The human body is full with chemicals and all of them are released and inhibited at particular times to maintain the homeostasis of the body. They are controlled by the brain and mainly by the secretion from glands in the brain area. Due to negative feedback and cascade systems, hormones secreted from the glands rule the body. They make us feel anger and happiness, sadness and joy. One of the main glands in the brain responsible for hormone regulation in the body is the pituitary gland. The pituitary gland is separated into to regions, the anterior and posterior part. I will discuss the posterior pituitary starting with the location of it, association with other parts of the brain, and the components of it. Next I will discuss the function and the two hormones that are secreted, oxytocin and vasopressin, and their function and regulation from the posterior pituitary. Finally I will talk about an illness associated with malfunction of the release of vasopressin.
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The pituitary gland is a very important part of the brain that is essential for the regulation of some of the major body functions, including water homeostasis, blood pressure, growth, development, and reproduction. It is about the size of a pea and is located at the base of the brain. Together with the hypothalamus the pituitary gland works in a feedback loop which allows the pituitary to communicate with cells in the body that send information to the brain and letting it know when hormones or neurotransmitters are needed. The pituitary gland is composed of an anatomically distinct anterior and posterior portion. The posterior pituitary, or neurohypophysis, is located on the posterior part of the pituitary gland and is part of the endocrine system as well as the anterior pituitary. The neurohypophysis is not a gland but rather collects the hormones and secretes them when needed. Axons project to the posterior pituitary from the hypothalamus and the anterior pituitary gland. The neurohypophysis is divided into three regions: Pars nervosa, Infundibular stalk, and Median eminence. Pars nervosa is the biggest part of the posterior pituitary and is made of Herring bodies and pituicytes. Herring bodies are axon terminals coming from the hypothalamus and are used for the storage of oxytocin and vasopressin. The pituicytes are just basic glial cells. The infundibular stalk is used to connect the hypothalamic and hypophyseal systems.
Currently, only two hormones are known to be released by the posterior pituitary, oxytocin and vasopressin, and they differ only by a single amino acid substitution. The two hormones are produced in the the paraventricular nucleus (PVN), which is a neuronal nucleus in the hypothalamus, by Magnocellular neurosecretory cells, and transferred to the neurohypophysis for storage. Oxytocin is a mammalian hormone and a neurotransmitter that is associated with menstruation, nipple stimulation, and parturition. New researchers are trying to find the relation between oxytocin and orgasms, bonding, love, and maternal behavior. Oxytocin has actions in the brain as a neurotransmitter, and as a hormone throughout the body. As a hormone it travels in the blood stream and it reaches an oxytocin receptor. Oxytocin receptors that are located on the mammary glands, and uterus help during labor and milk secretion. During labor injections of oxytocin are given to induce labor. When a newborn starts sucking a nipple they are rewarded with some milk as a result of the oxytocin hormone. The cell bodies in the nipple, once stimulated by the sucking and biting of the baby, send signals about every five minutes to the brain by action potential down the axon and signal the release of oxytocin by exocytosis. The hormone then travels in the blood stream to the mammary gland and secretes milk in a collecting duct from which is sucked by the newborn (Rossoni et al 2008). The relationship between oxytocin and sexual acts is still unclear. Some researchers have found an increase in plasma oxytocin in men and female. The authors believe that it might be due to stimulation of the nipples, in women, during the act that leads to the secretion of the hormone (Charmicael et al 1994). More research on oxytocin levels during sex has been done on women than men due to the more complex endocrine system with clearly defined cycles and longer orgasms that allows for a better examination of the hormones. (Bancroft 2005). The other hormone that is released by the posterior pituitary and differs from oxytocin by only two amino acids is vasopressin or antidiuretic hormone. Vasopressin is a hormone that targets the kidneys, and the arterioles. Vasopressin controls homeostasis of the body by controlling the water, salts, and glucose in the blood. The most important function is the water regulation of the body. Once the body is dehydrated vasopressin is released and travels to the tubules of the kidneys and causing a higher concentration of the urine to conserve water. Vasopressin affects the kidneys in three different ways. First, it increases the permeability of water to the collecting duct in the kidney, which allows the water to be reabsorbed and thus further concentrate the urine. Second, it increases the permeability of the medullary portion of the collecting duct to urea, allowing increased reabsorbtion of urea. And finally, in the Loop of Henle vasopressin stimulates sodium and chloride reabsorbtion. The release of the hormone can be stimulated by less fluid intake, diarrhea, dehydration, cardiac shock, and increase in perspiration. It is interesting to know that alcohol will inhibit vasopressin and will not allow the water to be reabsorbed which further dehydrates the body since water is directly excreted as urine. Vasopressin also acts on the cardiovascular system. It will increase the vasoconstriction in the vascular smooth muscle and raise blood pressure. Vasopressin also acts in the central nervous system. More interesting is the finding that the hormone plays a role in social behavior. When vasopressin is released into the brain during sexual activity, it initiates and maintains a model of activity that supports the pair-bond between the sexual partners. Vasopressin seems to have an affect on males to become aggressive towards other males as well (Wallum et al 2008).
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Decreased vasopressin release or decreased renal sensitivity to the hormone can lead to diabetes insipidus. Diabetes insipidus is a condition when the urine excreted is in large amounts and extremely diluted, and the patient is thirsty all the time due to the lack of fluids in the body. The most common type of diabetes insipidus is caused by deficit in both, production and secretion of vasopressin. The second most common is caused by insensitivity to the hormone in the kidneys. The signs and the symptoms of the illness are almost the same as the more prevalent Diabitis mellitus. Unnecessary secretion of urine and extreme thirst are two of the very typical signs for Diabitis insipidus although sometimes even blurred vision can be diagnosed in some of the patients (Akarsu et al 2006). The Symptoms of diabetes insipidus are quite similar to those of untreated diabetes mellitus. The difference is that the urine is not sweet, as it does not contain glucose and there is no hyperglycemia. Dehydration is not very common but it may also appear in some individuals since the body cannot conserve much of the water that has been consumed. The only way that the two diseases can be distinguished from each other is to test the blood glucose levels, bicarbonate levels, and calcium levels. Measurement of blood electrolytes can reveal a high sodium level that is caused by the lack of water. A urinalysis demonstrates dilute urine with a low specific gravity. In a patient with no significant renal disorder, all glucose present in the plasma passes through the glomerulus and is then absorbed by the tubular transport system (Akarsu et al 2006). Once it has been diagnosed the treatment can be just a simple delivery of the hormone Vasopressin.
The endocrine system, composed of the glands and hormones, I think is one of the most important system in the body. Because the body cannot exist without the presence of either of the systems, the endocrine system is what makes us human. We have feelings, we care about others, we feel anger and we get that butterfly feeling in the stomach, all because of some little chemical secreted by a small gland.
Rossoni E, Feng J, Tirozzi B, Brown D, Leng G, et al. 2008. Emergent Synchronous Bursting of Oxytocin Neuronal Network. PLoS Comput Biol 4(7):
Carmichael MS, Warburton VL, Dixen J, Davidson JM. 1994. Relationships among cardiovascular, muscular, and oxytocin responses during human sexual activity. Pubmed.gov. Available from: http://www.ncbi.nlm.nih.gov/pubmed/8135652
Bancroft J. 2005. The endocrinology of sexual arousal. Pubmed.gov. Available from: http://www.ncbi.nlm.nih.gov/pubmed/16135662
Wallum H, Westberg L, Henningsson S, et al. 2008. Genetic variation in the vasopressin receptor 1a gene (AVPR1A) associates with pair-bonding behavior in humans. Pubmed.gov. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2533683/?tool=pmcentrez
Akarsu E, Buyukhatipoglu H, Aktaran S, et al. 2006. The Value of Urine Specific Gravity in Detecting Diabetes Insipidus in a Patient with Uncontrolled Diabetes Mellitus
Urine Specific Gravity in Differential Diagnosis. Pubmed.gov. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1831666/
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