CorpusHuman Anatomy

Hypothalamic–Pituitary Axis

Sagittal schematic — portal system, neurohypophysis and target organs

A midline (sagittal) schematic of the hypothalamus and pituitary, anterior to the left. The hypothalamus controls the anterior pituitary through blood: parvocellular neurons release releasing/inhibiting hormones onto the primary capillary plexus of the median eminence, and the hypophyseal portal veins carry them down the stalk to the sinusoids of the anterior lobe (violet particles). It controls the posterior pituitary through nerves: magnocellular neurons of the supraoptic and paraventricular nuclei send neurosecretory granules of ADH and oxytocin down the hypothalamo-hypophyseal tract to be stored and released in the posterior lobe (yellow particles).

On the right, pituitary hormones leave via the hypophyseal veins and reach their targets through the systemic circulation — violet arrows for anterior-lobe hormones, yellow for posterior-lobe hormones. The red dashed arc represents negative feedback from target-gland hormones. Every nucleus, vessel, hormone arrow and target organ is clickable; use the guided walkthrough to follow each axis.

Step-by-step

  1. The hypothalamus — command centre

    The hypothalamus forms the floor of the third ventricle above the pituitary. Its parvocellular neurons (paraventricular, arcuate and periventricular nuclei) make releasing and inhibiting hormones; its magnocellular neurons (supraoptic and paraventricular nuclei) make ADH and oxytocin. The suprachiasmatic nucleus above the optic chiasm sets circadian timing for the whole axis.

  2. The hypophyseal portal system

    Superior hypophyseal arteries (from the internal carotid) form the primary capillary plexus in the median eminence. Releasing-hormone axons end on these fenestrated capillaries. Long portal veins run down the stalk to a secondary plexus of sinusoids in the anterior lobe — two capillary beds in series, so the hormones arrive concentrated and undiluted by the systemic circulation (violet particles).

  3. HPT axis: TRH → TSH → T₄/T₃

    Paraventricular neurons release TRH into the portal blood; thyrotrophs respond with TSH, which stimulates every step of thyroid hormone synthesis. T₃ (made locally from T₄) feeds back to inhibit both TRH and TSH — so in primary hypothyroidism TSH rises, and in Graves disease (TSH-receptor antibodies) TSH is suppressed.

  4. HPA axis: CRH → ACTH → cortisol

    Stress and the circadian clock drive paraventricular CRH (with ADH as a co-secretagogue). Corticotrophs cleave POMC to ACTH, which stimulates the zona fasciculata and reticularis of the adrenal cortex. Cortisol feeds back on CRH and ACTH; exogenous steroids therefore suppress the axis and atrophy the adrenal cortex.

  5. HPG axis: pulsatile GnRH → FSH and LH

    Arcuate (KNDy-driven) GnRH pulses every 60–120 min stimulate gonadotrophs to secrete FSH (granulosa and Sertoli cells) and LH (theca and Leydig cells). Sex steroids and inhibin feed back negatively — except the sustained estradiol peak of the late follicular phase, which flips to positive feedback and triggers the LH surge.

  6. GH axis and the inhibitory hormones

    GHRH (stimulatory) and somatostatin (inhibitory) together set pulsatile GH secretion; GH acts on the liver to make IGF-1, which mediates growth and feeds back negatively. Prolactin is unusual: tonic dopamine from the arcuate nucleus inhibits it, so a damaged stalk raises prolactin while every other anterior hormone falls.

  7. Posterior pituitary: ADH

    Magnocellular neurons of the supraoptic (and paraventricular) nucleus synthesise ADH, which travels in neurosecretory granules down the hypothalamo-hypophyseal tract (yellow particles) and is stored in the posterior lobe. A rise in plasma osmolality (or a fall in blood volume) triggers action potentials and release into the inferior hypophyseal capillaries; ADH inserts aquaporin-2 into renal collecting ducts.

  8. Posterior pituitary: oxytocin and positive feedback

    Oxytocin, made mainly in the paraventricular nucleus, is released when the cervix is stretched in labour or the nipple is suckled. It contracts the myometrium and mammary myoepithelial cells; the resulting stretch or suckling releases more oxytocin — the Ferguson and milk-ejection reflexes, classic examples of positive feedback.

Labelled parts