The endocrine secretions are controlled by the nervous system through a complex chain of command. Receptors around the body monitor sensory signals and alert the brain, which then relays the information to specific cells in the median eminence of the hypothalamus. For example, temperature receptors in the skin detect cold and inform the brain of potential body cooling. The brain then relays the information to cells in the hypothalamus, which secrete a molecule called thyrotropin-releasing hormone into a blood vessel called the hypothalamo-hypophysial portal vessel. This blood vessel delivers the releasing hormone to the anterior pituitary gland, which in turn secretes a hormone called thyroid-stimulating hormone (TSH), or thyrotropin, into the blood. The TSH travels to the thyroid gland to stimulate the secretion of thyroid hormones, which stimulate metabolism in liver, muscle, and other cells. Heat produced as a by-product of metabolism warms the body. Some hormones are under dual control. Growth hormone (somatotropin) is stimulated by a releasing hormone called somatocrinin and inhibited by somatostatin. There are about seven anterior pituitary hormones that are controlled by similar mechanisms. Adrenocorticotropic hormone (ACTH) is controlled by corticotropin-releasing hormone. Melanocyte-stimulating hormone (MSH) and prolactin are under dual control by both releasing hormones and inhibiting hormones. The gonadotropins—follicle-stimulating hormone (FSH) and luteinizing hormone (LH)— are under the control of a single releasing hormone called gonadotropin releasing hormone. All of these control systems are subject to feedback loops which usually involve negative feedback (for example, TSH secretion being inhibited by thyroid hormone), but positive feedback loops exist (estrogen feeding back positively to stimulate LH secretion).
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