Hypothalamic-pituitary axis

Axis Anatomy - Command & Control

• Hypothalamus: The "CEO," linking the CNS to the endocrine system.
  • Parvicellular Nuclei: Secrete releasing/inhibiting hormones (e.g., TRH, CRH, GnRH) into the portal system.
  • Magnocellular Nuclei: Synthesize ADH and oxytocin for transport to the posterior pituitary.
• Pituitary (Hypophysis): The "manager."
  • Anterior: Synthesizes & releases hormones (e.g., TSH, ACTH, FSH/LH).
  • Posterior: Stores & releases ADH and oxytocin.

⭐ The hypothalamic-hypophyseal portal system is a true portal system (capillary bed → portal veins → capillary bed), allowing minute amounts of hypothalamic hormones to exert powerful, direct control over the anterior pituitary.

Anterior Pituitary - The FLAT PEG Crew

📌 Mnemonic: FLAT PEG

• Tropic Hormones (FLAT): Stimulate other endocrine glands.
  • FSH (Follicle-Stimulating Hormone): Targets gonads → gamete development.
  • LH (Luteinizing Hormone): Targets gonads → hormone production (testosterone, estrogen/progesterone).
  • ACTH (Adrenocorticotropic Hormone): Targets adrenal cortex → cortisol release.
  • TSH (Thyroid-Stimulating Hormone): Targets thyroid → thyroid hormone release.
• Direct Hormones (PEG): Act directly on target tissues.
  • Prolactin: Targets breast → milk production.
  • Endorphins: Natural analgesics.
  • GH (Growth Hormone): Targets liver, bone, muscle → growth (via IGF-1).

⭐ Exam Pearl: The anterior pituitary is derived from oral ectoderm, specifically Rathke's pouch. Remnants can cause craniopharyngiomas.

Posterior Pituitary - The Hypothalamic Storeroom

• Function: Stores and releases hormones synthesized in the hypothalamus. Not a true endocrine gland.
• Hormones: Synthesized in hypothalamic nuclei, transported via neurophysins down axons.
  • Antidiuretic Hormone (ADH/Vasopressin): From Supraoptic Nucleus (SON). Regulates plasma osmolality by ↑ water reabsorption in collecting ducts.
  • Oxytocin: From Paraventricular Nucleus (PVN). Controls uterine contractions and milk letdown. 📌 Mnemonic: PVN → Parturition.

⭐ Central Diabetes Insipidus: Caused by ADH deficiency due to hypothalamic or posterior pituitary damage. Leads to polyuria and polydipsia with dilute urine.

Feedback Loops - The Endocrine Boomerang

• Negative Feedback: The most common type. The final hormone (e.g., cortisol) inhibits its own production by suppressing the hypothalamus and pituitary. This maintains homeostasis.
• Positive Feedback: Less common. The final hormone stimulates its own production (e.g., oxytocin during labor).

⭐ Dexamethasone Suppression Test: This test assesses the integrity of the negative feedback loop in the HPA axis. Low-dose dexamethasone (a synthetic cortisol) should suppress ACTH and cortisol production in healthy individuals. Failure to suppress suggests Cushing's syndrome.

High‑Yield Points - ⚡ Biggest Takeaways

• The hypothalamus secretes releasing hormones (e.g., GnRH, CRH) to control the anterior pituitary.
• The anterior pituitary produces stimulating hormones (e.g., TSH, ACTH) to target endocrine glands.
• The posterior pituitary only stores and releases ADH and oxytocin, which are made in the hypothalamus.
• Negative feedback is crucial: end-organ hormones (e.g., cortisol, T4) inhibit the hypothalamus and pituitary.
• Dopamine tonically inhibits prolactin secretion; somatostatin inhibits GH and TSH release.
• Pituitary adenomas are a common cause of hormone hypersecretion (e.g., prolactinomas).