02.01 Hypothalamic–Pituitary–Thyroid Axis.mp4

• Hypothalamic hormones are synthesized in the neuron cell body, packaged in vesicles, and transported down the axons to be stored and released from the nerve terminals.
• In the hypothalamic-anterior pituitary system, hormonal release is mainly pulsatile, with a possible exception being the thyroid system.
• The pulsatile release of hypothalamic hormones prevents downregulation of its receptors on the cells of the anterior pituitary. An example of this is GnRH, which prevents downregulation of its receptors on the gonadotrophs of the anterior pituitary. A constant infusion of GnRH will cause a decrease in the release of both LH and FSH.
• The nerve endings all converge in the median eminence region of the hypothalamus. Here, the hormones are secreted into the hypophyseal-portal system and transported to the anterior pituitary.
• The hypophyseal-portal system facilitates the delivery of releasing and inhibitory hormones from the hypothalamus directly to the anterior pituitary gland. This controls the production and release of trophic hormones into systemic circulation.
• Hypothalamic hormones bind to receptors on cells of the anterior pituitary, modifying the secretion of its hormones.

<aside> 💡 The pulsatile release refers to the manner in which hormones are released in pulses or bursts, rather than continuously. This type of release helps to prevent the downregulation of hormone receptors on target cells.

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