Hypothalamic Pituitary Axis Notes

Hypothalamic Pituitary Axis

Hypothalamus

  • Sends Liberins and Statins to the Anterior Pituitary gland via Hypophysial portal veins.
    • Liberins: Stimulatory hormones.
    • Statins: Inhibitory hormones.
    • Mainly releases/produces "Releasing Hormones."

Pituitary Gland (Hypophysis)

  • Divided into Anterior and Posterior pituitary.

Anterior Pituitary (Adenohypophysis)

  • Controlled by Liberins and statins from the hypothalamus.
  • Produces and Releases "Stimulating Hormones."
  • Hormones produced:
    • TSH (Thyroid-stimulating hormone)
    • ACTH (Adrenocorticotropic hormone)
    • LH (Luteinizing hormone)
    • FSH (Follicle-stimulating hormone)
    • GH (Growth hormone)
    • PRL (Prolactin)

Posterior Pituitary (Neurohypophysis)

  • Controlled via Hypothalamic Neurons.
  • Stores ADH (Vasopressin) and Oxytocin, which are produced by the hypothalamus.

Hormones

Adrenocortical Hormones

  • Cortisol:
    • Function: Carbohydrate metabolism and stress response.
    • Mechanism: Counteracts insulin, increases heart rate (HR), aids in fat metabolism.
    • Disease related: Cushing’s Syndrome (caused by a tumor in the pituitary or adrenal gland).
  • Aldosterone:
    • Function: Water and salt homeostasis (via the Renin-Angiotensin-Aldosterone System - RAAS).
  • Androgens:
    • Function: Male secondary sex characteristics (e.g., testosterone).

Mineralocorticoids

  • Function: Control concentration of sodium and potassium.
  • Primary Mineralocorticoid: Aldosterone.
    • Mechanism: Increases Na+/K+ channels in the epithelial membrane and increases Na+/K+ ATPase molecules.
    • Diseases:
      • Addison’s Disease: Inadequate mineralocorticoids produced, resulting in low mineralocorticoid levels.
      • Hyperaldosteronism: Opposite of Addison’s Disease (excessive aldosterone production).

Gonadotrophins

  • Function: Induce release of Androgens (Sex Hormones).
  • Weak precursors released by the adrenal gland are converted to more potent hormones by secondary glands (testes/ovaries).

Growth Hormone (GH)

  • Synthesized by Somatotropin cells in the Anterior Pituitary.
  • Direct Action: Diabetogenic (opposes insulin).
  • Indirect Action: Secretion of IGF-1 from the liver which induces bone growth, lipolysis, and protein synthesis.
  • Triggers production of Growth Factors (such as IGF-1) which cause permanent growth.
  • Disorders:
    • Dwarfism
    • Acromegaly

Posterior Pituitary Hormones

Anti-Diuretic Hormone (Vasopressin, ADH)

  • Synthesized by the hypothalamus then stored and released by the posterior pituitary.
  • Function: Increases water reabsorption in the collecting duct of the kidney.
  • Lack of ADH leads to Diabetes Insipidus.
  • Other actions: ACTH & TSH release.

ADH Receptors

  • V1A: Mediate Gluconeogenesis.
  • V1B: In the anterior Pituitary, induce ACTH and TSH release.
  • V2: In kidneys, increase water permeability in the collecting duct.

Oxytocin

  • Functions: Milk ejection, maternal (mother) behavior, associated with feelings of love.