin is a small molecule that functions both as a neurotransmitter in the CNS (Central Nervous System) and as a hormone in the periphery.
Serotonin is generally known as the “feel good” hormone, as low levels are associated with depression. Apart from the connection with mood, serotonin is an important hormone that impacts the entire body and plays a role in regulating almost every system in the body.
What is serotonin?
Serotonin is one of dozens of neurotransmitters in the brain, each playing a specific role. Some of the better-known neurotransmitters are histamine, which plays a role in allergic reactions; adrenalin, which is the body’s defense mechanism against stress; and oxytocin, which plays a role in social bonding between people.
Serotonin plays an important role in modulating virtually all the behavioral and neuropsychological processes in the brain, such as mood, emotion, attention, consciousness, perception, reward, anger, aggression, appetite, memory, sexuality, and cognition. Serotonin is also involved in the functioning of all major organ systems in the body, such as the cardiovascular, pulmonary, gastrointestinal, urogenital, and the central nervous systems.
Communication in the body is important to your health and neurotransmitters are key communicators. In the brain, for example, they relay instructions from one brain cell to the next and are active in transferring information throughout the brain and body. These chemical messengers are stored in tiny compartments, called synaptic vesicles, at the end of neurons until the brain needs them to relay a message between neurons and other target cells, such as other neurons, glands, or muscles, throughout the body.
Neurons communicate with each other via electrochemical signals. The electric events that take place across the connections (known as synapses) between neurons are called action potentials.The chemical part of this communication process starts when an action potential reaches the synaptic cleft (“gap”) between neurons and causes neurotransmitters to be released from the neuron’s axon into the cleft. After crossing the cleft, the neurotransmitters attach to receptors on the dendrites of the receiving neuron.
The synapse can be seen as the junction between the message-sending axon of one neuron and the message-receiving dendrite of another neuron, where the electrochemical communication processes take place and signals are transmitted. As there are many different neurotransmitters in the body, receptors, and neurotransmitters act like a lock-and-key system. A specific neurotransmitter (the key) will only bind to a specific receptor (the lock) and the unlocking action means it will trigger changes in the receiving cell.
Serotonin is not only a neurotransmitter, but it also does duty as a hormone, which is involved with numerous physiological processes such as sleep, the movement of food through the gastrointestinal tract, eating behaviour, blood clotting, and sexual function.
Although serotonin plays a crucial role in the brain, at least 90% of the serotonin in the body is made and used by cells in the digestive tract. Even gut bacteria are important in the production of serotonin, as particular gut bacteria stimulate intestinal cells to produce serotonin.
Functions of serotonin in the body:
- Mood: Serotonin is thought to be a natural mood stabilizer, which regulates anxiety, mood, and happiness. Normal levels contribute to feeling emotionally stable, happy, calm, less anxious, and more focused. Scientific theory claims that when dealing with stress related adversity in the brain, serotonin receptor signaling follows two pathways, with one group of receptors showing signs of passive coping by tolerating, but not necessarily dealing with a source of stress or psychological pain, which is characterized by stress moderation. The other pathway shows signs of active coping, by actively addressing a more serious source of stress or psychological pain by changing one’s relationship to it, which is characterized by enhanced brain plasticity (the brain’s capacity to adapt) to change outlook and behaviour. In this way serotonin enhances adaptive responses to adverse conditions along the two specific pathways, depending on the severity of the stress or psychological pain. Low levels of serotonin are associated with mood disorders, such as depression, anxiety, suicidal behaviour, and obsessive-compulsive disorder. While there is a connection between serotonin and mood, scientists are not sure if low levels of serotonin can cause depression, or – like the chicken and egg situation – whether depression can cause low levels of serotonin, or whether there are other factors that may cause both depression and low levels of serotonin.
- Digestion and appetite: With at least 90% of serotonin made and found in the gut, it regulates digestion at multiple levels in the digestive system, starting the moment when food enters the mouth and activates taste-bud cells on the tongue, which causes serotonin release onto sensory afferent nerves (which transmit information from peripheral organs) about taste to the central nervous system. Serotonin plays a role in controlling the movement of food through the digestive system and controlling bowel movements. When we eat food that upsets the stomach, extra serotonin is released to speed up the passing of the unwanted food along the gastrointestinal tract. More irritating or toxic substances in the food leads to the release of even more serotonin, which stimulates the part of the brain that triggers nausea and vomiting. Serotonin also plays a role in reducing our appetite and telling us to stop eating when we have reached satiety.
- Blood clotting: Blood platelets absorb serotonin and release it to assist in blood clotting whenever platelets bind to damaged tissue, for example a cut, by causing tiny arteries in the affected area to constrict and slow blood flow. The narrowing and slowing down of blood flow help with blood clotting.
- Vascular biology: Serotonin is involved in various forms of vascular biology, such as the control of vascular resistance (the resistance that must be overcome to push blood through the circulatory system) and blood pressure, as well as the control of hemostasis (the prevention of blood loss) and platelet function, as discussed above. Serotonin is involved in modulating heart function, such as electrical conduction and cardiac valvular function (opening and closing of heart valves to regulate the blood flowing into and out of the heart).
- Pulmonary system: Serotonin’s effects on brainstem respiratory control centers helps control breathing and the functioning of the pulmonary vasculature (the exchange of gases in the lungs where oxygen is added to and carbon dioxide is removed from circulating blood).
- Endocrine system: Serotonin has certain functions in the endocrine system and in metabolism, such as involvement with the central control of energy balance, the central modulation of the hypothalamic-pituitary-adrenal (HPA) axis (which is a biological system that mediates the effects of stressors, by regulating physiological processes such as metabolism, the autonomous nervous system, and the immune response), as well as mammary gland development and milk release.
- Bone health: Abnormally high levels of serotonin have been linked to low bone density (weaker bones), which increases the risk of fractures and osteoporosis.
- Sexual function: Serotonin plays various roles – in both the brain and the spinal cord – in the central and peripheral control of organs in the reproductive and urinary systems. Certain serotonin receptors increase ejaculation latency and delays orgasm. Voiding the bladder is regulated by the nervous system (and the muscles of the bladder and urethra) and serotonin modulates the act of urination in a similar way as it does ejaculation. Antidepressants that boost serotonin to higher-than-normal levels may affect the libido and inhibit sexual desire and function.
- Sleep: Serotonin is a chemical precursor to the secretion of the sleep hormone melatonin, as the body needs serotonin to produce melatonin in the pineal gland, which has receptors for serotonin. Serotonin also plays a role in the part of the brain that controls the body’s sleep wake cycle. Too much or too little serotonin can affect the quality of sleep and sleep patterns. Melatonin and serotonin work in harmony to keep the body balanced, but perform opposite jobs, as melatonin is produced at nighttime and kickstarts the sleep cycle, while serotonin is active during the day and kickstarts us to get going in the morning.
Some natural ways have been shown to boost serotonin levels, such as food containing tryptophan (an amino acid used to produce serotonin), for example eggs, bananas, turkey, and salmon. Exercise, especially cardio, increases serotonin levels. Light therapy or regular exposure to sunlight can also have a positive effect on mood.
Serotonin does so much more than merely regulating mood, as it is not only an important brain chemical, but also highly active in the digestive tract, as well being involved in a variety of biological processes in most human organs. A serotonin imbalance can affect your mental, emotional, and physical wellbeing. Research has indicated that each individual serotonin receptor is involved in multiple physiologic processes, pointing to the richness and complexity of the various functions of serotonin in the body.
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