Anatomy & Physiology

Anatomy & Physiology Study Guide , or thyrotropin, targets the thyroid gland and triggers the release of thyroid hormones. Adrenocorticotropic hormone (ACTH) , also known as corticotropin, stimulates the release of steroid hormones by the suprarenal cortex, the outer portion of the suprarenal gland. ACTH specifically targets cells that produce glucocorticoids, hormones that affect glucose metabolism. ACTH release occurs under the stimulation of corticotropin-releasing hormone (CRH) from the hypothalamus. Gonadotropins regulate the activities of the gonads. These organs—the testes and ovaries in males and females, respectively—produce reproductive cells as well as hormones. The production of gonadotropins occurs under stimulation by gonadotropin-releasing hormone (GnRH) from the hypothalamus. The two gonadotropins are follicle-stimulating hormone (which promotes follicle development in females and, in combination with a luteinizing hormone, stimulates the secretion of estrogens) and luteinizing hormone (which induces ovulation, the production of reproductive cells in females). Prolactin (PRL) , or mammotropin, functions with other hormones to stimulate mammary gland development. In pregnancy and during the nursing period following delivery, PRL also stimulates milk production by the mammary glands. Growth hormone (GH) , or somatotropin , stimulates cell growth and replication by accelerating the rate of protein synthesis. Although virtually every tissue responds to some degree, skeletal muscle cells and chondrocytes (cartilage cells) are particularly sensitive to GH. In epithelial and connective tissues, GH stimulates stem cell divisions and the differentiation of daughter cells. In adipose tissue, GH stimulates the breakdown of stored triglycerides by adipocytes (fat cells), which then release fatty acids into the blood. As circulating fatty acid levels rise, many tissues stop breaking down glucose and start breaking down fatty acids to generate ATP. In the liver, GH stimulates the breakdown of glycogen reserves by liver cells, which then release glucose into the bloodstream. The production of GH is regulated by growth hormone- releasing hormone and growth hormone–inhibiting hormone (GH–IH) from the hypothalamus. Melanocyte-stimulating hormone (MSH) stimulates the melanocytes of the skin, increasing their production of melanin, a brown, black, or yellow-brown pigment. Dopamine prevents the release of MSH. MSH is released by the human pars intermedia during fetal development, in very young children, in pregnant women, and in the course of some diseases. T e N urohypophysis The neurohypophysis is also called the posterior lobe of the pituitary gland, or pars nervosa (nervous part) because it contains the axons of hypothalamic neurons. These neurons manufacture antidiuretic hormone (ADH) and oxytocin. Antidiuretic hormone (ADH) , also known as vasopressin, is released in response to a variety of stimuli, most notably a rise in the solute concentration in the blood or a fall in blood volume or blood pressure. The primary function of ADH is to decrease the amount of water lost at the kidneys. With losses minimized, any water absorbed from the digestive tract will be retained, reducing the concentrations of electrolytes in the extracellular fluid. Oxytocin (OXT) causes smooth muscle contraction in the wall of the uterus, promoting labor and delivery. After delivery, oxytocin stimulates the contraction of myoepithelial cells around the secretory alveoli and the ducts of the mammary glands, promoting the ejection of milk. Oxytocin secretion and milk ejection are part of a neuroendocrine reflex. The standard stimulus is an infant Achieve Page 210 of 368 Thyroid-stimulating hormone (TSH) ©2018