Hypothalamus & Pituitary Gland: Structure, Hormones, and Regulation

Hypothalamus: Master Control Center

• Traditionally the pituitary was called the “master gland,” but modern understanding places primary control in the hypothalamus.
• Anatomical position: in the brain, directly inferior to the thalamus and superior to the pituitary gland.
• Produces 9 distinct hormones (notation: h^{\circ} used by lecturer to denote “hormone”).
• Acts as the crucial link between the nervous system (electrical signaling) and the endocrine system (chemical signaling).

Pituitary Gland: Location, Structure, Connectivity

• Sits just inferior to the hypothalamus in the hypophyseal fossa of the sphenoid bone.
• Size ≈ a pea (very small but physiologically essential).
• Connected to the hypothalamus via the infundibular stalk (infundibulum).
• Composed of two morphologically and functionally distinct lobes:
◦ Anterior pituitary (adenohypophysis).
◦ Posterior pituitary (neurohypophysis).

Anterior Pituitary (Adenohypophysis)

• Larger lobe ≈ \frac{3}{4} of total pituitary mass.
• Tissue type: glandular (classic endocrine tissue).
• Capabilities: synthesizes, stores, and releases its own hormones (total of 7 principal hormones; names not listed in transcript).
• Communication with hypothalamus: hypothalamo-hypophyseal portal system (vascular route).
◦ A double-capillary (portal) network that allows hypothalamic releasing/inhibiting hormones to directly reach anterior pituitary cells without dilution in systemic circulation.

Posterior Pituitary (Neurohypophysis)

• Smaller lobe ≈ \frac{1}{4} of pituitary mass.
• Tissue type: nervous tissue (primarily axons of hypothalamic neurons).
• Function: does NOT synthesize hormones; only stores and releases hormones produced in hypothalamus.
• Communication with hypothalamus: hypothalamo-hypophyseal tract (axonal/neuronal pathway) — NOT vascular.
• Hormones released (only two):
◦ Oxytocin (OXT).
◦ Antidiuretic hormone (ADH, a.k.a. vasopressin).

Neurosecretory Pathway & Blood Flow to Posterior Pituitary

• Cell bodies reside in two hypothalamic nuclei:
◦ Paraventricular nucleus (PVN).
◦ Supraoptic nucleus (SON) — “supra-optic” = above the optic chiasm.
• Both nuclei synthesize BOTH oxytocin and ADH (no one-to-one correspondence).
• Axons from PVN & SON form the hypothalamo-hypophyseal tract, descend through the infundibulum, and terminate in posterior pituitary.
• Hormones are packaged into vesicles and stored in axon terminals (awaiting release signal).
• Vascular pickup:
◦ Arterial inflow: internal carotid artery → inferior hypophyseal artery → capillary plexus in posterior pituitary.
◦ Upon stimulation, OXT/ADH are exocytosed into this capillary bed.
◦ Venous drainage: hypophyseal veins → systemic circulation, exposing all body tissues (but only target cells with appropriate receptors respond).

Regulation of Hormone Release from Posterior Pituitary

1. Hypothalamic neurons (PVN & SON) synthesize OXT and ADH.

2. Vesicular transport down axons of the hypothalamo-hypophyseal tract.

3. Arrival & storage in axon termini within posterior pituitary.

4. Appropriate neural stimuli reach hypothalamus → action potentials travel down tract → Ca^{2+}-dependent exocytosis of hormone vesicles.

5. Hormones enter capillary plexus → hypophyseal veins → systemic blood flow.

6. Target-organ effects:
   • Oxytocin → uterus & mammary glands.
   • ADH → kidneys (collecting ducts) & systemic arterioles.

Oxytocin: Functions & Clinical Relevance

• Uterine smooth-muscle contraction during parturition (positive-feedback loop initiated by fetal signals & cervical stretch).
• Myoepithelial cell contraction in mammary glands → milk ejection (NOT milk production).
• Post-delivery roles:
◦ Nursing/ nipple stimulation continues to trigger OXT release, aiding placental expulsion and uterine involution.
• Historical/clinical notes:
◦ Midwives had laboring mothers nurse the first of twins to hasten second twin’s delivery (pre-discovery of OXT).
◦ Synthetic OXT = Pitocin; administered intravenously to induce or augment labor when medically indicated.
• Cesarean section contexts:
◦ Scheduled (no labor) → minimal endogenous OXT until breastfeeding.
◦ Emergent after partial labor → patient already exposed to endogenous OXT.
• Ethical/practical implication: judicious use of Pitocin avoids unnecessary hyperstimulation, fetal distress, or uterine rupture.

Antidiuretic Hormone (ADH): Overview

• Biochemical identity: peptide hormone also called vasopressin.
• Primary targets/effects (detailed in earlier coursework):
◦ Kidneys: binds receptors on collecting duct cells → insertion of aquaporin-2 channels → increased water reabsorption → decreased urine volume.
◦ Arterioles: vasoconstriction at higher concentrations → elevates systemic blood pressure.
• Physiological relevance: critical in water-balance homeostasis, blood-osmolarity regulation, and short-term blood-pressure control.
• Pathophysiological note: deficiency → diabetes insipidus; excess → syndrome of inappropriate ADH secretion (SIADH).

Comparative Summary (Anterior vs. Posterior Pituitary)

• Size: anterior > posterior.
• Tissue type: glandular vs. neural.
• Hormone synthesis: anterior YES (7 hormones); posterior NO (stores 2 hormones).
• Method of hypothalamic communication:
◦ Anterior: vascular portal system.
◦ Posterior: neuronal tract.
• Endocrine output control:
◦ Anterior: hypothalamic releasing/inhibiting hormones via blood.
◦ Posterior: direct neural impulses triggering exocytosis.

Connections to Previous Material / Real-World Significance

• Reinforces the principle that endocrine glands often operate under hierarchical control (hypothalamus → pituitary → peripheral glands).
• Illustrates dual signaling modalities: vascular chemical messengers vs. direct axonal release.
• Practical clinical relevance in obstetrics (Pitocin protocols), nephrology (ADH-related water disorders), and neurosurgery (damage to pituitary stalk affects both lobes differently).

Numerical / Structural Quick-Facts Cheat-Sheet

• Hypothalamic hormones: 9.
• Pituitary hormones: 7 anterior-origin + 2 posterior-stored = 9 total handled by pituitary.
• Anterior : posterior volume ratio ≈ 3:1.
• Blood pathway: internal carotid → inferior hypophyseal artery → capillary plexus → hypophyseal veins.
• Positive-feedback loops: oxytocin during labor & breastfeeding (unique among endocrine pathways dominated by negative feedback mechanisms).