QS Study

Role of Hypothalamus in the regulation of Pituitary Gland Secretion

The hypothalamus plays a key role in regulating pituitary function. This structure can be considered a central relay station for collecting and integrating signals from different parts of the body. Then much of this information is used to control secretions of the many globally important pituitary hormones. The hormones of the pituitary gland helps regulate the functions of other endocrine glands. The pituitary consists of an anterior lobe and a posterior lobe, each of which has distinct functions.

Anatomic Considerations:

Fig: Human hypothalamus, with a superimposed diagrammatic representation of the portal hyperphysical vessels.

Posterior Lobe

There are neural connections between the hypothalamus and the posterior lobe of the pituitary gland. While the anterior lobe shoulders most of the work in producing hormones, the posterior lobe stores and releases only two: oxytocin and antidiuretic hormone (ADH), or vasopressin. Axons from cell bodies in the supraoptic and paraventricular nuclei pass to posterior pituitary via hypothalamohypophyseal tract.

  • Most Supraoptic fibers end in the posterior lobe of the pituitary gland.
  • Some of the paraventricular fibers end in the anterior pituitary gland.

Anterior Lobe

The anterior lobe has vascular connections with the hypothalamus. The anterior lobe (or adenohypophysis) secretes hormones that regulate a wide variety of bodily functions. The portal hypophyseal vessels form a direct vascular link.

Internal Carotid arteries and Circle of Willis → Arterial twigs → a network of fenestrated capillaries (These loops also penetrate median eminence) → Ventral surface of the hypothalamus → Sinusoidal portal hypophyseal vessels along the pituitary stalk → Capillaries of the anterior pituitary.

Embryologically,

  • Posterior lobe arises as an evagination of the floor of the third ventricle,
  • Anterior and intermediate lobe arises from Rathke’s pouch.

The hypothalamic-hypophvseal portal system is a true portal system. Because it begins and ends in capillaries without going through the heart. Median eminence is generally defined as the portion of ventral hypothalamus from which the portal vessels arise. This region is outside ‘Blood-Brain Barrier’.

Mechanism:

  • Synthesis of hypophysiotropic hormone. In the special neurons in arcuate and other nuclei.
  • Secretion of hypophysiotropic hormones at nerve endings in the median eminence in response to electrical activity in the endings.
  • Absorption of these hormones in the hypothalamic-hypophysial portal system.
  • Transport of these hormones to the anterior pituitary gland.
  • Action in the glandular cells of an anterior pituitary gland.
  • Control of anterior pituitary gland.

Hypothalamic control of posterior pituitary secretion:

Mechanism:

  • Synthesis of posterior pituitary hormone in the body of neurons of supraoptic and paraventricular nuclei.
  • Transport of these hormones along the axon combines with carrier protein called neurophysin.
  • Secretion of these hormones at nerve endings of the posterior pituitary gland.
  • When signal arises, the secretion from the posterior pituitary gland occurs.

Endocrine gland without pituitary control:

  • Parathyroid hormone,
  • Adrenal medulla,
  • Placenta,
  • Kidney,
  • Heart,
  • Stomach,
  • Small intestine.