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Learning Outcomes

  • Compare functional classes of chemical messengers: release source and target cell pathways.
  • Describe structure, function, synthesis, release, transport, and signal transduction of each chemical class.
  • Differentiate signal transduction for lipophilic and lipophobic messengers.
  • Classify receptors: channel-linked, enzyme-linked, G protein-linked.
  • Contrast communication via nervous vs. endocrine systems.

Chemical Messengers

Classification

  • Direct Communication: via Gap Junctions.
  • Indirect Communication:
    • Paracrine
    • Neurotransmitters
    • Hormones
    • Neurohormones

Types of Chemical Classes

  1. Lipophilic (hydrophobic)
  2. Lipophobic (hydrophilic)

Functional Classes of Chemical Messengers

  • Amino Acids: Neurotransmitters only.
  • Amines: Paracrines, neurotransmitters, hormones.
  • Peptides/Proteins: Paracrines, neurotransmitters, hormones.
  • Steroids: Hormones only.

Mechanisms of Indirect Communication

Paracrine Communication

  • Local signaling within interstitial fluid.
  • Examples: Histamine, Prostaglandins, Cytokines.

Neurotransmitter Communication

  • Fast signaling across synapses.
  • Examples: Glutamate, Acetylcholine.

Hormonal Communication

  • Systemic signaling via blood.
  • Examples: Insulin, Thyroid hormone.

Neurohormonal Communication

  • Released into bloodstream.
  • Examples: Vasopressin, Epinephrine.

Chemical Properties

  • Lipophilic Ligands: Diffuse easily through membranes, slow release.
  • Lipophobic Ligands: Cannot diffuse, stored in vesicles, rapid release.

Synthesis and Transport of Ligands

  • Amines and Amino Acids: Derived from specific amino acids; synthesis and transport mechanisms vary.
  • Peptides and Proteins: Synthesized from larger proteins, processed in the ER and Golgi.
  • Steroids: Synthesized from cholesterol.

Half-Life of Messengers

  • Varies based on solubility: Plasma dissolved messengers typically have shorter half-lives.
  • Degradation usually in the liver, excretion via kidneys.

Summary of Transport Mechanisms

  • Diffusion: Short distance (e.g., paracrines).
  • Blood Transport: Long distance, often requires carrier proteins for stability.