Anatomy & Physiology

Anatomy & Physiology Study Guide over the endocrine cells of the suprarenal medullae. When the sympathetic division is activated, the suprarenal medullae release hormones into the bloodstream. 17.2 Pituitary Gland The pituitary gland, or hypophysis , lies nestled within the sella turcica, a depression in the sphenoid. The pituitary gland hangs inferior to the hypothalamus, connected by the slender, funnel-shaped structure called the infundibulum . The base of the infundibulum lies between the optic chiasm and the mamillary bodies. The pituitary gland is cradled by the sella turcica and held in position by the diaphragma sellae , a dural sheet that encircles the infundibulum. The diaphragma sellae locks the pituitary gland in position and isolates it from the cranial cavity. The pituitary gland is divided into anterior and posterior lobes on the basis of function and developmental anatomy. Nine important peptide hormones are released by the pituitary gland—seven by the adenohypophysis (anterior lobe) and two by the neurohypophysis (posterior lobe). The Adenohypophysis The adenohypophysis, or anterior lobe of the pituitary gland, contains a variety of endocrine cells. The adenohypophysis has three regions: the pars distalis , the largest and most anterior portion of the pituitary gland; the pars tuberalis , which wraps around the adjacent portion of the infundibulum; and the slender pars intermedia , which forms a narrow band bordering the neurohypophysis. An extensive capillary network radiates through these regions, giving every endocrine cell immediate access to the circulatory system. Blood vessels that link two capillary networks are called portal vessels; in this case, they have the histological structure of veins. The entire complex is a portal system. Portal systems are named after their destinations. The hypophyseal portal system is the system of blood vessels that link the hypothalamus and the anterior pituitary in the brain. It allows endocrine communication between the two structures. Portal systems provide an efficient means of chemical communication by ensuring that all the hypothalamic hormones entering the portal vessels will reach the target cells in the adenohypophysis before being diluted through mixing with the general circulation. Hypothalamic Control of the Adenohypophysis Two classes of hypothalamic regulatory hormones exist: releasing hormones and inhibiting hormones. A releasing hormone (RH) stimulates the synthesis and secretion of one or more hormones at the adenohypophysis, whereas an inhibiting hormone (IH) prevents the synthesis and secretion of hormones from the adenohypophysis. The regulatory hormones released from the hypothalamus are transported directly to the adenohypophysis by the hypophyseal portal system. Hormones of th Adenohypophysis There are seven hormones of the adenohypophysis: thyroid-stimulating hormone; adrenocorticotropic hormone; two gonadotropins called follicle-stimulating hormone and luteinizing hormone; prolactin; growth hormone; and melanocyte-stimulating hormone. The hormones of the adenohypophysis are also called tropic hormones . Achieve Page 209 of 368 ©2018