Hormones: Chemical Signals and Their Mechanisms

Chemical Signals and Hormones

• Chemicals secreted into the blood by a cell travel to distant targets
• The hormone affects growth, development, homeostasis, or metabolism at very low concentrations

Peptide and Steroid Hormone Comparison

• Peptide Hormones:
  • Synthesized in advance as preprohormones, then cleaved into active hormones
  • Stored in secretory vesicles
  • Released via exocytosis (e.g., insulin)
• Steroid Hormones:
  • Synthesized on demand from cholesterol
  • Not stored, released immediately after synthesis
  • Diffuse through the cell membrane (e.g., cortisol)

Hormone Receptor Location and Mechanism

• Peptide Hormones:
  • Receptors located on the cell surface
  • Activate second messenger pathways, leading to rapid responses (e.g., insulin)
• Steroid Hormones:
  • Receptors located within the cytoplasm or nucleus
  • Regulate gene expression, leading to slower but longer-lasting effects (e.g., testosterone)

Amine Hormone Groups

• Catecholamines:
  • Derived from tyrosine
  • Act like peptide hormones (e.g., epinephrine, norepinephrine)
• Thyroid Hormones:
  • Derived from two tyrosine molecules
  • Act like steroid hormones (e.g., thyroxine [T4], triiodothyronine [T3])
• Melatonin:
  • Derived from tryptophan
  • Regulates circadian rhythms

Nervous System and Endocrine Reflexes

• The nervous system integrates with the endocrine system through neurohormones and control pathways
• Hypothalamus: Produces neurohormones regulating the pituitary gland
• Neuroendocrine Reflexes: Electrical signals trigger hormone release (e.g., oxytocin)
• Feedback Loops: Coordinate hormonal and neural responses for homeostasis

Anterior Pituitary Hormones

• Prolactin (PRL): Controlled by prolactin-releasing factors and dopamine. Targets mammary glands.
• Thyroid-Stimulating Hormone (TSH): Controlled by thyrotropin-releasing hormone (TRH). Targets thyroid gland.
• Adrenocorticotropic Hormone (ACTH): Controlled by corticotropin-releasing hormone (CRH). Targets adrenal cortex.
• Growth Hormone (GH): Controlled by growth hormone-releasing hormone (GHRH) and somatostatin. Targets liver and other tissues.
• Follicle-Stimulating Hormone (FSH): Controlled by gonadotropin-releasing hormone (GnRH). Targets gonads.
• Luteinizing Hormone (LH): Controlled by gonadotropin-releasing hormone (GnRH). Targets gonads.

Negative Feedback Loops

• Long-Loop Negative Feedback: Hormones from target endocrine glands inhibit the anterior pituitary and hypothalamus (e.g., cortisol).
  • Examples: Insulin (blood glucose), Parathyroid Hormone (calcium)
• Other: Short-loop, Ultra-short-loop

Hormone Interactions

• Permissiveness: One hormone enables another's full effect (e.g., thyroid hormone and epinephrine)
• Synergism: Two hormones together produce a greater effect than their individual effects (e.g., glucagon and epinephrine on blood glucose)
• Functional Antagonism: Two hormones have opposing effects (e.g., insulin and glucagon on blood glucose)

Endocrine Pathologies

• Hypersecretion: Excess hormone production (e.g., hyperthyroidism)
• Hyposecretion: Deficient hormone production (e.g., hypothyroidism)
• Abnormal Target Response: Target cells fail to respond appropriately (e.g., type 2 diabetes)

This quiz explores the fascinating world of hormones, focusing on their chemical signals, types, and mechanisms of action. You'll learn about the differences between peptide and steroid hormones, their synthesis, and how they interact with cell receptors. Test your knowledge on how these essential substances regulate various physiological processes.