Craig C. Freudenrich
Hormones are chemical signals that are produced by tissues called endocrine glands, such as the thyroid, pituitary and adrenal glands. These hormones affect many body tissues, including those of the nervous system. But did you know that your brain makes hormones as well? An area of the brain called the hypothalamus contains nerve cells that secrete hormones; these specialized nerve cells are called neurosecretory cells. The neurosecretory cell bodies act in two distinct ways.
First, some cells send axons to secrete hormones called releasing hormones into the blood supply to the anterior pituitary gland. There, these releasing hormones cause the anterior pituitary cells to secrete other hormones into the bloodstream. For example, the hypothalamus secretes gonadotropin-releasing hormone (GnRH), which acts on anterior pituitary cells to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH and LH control egg development and estrogen secretions that promote women's ovulation. In men, the hormones control sperm development and testosterone secretions. The sex hormones (estrogen, testosterone) act on the hypothalamus and pituitary to regulate GnRH, FSH and LH secretions. Furthermore, the sex hormones influence areas of the brain that control sex drive. Neurosecretory cells that control secretions of thyroid hormone, growth hormone, cortisol and prolactin act in a similar manner.
Second, other hypothalamic neurosecretory cells send axons into the posterior pituitary gland, where the axons release hormones directly into the blood. For example, hypothalamic neurosecretory cells secrete antidiuretic hormone, which increases water retention by the kidneys, decreases sweat production, stimulates the drinking centers of the brain and increases blood pressure. Together, these effects help your body conserve water and increase water intake when you are dehydrated. When your water levels return to normal, the neurosecretory cells stop secreting antidiuretic hormone. Another hormone that the pituitary gland secretes in this manner is oxytocin, which stimulates uterine contractions during labor, childbirth and release of breast milk.
Finally, neurons from the sympathetic nerve centers in the medulla directly stimulate cells of the adrenal medulla to secrete epinephrine into the bloodstream. This hormone increases the heart rate and blood pressure, mobilizes energy reserves, dilates the airways and causes sweating in the fight-or-flight syndrome. This is the body's helpful response to fear or danger.
Since hormones act as chemical signals in the body, it's likely they interact with the nervous system, too. The nervous system and endocrine system often work together as the body matures and maintains itself [source: Farabee]. In addition, the nervous system helps humans and other organisms react to the surrounding world -- a task that often involves the release of hormones. During a "fight or flight" response, for example, the nervous system prompts the release of an adrenaline hormone and neurotransmitter called epinephrine. Though the endocrine system seems responsible for the reaction, the nervous system plays an equal role in triggering a response based on sensory information from the organism's environment.
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