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Posterior Pituitary Lobe Anatomy & Physiology

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Posterior Pituitary Lobe Anatomy & Physiology

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Overview

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.

Hormone 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.

Posterior Pituitary Hormones

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.

Oxytocin stimulates 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.

Many authors report a possible, but uncertain, role for oxytocin in the male sexual response, as well.

Oxytocin is also thought to enhance emotional and behavioral responses, particularly in romantic and mother-child interactions; however, these relationships can be difficult to ascertain, and are omitted, here.

Full-Length Text

  • Here we will learn about hypothalamic regulation of the posterior lobe of the pituitary gland.
  • To begin, start a table, and denote that:
  • 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, which is discussed in detail, elsewhere, and,
    • The posterior lobe (aka neurohypophysis), which directly connects to the hypothalamus.
  • This 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").
  • Contrast this to the anterior lobe, which communicates with the hypothalamus via portal blood vessels.
  • Lastly, denote that the posterior lobe does not synthesize hormones but rather stores and secretes neurohormones synthesized by the hypothalamus.
  • Next, let's draw the anatomy of the hypothalamus and posterior lobe of the pituitary gland.
  • To begin, draw the anterior hypothalamus and pituitary gland; differentiate the anterior and posterior lobes.
  • We shade the posterior lobe and hypothalamus in similar colors because of their similar neural origins.
  • Indicate that the infundibulum, also called the pituitary stalk, connects the hypothalamus and posterior lobe of the pituitary gland.

Next, we'll show the hypothalamo-hypophyseal tracts:

  • First, indicate the paraventricular and supraoptic nuclei of the hypothalamus, which house the cell bodies of neurosecreting cells.
  • Draw a representative enlarged neurosecretory cell traveling from the hypothalamus to the posterior lobe.
  • Indicate the cell body, axon and the terminal of the hypothalamohypophyseal tract.
  • Then, show venous capillary drainage from the posterior lobe.

With these structures in place,

    1. Indicate that the cell body synthesizes and packages neurohormones in vesicles;
    1. Show that the axon delivers the vesicles to its terminal in the posterior lobe, where it stored until its release is signaled.
    1. When signaled to do so, the vesicles release the neurohormone.
    1. The hormone which then enters the venous blood so that it can travel within the systemic circulation to reach its target organs.
  • Now that we know the general pathway of neurohormone synthesis and release, let's learn about the two hormones that are secreted from the posterior lobe of the pituitary gland.
  • First, write that anti-diuretic hormone, ADH, is released in response to low blood pressure and/or water volume contraction.
  • Indicate that 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.
  • As a clinical correlation, write that 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.
  • The other hormone secreted by the posterior lobe is oxytocin;
    • It induces smooth muscle contraction in lactating mammary glands and uterus.
    • In the breast, oxytocin promotes myoepithelial cell contraction and milk ejection;
    • Write that 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.
    • Many authors report a possible, but uncertain, role for oxytocin in the male sexual response, as well.
    • Oxytocin is also thought to enhance emotional and behavioral responses, particularly in romantic and mother-child interactions; however, these relationships can be difficult to ascertain, and are omitted, here.