Positive feedback: the LH surge and the Ferguson reflex
Effect and feedback · LH surge Ferguson reflex Self-limiting
Two loops in this system run the other way. Sustained high oestradiol flips the gonadotroph from inhibition to stimulation, and cervical stretch in labour recruits more oxytocin, which stretches the cervix further.
Traced from the start
- Gonadotrophin-releasing hormone pulse generator (arcuate nucleus)
- Gonadotroph — luteinising hormone and follicle-stimulating hormone
- Leydig cells and ovarian theca cells (the LH target)Sertoli cells and ovarian granulosa cells (the FSH target)
- Tuberoinfundibular dopamine (prolactin-inhibiting factor)Thyrotrophin-releasing hormone (paraventricular nucleus)
- Supraoptic and paraventricular magnocellular neurones
- Lactotroph — prolactinPosterior pituitary — oxytocin release
- Mammary alveolus and myometrium
- Testosterone and oestradiolMilk ejection and uterine contraction
- Positive feedback: the LH surge and the Ferguson reflex
Detail
- Ovulation
- Oestradiol held above roughly 200 pg/mL (about 700 pmol/L) for around 36-48 hours triggers the LH surge; ovulation follows about 36 hours after the surge begins, some 10-12 hours after its peak
- Termination
- Progesterone from the corpus luteum restores negative feedback and slows GnRH pulses, ending the surge
- Labour
- The oxytocin loop is broken only by delivery, which removes the stretch stimulus
- Clinical use
- hCG substitutes for the surge in ovulation induction, and a mid-luteal progesterone confirms that ovulation occurred
When it goes wrong
Polycystic ovary syndrome
Oestradiol never reaches a sustained surge threshold: chronic anovulation with unopposed oestrogen and a raised risk of endometrial hyperplasia
Combined oral contraceptive
Steady exogenous steroid keeps feedback negative, so there is no surge and no ovulation
Practise this structure
2 questions in the bank tagged Feedback Axis.
A 35-year-old woman has palpitations and a 9 kg weight loss over 3 months. Her free thyroxine is 48 pmol/L (reference 10–22 pmol/L), TSH is less than 0.01 mIU/L…
Mechanisms of disease · common
A 41-year-old woman has progressive weight gain, proximal muscle weakness and easy bruising. Her 24-hour urinary free cortisol is 780 nmol/day (reference less t…
Mechanisms of disease · middle twist