Hormones and Their Functions

Overview of Hormones

  • Introduction to hormones:
    • Discussed the function and significance of hormones in the body.
    • Mentioned the three main pathways related to hormonal actions that will appear in upcoming assessments.

Types of Hormones

  • Understanding local and circulating hormones:
    • Circulating hormones:
    • Released by endocrine glands into the bloodstream.
    • Travel through the blood to reach target cells and organs.
    • Bind to specific receptors to exert their effects:
      • Water-soluble hormones bind to cell surface receptors.
      • Steroid hormones (fat-soluble) can diffuse through the cell membrane and bind to intracellular receptors.
    • Local hormones:
    • Act locally on nearby cells:
      • Paracrine action: affects neighboring cells.
      • Autocrine action: affects the cell that secreted the hormone itself.

Local Hormonal Pathways and Inflammation

  • Phospholipase A2 and eicosanoids:
    • Phospholipase A2 cleaves arachidonic acid from cell membrane phospholipids.
    • Arachidonic acid is converted by enzymes into substances that mediate inflammation and pain, including:
    • Prostaglandins
    • Thromboxanes
    • Leukotrienes
    • Corticosteroids (e.g., prednisone) block phospholipase A2, reducing inflammation and the synthesis of eicosanoids.
    • NSAIDs (e.g., aspirin) block the effects of prostaglandins, providing pain relief.

Mechanisms of Hormone Action

  • Differences in mechanisms between steroid hormones and water-soluble hormones:
    • Steroid hormones:
    • Require transport proteins for circulation.
    • Once they enter target cells, they bind to intracellular receptors and influence gene expression, leading to the synthesis of new proteins.
    • Water-soluble hormones:
    • Bind to surface receptors on target cells, activating a second messenger system (e.g., cAMP or phospholipase C pathways).
    • These hormones typically activate existing enzymes and cellular processes, rather than directly leading to new protein synthesis.

Receptor Regulation

  • Upregulation and downregulation of receptors:
    • Cells can increase the number of receptors (upregulation) in response to low hormone levels to enhance sensitivity.
    • Conversely, downregulation occurs in response to high hormone levels, reducing the number of receptors and sensitivity to that hormone.

Types of Hormonal Interactions

  • Synergistic Effects: Hormones work together to produce a more significant effect.
  • Permissive Effects: One hormone's presence is necessary for another hormone to exert its effect.
  • Antagonistic Effects: One hormone opposes the action of another hormone.

Feedback Mechanisms

  • Hormonal regulation operates primarily through feedback mechanisms:
    • Negative Feedback: Common throughout the body, where an increase in a hormone leads to actions that reduce its further production.
    • Positive Feedback: Less common, where a hormone increases its own production until a specific event occurs (e.g., childbirth).

Pituitary Gland Functions

  • Structure of the Pituitary Gland:
    • Divided into anterior and posterior lobes:
    • Anterior Pituitary: Produces hormones in response to releasing/inhibiting hormones from the hypothalamus.
    • Posterior Pituitary: Stores and releases hormones (oxytocin, ADH) produced in the hypothalamus.
  • Key Hormones of the Anterior Pituitary:
    • TSH (Thyroid Stimulating Hormone)
    • Prolactin: Milk production activation.
    • LH/FSH: Gonadotropic hormones,
    • ACTH: Stimulates adrenal cortex.
    • GH: Growth hormone, affects overall growth.

Hypothalamus and Pituitary Communication

  • Hypothalamo-hypophyseal tract: refers to the neural connection to the posterior pituitary.
  • Hypothalamic hypophyseal portal system: a vascular connection to the anterior pituitary, allowing hormonal communication and control.

Hormonal Regulation of Growth

  • Growth Hormone:
    • Influences growth and metabolism within multiple body tissues.
    • Triggered by factors such as sleep, exercise, and nutrient availability.
    • GH can lead to increased muscle and bone growth and influence glucose metabolism.