There is pulsatile secretion of GnRH (Gonadotropin releasing hormone) from the hypothalamus.
This stimulates the release of LH and FSH from the anterior pituitary. The alpha chain of FSH and LH are identical. It is the beta chain that differentiates them.
The effects of LH and FSH on the gonads are different in men and women
Inhibition of GnRH, LH, and FSH secretion by:
Androgens, estrogen, progesterone
Inhibin
Pathway in men
LH stimulates the production of testosterone from the Leydig cells in the testis. Testosterone acts in a negative feedback loop on the hypothalamus to reduce the secretion of GnRh.
FSH stimulates the seminiferous tubules within the Sertoli cells to produce mature sperm and the inhibins A and B.
Inhibin feeds back to the pituitary to decrease FSH secretion.
Pathway in women (HPO Axis)
LH acts on theca cells to stimulate the production of the androgen androstenedione.
FSH stimulates follicular development and acts on theca cells to activate the enzyme aromatase which is responsible for converting the androstenedione into oestrogen.
FSH also stimulates the release of inhibin from ovarian stroma cells, which inhibits FSH release.
Oestrogen is interesting in that initially, it acts as negative feedback, reducing GnRh production. However once a threshold level is reached, the switch is flipped and it exerts positive feedback increasing GnRH levels.
The increased GnRH levels and increased LH sensitivity to GnRh, causes a mid cycle LH surge responsible for ovulation.
The Hypothalamic-Pituitary-Adrenal Axis
Hypothalamus is stimulated to release CRH in response to stress, circadian rhythm and other stimuli.
CRH travels down the portal system and acts on the anterior pituitary to stimulate the production and release of ACTH.
ACTH then acts on the adrenal cortex to produce cortisol.
Cortisol acts in a negative feedback loop, on the hypothalamus and pituitary to reduce the production of CRH and ACTH respectively.
The set point varies throughout the day and is usually overridden by severe stress.
