Endocrine system posterior pitutitary gland

Posterior pituitary Hormones

• oxytocin

• antidiuretic hormone

Oxytocin

• used to induce labor

• produced by neurons in the hypothalamus
  - the hormone travels down to the axon terminal in vesicles that are stored within the axon terminals in the posterior lobe
  - only when neurons are stimulated and action potential occurs in the axon that oxytocin is secreted into the inferior hypophyseal tract and released into general circulation

• amino acid based hormone

• stimulates uterine contraction
  - when its under uterine contractions its under positive feedback control
  - this means that oxytocin stimulates uterine contractions, uterine contractions stimulate secretion of more oxytocin, which stimulates more uterine contractions which stimulates secretion of more oxytocin
  - since its a positive feedback loop, the pathway leads to more oxytocin secretion and stronger and more vigorous uterine contractions

• when braxton hicks contractions happen, the smooth muscle in the walls of the uterus are proliferating and more smooth muscle cells in order to be able to have enough force to expel the child during parturition (birthing)
  - oxytocin’s small role in this: reflex

• the regular pattern of woman’s contractions is what induces the positive feedback loop

• The requirements for the oxytocin to be under feedback loop is:
  - contractions 5 minutes apart and regular and they have to be strong

• until the oxytocin gets into the positive feedback loop, it is not strong enough to perpetuate itself

What stops the positive feedback loop of oxytocin

• uterus is contracting and getting smaller until its gets to its most contracted point and the uterus can not contract anymore and produce more force, the oxytocin secretion goes away and the uterus will cease to contract

What does oxytocin trigger?

• Let down reflex
  - colostrum is released from the mammary glands so the child will be able to nurse and get the colostrum and nutrients

Oxytocin during s*x

• plays a role in sexual arousal as well as sexual satisfaction in both males and females

• associated to some extent with orgasm in females

Antidiuretic Hormone (ADH)

• diuretic drug: refers to drug that promotes the production of urine and decreases body water
  - can decrease blood pressure and used to treat hypertension in adults

• Antidiuretic: conserves water and helps to avoid dehydration
  - prevents urine formation
  - high ADH: produce very little urine
  - promotes thirst development because it means you need fluid

• Pathway:
  - neurons that originate in the hypothalamus have axons that project down into the posterior pituitary via the hypothalamic hypophyseal tract
  - its the neurons at the cell body of the dendrites that determine weather or not there is enough stimulus for action potential to cause the secretion of hormones from the axon terminals

• Osmoreceptors
  - in the dendrites and on the cell body
  - receptors that are sensitive to salt concentrations in the blood
  - if salt concentrations are high, it is called osmolarity
  - high osmolarity stimulates ADH secretion and stimulates the reabsorption of water in the kidneys
  - this keeps water inside of the body and prevents you from losing it to urine formation, urine is concentrated with little water
  - if your blood is dilute and salt concentrations are low and you have low osmolarity, the osmoreceptors are not activated and ADH is not secreted
  - this will cause you to increase urine formation and you will lose water to the production of urine

• alcohol inhibits ADH release and causes copious urine output

Absorption

• nephrons are in kidneys and its where we regulate body water

• water gets to the kidneys and to the nephrons via capillaries via the circulatory system that brings blood flow to a specific portion of the nephron known as the glomerulus

• the WATER in the blood will pass into the nephron and water is going to travel throughout this nephron and it will exit the nephrons and enter the collecting duct

• collecting ducts from different neurons merge and eventually the water in these collecting ducts ends up forming the urine that drains into your bladder

Reabsorption

• not all water in the kidneys get lost, some gets reabsorbed

• the water in the nephron does not remain in the nephron

• the water gets shuttled across the walls of the nephron back into the interstitial fluid of the kidneys where it can reenter the cardiovascular system and the water gets kept in our body

• Proximal Convoluted Tubule (PCT)
  - automatic process to help with reabsorption

• Loop of Henle absorbs more water back into the interstitial places

• water that is not recovered by the time it gets to the loop of henley, it passes thorough the Distal Convoluted Tubule (DCT), goes into the collecting ducts

• ADH enhances our ability to recover water from the nephron in the kidneys and this is how:
  - within the collecting ducts of the nephrons, ADH stimulates the production of something called aquaporins, they open and we cover more water when ADH is secreted

• ADH only promotes Water absorption absolutely nothing else

• result of reabsorption from ADH is that we maintain out body water and produce very small amounts of very concentrated urine

ADH deficiency

• Diabetes Insipidus: some people experience a lesion in the cells within the hypothalamus that produce ADH and this is when these cells become deficient
  - tasteless urine
  - people who suffer from diabetes insipidus produce large amounts of urine
  - they are also always thirsty and they tend to be hypovolemic
  - so they are filtering out all this water and they’re unable to stimulate aquaporin formation to cause water reabsorption
  - as a consequence from this they produce large amounts of urine

• Symptoms:
  - Dilate Urine (low osmolality)
  - Increased urinary frequency
  - Nocturia (frequent awakening at night to urinate)
  - Enuresis (involuntary urination during sleep or “bedwetting”)

• Cause:
  - Death of ADH production cells in hypothalamus posterior pituitary gland
  - cause of this could be viral, could be due to trauma, not really well known

ADH overproduction

• Syndrome of inappropriate ADH secretion (SIADH): overproduction of ADH
  - you have low blood salt concentrations because your reabsorbing all the water but not reabsorbing sodium
  - small amounts of urine and the urine is hyper concentrated

• Symptoms:
  - hyponatremia
  caused by: ADH acts upon the nephrons, we instal those aquaphorins and you recover most of the water that passes through the nephrons
  - inappropriately elevated urine osmolality (>200 mOsm/kg)
  - excessive urine sodium excretion (UNa >30 mEq/L)
  - decreased serum osmolality