Notes on The Endocrine System: HPG Axis

Learning Objectives

• Understand the roles of tropic hormones and feedback loops in hormone regulation.
• Draw and label structures of the hypothalamus and pituitary glands.
• Explain the Human Hypothalamic-Pituitary-Gonadal (HPG) axis.

The Hypothalamic-Pituitary-Gonadal (HPG) Axis

• The HPG axis is a central part of the endocrine system that involves communication between the hypothalamus, pituitary gland, and gonads (testes and ovaries).

Components of the HPG Axis:

• Hypothalamus: Produces Gonadotropin-Releasing Hormone (GnRH).
• Anterior Pituitary: Releases two key hormones in response to GnRH.
  • Luteinizing Hormone (LH): Stimulates the production of testosterone in males and estrogen/progesterone in females.
  • Follicle-Stimulating Hormone (FSH): Supports spermatogenesis in males and follicle maturation in females.
• Gonads:
  • Testes: Produce testosterone (T).
  • Ovaries: Produce estrogen (E) and progesterone (P).

Hormones and Their Half-Lives:

• Gonadotropin-Releasing Hormone (GnRH):
  • Function: Stimulates the anterior pituitary to release LH and FSH.
  • Half-life: 3 minutes.
• Luteinizing Hormone (LH):
  • Function: Regulates testosterone and estrogen production.
  • Half-life: 20 minutes.
• Follicle-Stimulating Hormone (FSH):
  • Function: Supports reproductive processes.
  • Half-life: ~22 minutes.
• Testosterone (T):
  • Half-life: ~45 minutes.
• Estrogen (E):
  • Half-life: ~30 minutes.
• Progesterone (P):
  • Half-life: ~66 minutes.

Regulation of Hormones:

• Hormones are regulated through feedback loops:
  • Positive Feedback: Enhances the action of a hormone to increase its levels.
  • Negative Feedback: Decreases the releasing hormone when levels are adequate to maintain homeostasis.
• Example of a Feedback Loop: Increased testosterone levels may inhibit the release of GnRH from the hypothalamus, reducing LH and FSH secretion, thus regulating testosterone production.

Structures Related to the HPG Axis:

• Optic Chiasm: Important anatomical landmark near the base of the hypothalamus.
• Infundibulum: Connects the hypothalamus to the pituitary glands.
• Superior and Inferior Hypophyseal Arteries: Supply blood to the pituitary gland.
• Portal Veins: Transport hormones from the hypothalamus to the anterior pituitary.
• Hypophyseal Veins: Carry hormones from the pituitary to the systemic circulation.

Nuclei of the Hypothalamus:

• Supraoptic Nucleus: Involved in the regulation of water balance and blood pressure.
• Arcuate Nucleus (AN): Plays a role in energy balance and reproductive function.