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The Hypothlamo-pituitary Unit I

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BIOL 1840U
Peter Cheung

The Hypothlamo-pituitary Unit I. Introduction a. Pituitary (“mucus”) = hypophysis (“small/under growth”) b. Brain acts as an endocrine gland thru hypothalamus via the pituitary gland c. Pituitary has a portal system—(vessels w/ capillary network on either side); blood flows from neurohypophysis to adenohypophysis via the vessels & then out to body II. Neurohypophysis—(posterior pituitary) releases ADH & oxytosin; hormones synthesized in hypothalamus & stored/released from pituitary a. ADH (arginine-vasopressin) i. Neurophysin II—transport protein carrying ADH from hypothalamus to neurohypophysis ii. circulates as free peptide; ½ life = 2 min iii. Fxn—increases H2O absorption from renal tubuli iv. Stimuli 1. changes in blood volume—volume receptors in carotid sinus, L atrium, & aortic arch; exponential relationship (quadratic) between ADH release & blood volume loss 2. changes in serum osmolality—↑ osmolality => ↑ ADH secretion; linear relationship+ 3. fear & pain b. Oxytocin i. Neurophysin I—transport protein carrying oxytocin from hypothalamus to neurohypophysis ii. circulates as free peptide; ½ life = 2 min iii. Fxn—stimulates contraction of uterine mm. & letdown of milk iv. Stimuli—parturition, suckling, coitus III. Adenohypophysis—(anterior pituitary) a. Cytology of adenohypophysis i. Somatotropes (GH-secreting) ~50% ii. Lactotropes (PRL-secreting) ~15-25% iii. Corticotropes (ACTH secreting) 15-20% iv. Gonadotropes (LH & FSH secreting) 10-15% v. Thyrotropes (TSH secreting) 3-5% b. PIT-1 Gene i. transcription factor that regulates (characterizes) pituitary stem cells ii. gives rise to GH, PRL & TSH hormones => determines differentiation of somatotropes, thyrotropes, & lactotropes IV. Hypothalamic Hypophysiotropic Hormones a. Growth Hormone releasing hormone (GRH) i. Fxn—Induces production of GH from pituitary; sleep induction ii. Two forms, both are equally active; ½ life = 5 min iii. Binds to a GPCR iv. Part of H-P-growth hormone axis b. Somatostatin 1 i. Fxn—blocks production of GH & TSH from ant. pituitary ii. Produced by “D cells” of stomach iii. Binds to a GPCR iv. Part of H-P-growth hormone axis c. Corticotrophin Releasing Hormone (CRH) i. Fxn—responsible for cyclic hormone levels (waxing & waning) of circadian rhythm ii. Key for H-P-adrenal axis d. Gonadotropin Releasing Hormone (GnRH) i. Fxn—stimulates 1) synthesis & storage of gonadotropins (LH & FSH); 2) movement from reserve pool to “ready for secretion” pool; 3) stimulates release of FSH & LH ii. Produced by neurons in arcurate nucleus iii. Secreted in pulsatile fashion; ½ life = 3 min.
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