Pituitary Gland: Posterior Lobe
Posterior lobe of the pituitary gland
- The hypothalamus regulates the pituitary's endocrine functions via hormonal and neural mechanisms.
- The pituitary gland, aka, hypophysis, divides structurally and functionally into the:
- Anterior lobe and posterior lobe (aka neurohypophysis), which directly connects to the hypothalamus.
- Direct connection allows the hypothalamus to communicate with the posterior lobe via neural connections – the posterior lobe is derived from neural tissue (hence its name "neurohypophysis").
- Infundibulum connects hypothalamus and posterior lobe.
- Posterior lobe does not synthesize hormones but rather stores and secretes neurohormones synthesized by the hypothalamus.
Pathway:
Hypothalamo-hypophyseal tracts:
- Paraventricular and supraoptic nuclei of the hypothalamus house the cell bodies of neurosecreting cells.
- Neurosecretory cell traveling from the hypothalamus to the posterior lobe.
- Cell body synthesizes and packages neurohormones in vesicles;
- Axon delivers the vesicles to its terminal in the posterior lobe, where it stored until its release is signaled.
- When signaled to do so, the vesicles release the neurohormone.
- The hormone then enters the venous blood so that it can travel within the systemic circulation to reach its target organs.
Two hormones secreted from the posterior lobe of the pituitary gland.
Anti-diuretic hormone, ADH, is released in response to low blood pressure and/or water volume contraction.
- ADH induces vasoconstriction, which counteracts low blood pressure; this explains its alternative name, vasopressin
- ADH also acts on the distal nephron tubules of the kidneys to increase water reabsorption, which counteracts water volume contraction.
- Central diabetes insipidus is caused by defects in the hypothalamic nuclei or in the mechanisms of axon transport. As a result, ADH is not secreted by the posterior pituitary, and individuals produce large quantities of dilute urine.
- Smooth muscle contraction in lactating mammary glands and uterus.
- In the breast, oxytocin promotes myoepithelial cell contraction and milk ejection.
- Suckling promotes oxytocin release to facilitate breastfeeding.
- In the uterus, oxytocin induces rhythmic myometrium contractions during parturition (to expel the fetus) and orgasm.
- Stretch receptors in the vagina trigger its release.