Pulsatility

The defining feature of the HPG axis is that its top-level signal is released in bursts, not as a steady stream. Roughly every 60–120 minutes in an adult, a small population of neurons in the hypothalamus fires together, releases a pulse of GnRH, and falls silent again. Continuous GnRH would shut the axis down. The rhythm is part of the signal.

The GnRH neurons

GnRH (gonadotropin-releasing hormone) is a tiny peptide — just ten amino acids — encoded by the GNRH1 gene. It has a circulating half-life of only 2–4 minutes, so each pulse is rapidly cleared, creating the trough between bursts.

The neurons that make it are unusual:

• There are surprisingly few — only about 1,000–2,000 GnRH neurons in the entire human brain, scattered through the preoptic area and arcuate nucleus of the hypothalamus.
• They are not born in the brain. During fetal development they form in the olfactory placode, the same structure that gives rise to olfactory neurons, then migrate along the olfactory nerves into the forebrain and down to the hypothalamus. This shared origin is why some genetic conditions that disrupt this migration affect both reproduction and the sense of smell (see Kallmann syndrome under deviations).
• Their axons all converge on the median eminence at the base of the hypothalamus, where they release GnRH into a specialized portal blood supply that carries it directly to the anterior pituitary.

Why pulses, not continuous release

The gonadotrope cells of the anterior pituitary carry a GnRH receptor on their surface. Each pulse of GnRH binds, triggers a burst of LH and FSH release, and the receptor then briefly resets.

If GnRH is delivered continuously — at the same average concentration but without troughs — the receptor desensitizes and downregulates. After a few weeks of continuous exposure the pituitary effectively stops responding, and LH, FSH, and downstream sex hormones fall to near zero.

This paradox is medically useful. GnRH agonist drugs (leuprolide, goserelin) work by overstimulating the axis: a long-acting analog of GnRH that floods the receptor continuously and shuts it down. They are used to suppress the axis in prostate cancer, endometriosis, certain forms of precocious puberty, and as part of IVF protocols.

Frequency encoding

The pituitary not only requires pulses — it reads their frequency as a distinct message:

• Faster pulses (about every 60–90 minutes) preferentially drive LH biosynthesis and release.
• Slower pulses (every 2–4 hours) preferentially drive FSH.

This is why LH and FSH can dissociate even though the same gonadotrope cell makes both. Many disorders of the axis present with a characteristic LH/FSH ratio because the underlying defect is a pulse-frequency abnormality rather than a missing hormone.

The KNDy pulse generator

GnRH neurons do not generate their own rhythm. They are driven by an upstream population in the arcuate nucleus of the hypothalamus called KNDy neurons — named for the three signaling molecules they co-express:

• Kisspeptin (gene KISS1) — released onto GnRH neurons, triggers their firing. Kisspeptin is the proximate “go” signal for the axis.
• Neurokinin B (gene TAC3) — provides positive feedback within the KNDy network itself, so once a burst starts it sustains briefly rather than dying immediately.
• Dynorphin — provides negative feedback within the KNDy network, terminating each burst after a short interval.

The interplay of these three — kisspeptin firing outward, neurokinin B sustaining the burst, dynorphin shutting it down — produces the pulsatile pattern. The whole HPG axis hangs off this small upstream circuit.

Mutations affecting any KNDy component (KISS1, KISS1R, TAC3, TACR3) produce specific forms of congenital hypogonadism with characteristic patterns — see deviations.

What sets the pulse generator’s pace

The KNDy network is not autonomous. It integrates signals from elsewhere in the body and the brain — energy status, stress, sex steroid levels, prolactin, thyroid hormone — and adjusts pulse frequency accordingly. This integration is covered separately under inputs to the axis.

Key idea

A pulsatile signal is a richer signal than a steady one: it carries information in its frequency, and it resets the receptor between pulses so the downstream cells stay responsive. The HPG axis is built around this principle, and most of its disorders trace back, one way or another, to a problem with the rhythm.