Lec 3 Dr Shaimaa Munther The posterior pituitary The posterior pituitary in mammals consists predominantly of nerves that have their cell bodies in the hypothalamus and stores oxytocin and vasopressin in the termini of these neurons to be released into the bl ID: 1006563
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1. The Posterior Pituitary Gland Lec .3 Dr . Shaimaa Munther
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4. The posterior pituitaryThe posterior pituitary in mammals consists predominantly of nerves that have their cell bodies in the hypothalamus, and stores oxytocin and vasopressin in the termini of these neurons, to be released into the bloodstream. These neurons are referred to as neurosecretory cells because they generate action potentials as well as synthesize hormones. They produce primarily antidiuretic hormone (ADH) and oxytocin. These hormones are then transported down the axons to the neurohypophysis and stored in membrane-bound vesicles in the neuron terminals. Much like neurotransmitters, the hormones are released in response to the arrival of action potentials at the neuron terminal.
5. Hormones of the Neurohypophysis(Antidiuretic hormone (ADH))Antidiuretic hormone (ADH) also referred to as vasopressin or arginine vasopressin.It has two major effects, both of which are reflected by its names: Antidiuresis (decrease in urine formation by the kidney); and vasoconstriction of arterioles.
6. Antidiuretic HormoneAntidiuretic hormone promotes the reabsorption of water from the tubules of the kidney .Specifically, it acts on the collecting ducts and increases the number of water channels, which increases the diffusion coefficient for water. This results in the body’s conservation of water and the production of a low volume of concentrated urine. The reabsorbed water affects plasma osmolarity and blood volume. This effect of ADH on the kidney occurs at relatively low concentrations of ADH. At higher concentrations, ADH causes constriction of arterioles , which serves to increase blood pressure.
7. Regulation of OsmolalityOsmolality of ECF is controlled by :Hypothalamus by osmoreceptor via :Thirst MechanismRegulation of vasopressin ( antidiuretic hormone ADH) The Kidney via the control of Na concentration & regulation of blood volume (Renin-angiotensin-aldosterone system)
8. To maintain a normal plasma osmolality ( 275– 295 mOsm/kg of plasma H2O), osmoreceptors in the hypothalamus respond quickly to small changes in osmolality. A 1%–2% increase in osmolality causes a fourfold increase in the circulating concentration of ADH, and a 1%–2% decrease in osmolality shuts off ADH production. ADH acts by increasing the reabsorption of water in the collecting tubules. glucose (mg/dl) BUN (mg/ dl)Osmolality = 2Na + + 18 2.8
9. Osmoreceptors in the hypothalamus are located in close proximity to the ADH-producing neurosecretory cells. Stimulation of these osmoreceptors by an increase in plasma osmolarity results in stimulation of the neurosecretory cells; an increase in the frequency of action potentials in these cells; and the release of ADH from their axon terminals in the neurohypophysis .Osmoreceptors
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11. Regulation Antidiuretic hormone secretion is regulated by several factors:Plasma osmolality (The primary factor that influences ADH secretion )Blood volumeBlood pressureAlcohol ( inhibit ADH release from the pituitary gland)
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13. Effects of VasopressinBecause one of its principal physiologic effects is the retention of water by the kidney, vasopressin is often called the antidiuretic hormone (ADH). It increases the permeability of the collecting ducts of the kidney by increasing the insertions of aquaporin. The urine becomes concentrated and its volume decreases. The overall effect is therefore retention of water in excess of solute, In the absence of vasopressin, the urine is hypotonic to plasma, urine volume is increased, and there is a net water loss. Consequently, the osmolality of the body fluid rises.High level of ADH result in blood pressure elevation
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15. OxytocinIn humans, oxytocin acts primarily on the breasts and uterus, though it appears to be involved in luteolysis as well .A G protein-coupled serpentine oxytocin receptor has been identified in human myometrium, and a similar or identical receptor is found in mammary tissue and the ovary. It triggers increases in intracellular Ca2+ levels.
16. Physiological Action of Oxytocin The Milk Ejection Reflex Oxytocin causes contraction of the myoepithelial cells, ( smooth muscle like cells) that line the ducts of the breast.This squeezes the milk out of the alveoli of the lactating breast into the large ducts (sinuses) and then out of the nipple (milk ejection).Many hormones acting in concert are responsible for breast growth and for the secretion of milk into the ducts , but milk ejection in most species requires oxytocin .
17. Other Actions of OxytocinOxytocin causes contraction of the smooth muscle of the uterus. The sensitivity of the uterine musculature to oxytocin is enhanced by estrogen and inhibited by progesterone. The inhibitory effect of progesterone is due to a direct action of the steroid on uterine oxytocin receptors. In late pregnancy, the uterus becomes very sensitive to oxytocin with a marked increase in the number of oxytocin receptors. Oxytocin secretion is increased during labor. After dilation of the cervix, descent of the fetus down the birth canal causing secretion of sufficient oxytocin to enhance labor .
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19. Reproductive oxytocin action In female : Oxytocin may also act on the non pregnant uterus to facilitate sperm transport.Oxytocin initiates a specialized uterine contractions that transport the sperm, facilitating the passage of sperm up the female genital tract to the uterine tubes, where fertilization normally takes place. In male : Circulating oxytocin increases at the time of ejaculation in males, and it is possible that this increase causes increased contraction of the smooth muscle of the vas deferens, propelling sperm toward the urethra.
20. Regulation of oxytocin secretion The secretion of oxytocin is increased by stressful stimuli and, like that of vasopressin, is inhibited by alcohol.
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