5.2 Hypothalamus and pituitary gland

The hypothalamus and pituitary gland are key players in the endocrine system. They work together to control hormone production and release, affecting many bodily functions like growth, metabolism, and reproduction.

These structures form a crucial link between the nervous and endocrine systems. The hypothalamus produces hormones that regulate the pituitary, which in turn releases hormones that control other glands and organs throughout the body.

Hypothalamus and Pituitary Anatomy

Hypothalamus Location and Structure

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• The hypothalamus is a small region of the diencephalon located inferior to the thalamus and superior to the pituitary gland
• Forms the floor and part of the lateral walls of the third ventricle
• Divided into several nuclei, each with specific functions related to endocrine regulation, autonomic control, and behavior
  • Nuclei include the supraoptic, paraventricular, ventromedial, dorsomedial, and arcuate nuclei

Pituitary Gland Location and Structure

• The pituitary gland (hypophysis) is a small endocrine gland located in the sella turcica of the sphenoid bone at the base of the brain, inferior to the hypothalamus
• Divided into two lobes: the anterior lobe (adenohypophysis) and the posterior lobe (neurohypophysis)
  • Anterior lobe is derived from oral ectoderm and consists of glandular tissue that synthesizes and secretes hormones
  • Posterior lobe is derived from neural tissue and consists mainly of axons and nerve terminals originating from the hypothalamus
• The infundibulum (pituitary stalk) connects the hypothalamus to the posterior lobe of the pituitary gland, allowing for direct neural communication between the two structures

Hypothalamus-Pituitary Relationship

Hypothalamic Regulation of Pituitary Function

• The hypothalamus acts as the main link between the nervous system and the endocrine system, regulating pituitary gland function through the release of hypothalamic hormones
• Synthesizes releasing and inhibiting hormones that are transported to the anterior pituitary via the hypophyseal portal system, a network of blood vessels connecting the hypothalamus to the anterior pituitary
• Hypothalamic hormones bind to specific receptors on anterior pituitary cells, stimulating or inhibiting the synthesis and secretion of anterior pituitary hormones

Posterior Pituitary Hormone Storage and Release

• The posterior pituitary does not synthesize hormones; instead, it stores and releases hormones produced by the hypothalamus
  • Neurons in the supraoptic and paraventricular nuclei of the hypothalamus synthesize vasopressin (antidiuretic hormone, ADH) and oxytocin
  • These hormones are transported along the axons of the hypothalamic neurons through the infundibulum and stored in the nerve terminals of the posterior pituitary until released into the bloodstream
• Feedback loops involving the hypothalamus, pituitary gland, and target organs help maintain homeostasis by regulating hormone levels in the body (negative feedback, positive feedback during menstrual cycle)

Pituitary Hormones and Targets

Anterior Pituitary Hormones and Their Functions

• The anterior pituitary secretes six major hormones:
  • Growth hormone (GH): Stimulates growth and cell reproduction in bones, muscles, and other tissues
  • Thyroid-stimulating hormone (TSH): Stimulates the thyroid gland to produce thyroid hormones (T3, T4)
  • Adrenocorticotropic hormone (ACTH): Stimulates the adrenal cortex to produce glucocorticoids (cortisol)
  • Follicle-stimulating hormone (FSH): Stimulates the development of ovarian follicles in females and spermatogenesis in males
  • Luteinizing hormone (LH): Triggers ovulation and stimulates the production of estrogen and progesterone in females; stimulates testosterone production in males
  • Prolactin (PRL): Stimulates milk production in the mammary glands and plays a role in reproductive behavior

Posterior Pituitary Hormones and Their Functions

• The posterior pituitary secretes two hormones:
  • Vasopressin (antidiuretic hormone, ADH): Increases water reabsorption in the kidneys and constricts blood vessels to increase blood pressure
  • Oxytocin: Stimulates uterine contractions during labor and milk ejection during lactation; plays a role in social bonding and stress reduction

Regulation of Pituitary Hormone Secretion

Hypothalamic Releasing and Inhibiting Hormones

• The hypothalamus produces releasing and inhibiting hormones that regulate the synthesis and secretion of anterior pituitary hormones
• Releasing hormones stimulate the anterior pituitary to secrete its hormones, while inhibiting hormones suppress hormone secretion
• Major hypothalamic releasing and inhibiting hormones include:
  • Thyrotropin-releasing hormone (TRH): Stimulates TSH secretion
  • Corticotropin-releasing hormone (CRH): Stimulates ACTH secretion
  • Gonadotropin-releasing hormone (GnRH): Stimulates FSH and LH secretion
  • Growth hormone-releasing hormone (GHRH): Stimulates GH secretion
  • Somatostatin (growth hormone-inhibiting hormone, GHIH): Inhibits GH secretion
  • Dopamine (prolactin-inhibiting factor, PIF): Inhibits prolactin secretion

Feedback Loops in Pituitary Hormone Regulation

• Negative feedback loops involving the hypothalamus, pituitary, and target organs help maintain stable hormone levels in the body
  • Example: When thyroid hormone levels are high, they inhibit the release of TRH from the hypothalamus and TSH from the anterior pituitary, reducing further thyroid hormone production
• Positive feedback loops, such as the one involving estrogen during the menstrual cycle, can also modulate pituitary hormone secretion
  • Rising estrogen levels during the follicular phase stimulate a surge in LH secretion, triggering ovulation