Senses and the Endocrine System
Location and normal physiology (function) of the pituitary gland
The pituitary gland (also known as hypophysis cerebri) is located within the hypophyseal fosa, which is a fibro-osseous compartment situated near the middle of the cranial base (Khurana, 2005). It is connected to the hypothalamus by the pituitary stalk, or the infundibulum, and is placed within the sella turcica found at the base of the skull and surrounded by sphenoid bones on both side.
Physiology of the pituitary gland
The normal physiology of the pituitary gland involves its anterior and posterior lobes, which are interlinked by the pars intermedius. The anterior lobe secrets 6 hormones (thyroid stimulating hormone, corticotropin, follicle stimulating hormone, growth hormone, prolactin) essential to metabolic functions throughout the body (Amar & Weiss, 2003).
Pituitary gland Hormones and their feedback mechanism
Thyoid stimulating hormone (TSH)
The TSH is secreted under the influence of the hypothalamic Throtropin-releasing hormone (TRH). Its release is pulsatile, with peak output around midnight. TSH acts on a cell-surface receptor which activates adenylyl cyclase via a GTP-C. the binding of TSH to the receptor on thyroid cells leads to increased synthesis of triiodothyronine (T3) and thyroxine (T4) along with increased release of stored thyroglobulin (Khurana, 2005). T3 and T4 act at pituitary and hypothalamic levels in blocking the secretion of TSH through feedback inhibition. Day-to-day maintenance of thyroid secretion is depended on the interplay between thyroid hormones and TRH.
Luteinizing hormone (LH) and follicle stimulating hormone (FSH)
LH and FSH stimulate the gonads of both males and females and are important in production of germ cells as well the release of estrogens and androgens by those tissues. The hormones are also significant in the ovulatory cycles of women. Their effects are mediated by adenylyl cyclase activity coupled to their receptors via a GTP-binding protein (Swearingen & Biller, 2008).
Pituitary gland Hormones and their feedback mechanism
The secretion of LH and FSH is a result of complex positive and negative feedback mechanisms that interact with GnRH.
Growth hormone (GH)
GH is the most abundant of all hormones secreted by the pituitary gland, with a molecular weight of 22,000. It is secreted under the influence of hypothalamic Growth Hormone-Releasing Hormone (GHRH). Almost half of GH in the plasma is bound to protein consisting a cleavage product of GH receptor, which provides a reservoir to compensate for wide fluctuations in secretion rate and short half-life of GH (6-20 minutes). Besides direct activation of several genes, GH is responsible for release of somatomedins – a group of polypeptide growth factors released by the liver, cartilage, among other tissues (Khurana, 2005). GH helps in growth of long bones and chondrogenesis prior to the age of epiphyseal fusion in young children. It also releases a positive nitrogen balance resulting in increase in lead body mass as well as decrease in body fat. GH also increases hepatic glucose output in addition to exerting anti-insulin effect in the muscle. The secretion is under feedback control as it is inhibited by the hypothalamic Somatostatin.
Proclactin (PRL)
The secretion of PRL is inconsistently mediated by hypothalamic TRH, Vasopressin, oxytocin, vasoactive intestinal peptide, neuropeptide Y, angiotensin II, substance P, galanin, bombesin-like peptides, and neurotensin (Amar & Weiss, 2003). PRL stimulates milk release from female breast. It also inhibits actions of gonadotropins on the ovary, preventing ovulation in lactating women or those having PRL-secreting tumors. Excessive levels of PRL in men may cause impotence. PRL enables secretion of dopamine from median eminence, thus acting in a negative feedback mechanism to inhibit its own secretion.
The posterior lobe secrets 2 hormones (oxytocin and vasopressin/antidiuretic) into the general circulation in reaction to electrical activity happening at the axon endings that originate from cell bodies in the hypothalamus. Oxytocin triggers contraction of myoepithelial cells lining the breast duct, causing milk flow to the nipple for excretion. It also causes contraction of smooth muscle of uterus, enhances labor, and facilitates passage of sperm in fallopian tube, and increase time of ejaculation by contracting the smooth muscle of vas deferens (Swearingen & Biller, 2008).
Pituitary gland Hormones and their feedback mechanism
. Vasopressin causes translocation of water channels in kidney ducts to increase their permeability, reducing urine volume in increasing its concentration.
Anatomy/physiology of Pituitary Apolexy
Pituitary apoplexy refers to a condition in which hemorrhage occurs to the pituitary gland or becomes infarcted. This condition is primarily caused by a transient or sustained alteration in the blow flow to the pituitary gland, resulting in either hyperperfusion or hypoperfusion (Amar & Weiss, 2003). Common initial symptoms include sudden headache, rapidly increasing double vision due to compression of nerves around the pituitary gland. This is often succeeded by acute symptoms due to adrenal insufficiency.
References:
Amar, P. A, & Weiss, H. M. (2003). Pituitary anatomy and physiology. Neurosurg Clin N Am 13 (2003) 11–23.
Swearingen, B., & Biller, B. M. K. (2008). Diagnosis and management of pituitary disorders. Totowa, N.J: Humana Press.
Khurana. (2005). Textbook of Medical Physiology. New Delhi: Elsevier India.
