Posterior Pituitary

Question 1

what are the 2 functions/roles of the posterior pituitary gland?

Answer 1

1. secretes several essential hormones on its own such as oxytocin, ADH
2. critical component of the HPA, which regulates hormone secretion from several endocrine glands

Question 2

give an endocrine based disorder caused by posterior pituitary gland dysfunction and describe what the hormone issue is for each

Answer 2

diabetes insipidus/water diabetes: decreased ADH

Question 3

describe the pituitary gland

Answer 3

1. a small gland located in the base of the brain, surrounded/protected by the stella turcica of the sphenoid bone
2. two major components: anterior lobe/adenohypophysis and posterior lobe/neurohypophysis

Question 4

describe pituitary gland development

Answer 4

derived from 2 sources during early embryonic development:
posterior lobe arises from the neuroepithelium of the developing brain (diencephalon) and the anterior lobe arises from an outpocketing of the oral cavity called Rathke’s pouch

two embryonic sources=two different functions and cellular compositions

Question 5

what is pituitary gland secretion dependent on? describe (2)

Answer 5

secretion is dependent on signaling from the hypothalamus; facilitated by:
1. hypothalamic neurons that project into the POSTERIOR pituitary and
2. the hypothalamic portal (vascular) system to the ANTERIOR pituitary

neurons synthesize trophic hormones and release them into the capillaries of the portal system; portal vessels carry the hormones directly to anterior pituitary

Question 6

describe the neurohypophysis (posterior pituitary) by explaining what is meant by PN2

Answer 6

PN2 = Posterior, Neural, 2 hormones

the 2 hormones are oxytocin, which acts on uterine muscles and mammary glands, and antidiuretic hormone/vasopressin, which acts on kidney tubules

Question 7

where are oxytocin and ADH synthesized? where are they stored? when are they released?

Answer 7

synthesized: in cell bodies of neurosecretory neurons within the hypothalamus

stored: in the axon terminals of the posterior pituitary

released: to posterior pituitary in response to an action potential from the brain that travels along the axon; hormones then enter circulation

Question 8

what kind of hormone is oxytocin? give its main function and stimulus for release and what type of feedback governs its release

Answer 8

oxytocin is a 9 AA long peptide hormone

function: stimulate myoepithelial cell contractions in the uterus during parturition and in the mammary gland during lactation to promote milk ejection (NOT milk production)

stimulus for release during lactation: suckling; sensory receptors in the nipple connect with nerve fibers to the spine, then relay impulses to the hypothalamus, then activate oxytocin neurons to stimulate hormone release from the posterior pituitary, forming an open feedback loop in which positive input is interrupted when suckling stops

stimulus for release during parturition: oxytocin stimulates the uterus to contract and causes the cervix to stretch; increased cervical stretch is sensed by neurons in the cervix and transmitted to the hypothalamus, which sends signals for more oxytocin secretion

release is controlled by positive feedback mechanisms to amplify all the way through the response

Question 9

what kind of hormone is ADH? give its function, stimulus for release, and feedback mechanism that governs its release

Answer 9

ADH is a 9 AA long peptide hormone

functions to control water balance and promotes vasoconstriction (in higher concentrations)

released: when the body is dehydrated, promotes increased permeability to water in the distal tubules and collecting ducts of kidneys, causing the kidneys to conserve water, concentrate urine, and decrease volume of urine output; also promotes vascular smooth muscle contraction to increase blood pressure (in higher concentrations)

stimulus for release:
primary: high plasma osmolarity, detected by osmoreceptors in the hypothalamus
secondary: low blood volume and pressure: detected by receptors in the heart and large arteries

acts on 3 subclasses of Arginine VasoPressin Receptors (AVPR) that are all G-coupled receptors that activate different secondary messengers (V2 is receptor in kidneys that functions to promote water reabsorption by increases cAMP to promote insertion of aquaporin-2 channels)

Question 10

INTEGRATION Q: what is diabetes insipidus and what are the 4 types? give clinical features and one distinguishing difference between diabetes mellitus

Answer 10

diabetes/water insipidus is also called water diabetes;
1. pituitary diabetes/central/neurogenic DI: caused by an ADH deficiency due to pituitary tumor, head injury, or infection; treated by administering ADH/vasopressin

2. gestational DI: reduced ADH during pregnancy, is transient and will resolve once baby gone
3. nephrogenic DI: kidneys do not respond to ADH due to a genetic defect, certain drugs, kidney disease, or hypercalcemia; treated by controlling fluid intake and using drugs to lower urine output
4. primary polydipsia: excess fluid intake reduces ADH

major clinical features: are excessive thirst and excretion of large volumes of dilute urine (not increased by fluid intake); risks are dehydration and electrolyte imbalance

the difference between DI and diabetes mellitus is the absence of hyperglycemia and glucosuria that is observed in diabetes mellitus