5.3 Hypothalamic-pituitary axis and major endocrine systems

The hypothalamic-pituitary axis is the control center of the endocrine system. It connects the brain to hormone-producing glands, regulating vital functions like growth, metabolism, and reproduction. This intricate system showcases how the nervous and endocrine systems work together.

Major endocrine glands, including the thyroid, adrenals, pancreas, and gonads, produce hormones that affect the whole body. These glands respond to signals from the hypothalamus and pituitary, forming a complex network of hormone interactions that maintain homeostasis.

Hypothalamic-Pituitary Axis

Hypothalamus and Pituitary Gland Anatomy

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• Hypothalamus located at the base of the brain, acts as the link between the nervous system and endocrine system
• Anterior pituitary (adenohypophysis) consists of glandular cells that synthesize and secrete hormones
• Posterior pituitary (neurohypophysis) is an extension of the hypothalamus, stores and releases hormones produced by the hypothalamus (oxytocin and antidiuretic hormone)
• Hypothalamus and pituitary gland are connected by the hypophyseal stalk (infundibulum)

Hypothalamic Hormones and Their Effects

• Releasing hormones produced by the hypothalamus stimulate or inhibit the secretion of hormones from the anterior pituitary
  • Examples include thyrotropin-releasing hormone (TRH), corticotropin-releasing hormone (CRH), and gonadotropin-releasing hormone (GnRH)
• Hypothalamus secretes hormones that are stored and released by the posterior pituitary, such as oxytocin and antidiuretic hormone (ADH or vasopressin)
  • Oxytocin stimulates uterine contractions during labor and milk ejection during lactation
  • ADH regulates water balance by promoting water reabsorption in the kidneys

Anterior Pituitary Hormones and Their Target Glands

• Tropic hormones secreted by the anterior pituitary stimulate other endocrine glands to produce and secrete their respective hormones
  • Thyroid-stimulating hormone (TSH) stimulates the thyroid gland to produce thyroid hormones (T3 and T4)
  • Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to produce glucocorticoids (cortisol)
  • Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) regulate gonadal function and reproductive processes
• Anterior pituitary also secretes growth hormone (GH) and prolactin (PRL), which have direct effects on target tissues
  • GH stimulates growth and development of bones, muscles, and other tissues
  • PRL stimulates milk production in the mammary glands

Endocrine Glands

Thyroid Gland and Adrenal Glands

• Thyroid gland located in the neck, produces thyroid hormones (T3 and T4) that regulate metabolism, growth, and development
  • Thyroid hormones are synthesized from iodine and the amino acid tyrosine
  • Calcitonin, another hormone produced by the thyroid, helps regulate calcium homeostasis
• Adrenal glands located above the kidneys, consist of the adrenal cortex and adrenal medulla
  • Adrenal cortex produces glucocorticoids (cortisol), mineralocorticoids (aldosterone), and androgens
  • Adrenal medulla produces catecholamines (epinephrine and norepinephrine) in response to stress

Pancreas and Gonads

• Pancreas is a dual-function gland with both endocrine and exocrine roles
  • Endocrine portion (islets of Langerhans) secretes insulin and glucagon to regulate blood glucose levels
  • Insulin lowers blood glucose by promoting glucose uptake and storage, while glucagon raises blood glucose by stimulating glycogen breakdown and gluconeogenesis
• Gonads (ovaries in females and testes in males) produce sex hormones and gametes
  • Ovaries secrete estrogens and progesterone, which regulate the menstrual cycle and support pregnancy
  • Testes secrete testosterone, which is responsible for male secondary sexual characteristics and spermatogenesis

Pineal Gland and Melatonin

• Pineal gland located in the brain, produces the hormone melatonin
• Melatonin plays a role in regulating the sleep-wake cycle (circadian rhythm) and seasonal reproductive cycles in some animals
  • Melatonin secretion is influenced by light, with higher levels produced at night and lower levels during the day
  • Melatonin supplements are sometimes used to treat sleep disorders and jet lag