Chemical Messengers

CHAPTER 5 - CHEMICAL MESSENGERS

I. Overview of Cellular Communication

  • The human body contains over 10 trillion cells.

  • Effective communication among these cells is essential for proper bodily function.

  • Focus will be on communication mechanisms both between adjacent cells and distant cells.

II. Mechanisms of Intercellular Communication

  • Definition: Intercellular communication refers to signaling between two or more cells, which can be either physically close or far apart.

A. Types of Intercellular Communication

  1. Direct Communication

    • Description: Cells communicate directly with each other.

  2. Indirect Communication

    • Description: One cell sends a signal (chemical or electrical) to another.

    • Examples of Signals: Chemical messengers, hormones, electrical impulses.

    • Focus: The chapter emphasizes chemical messengers.

B. Characteristics of Intercellular Communication

  • Always involves two or more cells.

  • Information is shared, generating a response in at least one cell.

C. Mechanisms of Intercellular Communication

  1. Direct Communication through Gap Junctions

    • Definition: Gap junctions are openings between adjacent cells, formed by plasma membrane proteins called connexins.

    • Function: Six connexins form a channel (connexon), allowing ions and small molecules to pass between cells.

    • Example: In cardiac muscle tissue, ion movement signals cells to contract synchronously.

    • Outcome: Allows synchronous cell function and electrical coupling.

  2. Indirect Communication

    • Process: Cells communicate via chemical messengers.

    • Definition of Ligands: Molecules that bind reversibly to proteins (receptors).

    • Target Cell: The cell that receives the chemical messenger.

    • Transport: Messengers move through interstitial fluid.

    • Response Strength Factors:

      • Concentration of messenger near the target cell.

      • Number of receptors available for binding.

      • Sensitivity of those receptors to the messenger.

III. Chemical Messengers

  • Chemical messengers can be classified by function or chemical structure.

A. Functional Classification of Chemical Messengers

  1. Paracrines

    • Function: Chemicals that communicate with nearby cells through diffusion.

    • Specific Type: Autocrines act on the same cell that secretes them, regulating their own secretions.

    • Types of Paracrines:

    1. Growth Factors - proteins that stimulate growth and development (e.g., nerve growth factor).

    2. Clotting Factors - proteins that stimulate blood clot formation.

    3. Cytokines - peptides released by immune cells to coordinate defense mechanisms.

      • Example: Histamine, released by mast cells, plays a role in allergic reactions and inflammation.

  2. Neurotransmitters

    • Definition: Chemicals released by neurons into interstitial fluid.

    • Release Mechanism: Released from axon terminals.

    • Targets: Other neurons, muscle fibers, glandular cells.

    • Synapse: The junction between a neuron and its target cell.

    • Example: Acetylcholine triggers contraction in skeletal muscle.

  3. Hormones

    • Definition: Chemicals released by endocrine glands that travel via the bloodstream to target cells.

    • Characteristics: Target cells must possess specific receptors.

    • Example: Insulin from the pancreas stimulates energy metabolism (glucose uptake).

    • Neurohormones: Released by neurosecretory cells, affecting distant cells (e.g., Vasopressin or Antidiuretic Hormone).

B. Chemical Classification of Chemical Messengers

  1. Chemical Structure Impact: Determines synthesis, release, and transport mechanisms.

    • Solubility Factors:

      • Ability to dissolve in plasma (mostly water).

      • Ability to cross the lipid bilayer of the plasma membrane.

    • Lipophilic (Hydrophobic) Messengers:

      • Lipid soluble, not plasma soluble. Easily pass through plasma membranes but not dissolved in plasma.

    • Lipophobic (Hydrophilic) Messengers:

      • Water-soluble, dissolve easily in plasma but cannot cross plasma membranes.

C. Major Classes of Chemical Messengers Based on Chemical Structure

  1. Amino Acid Messengers

    • Characteristics: Lipophobic, hydrophilic.

    • Function: Serve as neurotransmitters in the CNS.

    • Examples: Glutamate, aspartate, glycine, gamma-aminobutyric acid (GABA).

  2. Amine Messengers

    • Definition: Derived from amino groups. Predominantly lipophobic and hydrophilic.

    • Subclasses:

      • Catecholamines: Derived from tyrosine (e.g., dopamine, norepinephrine, epinephrine). Functions include neurotransmission (dopamine, norepinephrine) and hormone action (epinephrine).

      • Serotonin: Derived from tryptophan, influences mood, learning, and sleep.

      • Histamine: Produced from histidine, typically released via exocytosis.

  3. Peptide/Protein Messengers

    • Composition: 2-100 bonded amino acids; lipophobic and hydrophilic.

    • Production Location: Ribosomes on Rough ER.

  4. Steroid Messengers

    • Derivation: All derived from cholesterol; lipophilic and hydrophobic.

    • Examples: Testosterone, estrogen, aldosterone; synthesized in the smooth ER.

  5. Eicosanoid Messengers

    • Derivation: Derived from arachidonic acid; hydrophobic and lipophilic.

    • Characteristics: Cannot be stored; produced and used immediately as messengers.

    • Examples: Thromboxanes, prostaglandins.

D. Transport of Messengers

  1. Paracrines & Neurotransmitters: Released near target cells and actuate signal rapidly through diffusion. Quickly deactivated in interstitial fluid.

  2. Hormones: Transported in blood either in dissolved form or bound to carrier proteins.

    • Hydrophilic Hormones: Transported dissolved in blood.

  3. Hydrophobic Messengers: Often bound to carrier proteins for effective transport (99% bound).

    • Free Hormones: Less than 1% of free hormone in plasma can enter target cells and bind to receptors.

    • Equilibrium: Shifts to release more hormone upon leaving blood and entering cells.

  4. Half-Life: Duration required for half of the hormone in blood to degrade varies across different messengers.

IV. Signal Transduction by Chemical Messengers

A. Mechanism Overview

  • Chemical messengers exert signals by binding to target cell receptors located in:

    • Plasma membrane

    • Cytosol

    • Nucleus

B. Receptor Properties

  1. Specificity: Usually bind a single type of messenger.

  2. Interaction Nature: Brief and reversible encounters.

  3. Multiple Binding: A single messenger can bind to multiple receptors; a single cell can have multiple receptors for different messengers (e.g., skeletal muscle cells for insulin and acetylcholine).

C. Response Magnitude Influencers

  1. Messenger's Concentration: Higher concentrations increase interaction likelihood.

  2. Number of Receptors: Greater receptor presence enhances interaction probability.

  3. Receptor Affinity: Stronger affinity leads to a more substantial response.

D. Receptor Agonists vs. Antagonists

  1. Receptor Agonists: Bind to receptors to induce a response.

    • Example: Morphine, an opioid agonist, blocks pain signals.

  2. Receptor Antagonists: Bind to receptors to block a response.

    • Example: Naloxone prevents morphine from binding to its receptors.

E. Signal Transduction via Intracellular Receptors

  1. Mechanism: Lipophilic messengers easily pass through the plasma membrane to bind their receptors.

    • Transcription Activation: Typically bind to DNA to activate transcription, producing mRNA for protein translation.

  2. Receptor Locations:

    • Steroid Receptors: Found in cytoplasm and nucleus.

    • Thyroid Receptors: Located in the nucleus.

F. Signal Transduction via Membrane-Bound Receptors

  1. Mechanism: Lipophobic messengers cannot pass through the plasma membrane; hence, their receptors are on the extracellular side of the cell membrane.

G. Types of Receptors for Lipophobic Messengers

  1. Channel-Linked Receptors:

    • Functionality: Binding opens ion channels altering membrane potential leading to cell activity.

    • Examples and Speed: Fast Channels for immediate responses (e.g., acetylcholine in muscle fibers causes Na+ influx).

  2. Enzyme-Linked Receptors:

    • Functionality: Function as both receptors and enzymes; catalyze cellular reactions upon ligand binding.

    • Example: Insulin uses tyrosine kinases to initiate phosphorylating reactions, altering target cell activity.

  3. G-Linked Receptors:

    • Activation Mechanism: Binding activates G Proteins producing GTP which then activates effector molecules.

    • Response Duration: Can generate a sustained response compared to fast channels.

V. Long-Distance Communication via Nervous and Endocrine Systems

A. System Functionality

  • Nervous and endocrine systems need to communicate with distant body regions for homeostasis maintenance.

B. Neuronal Communication

  • Neurons transmit electrical signals (impulses/action potentials) across the length of the neuronic body.

  • Axon Terminals: Where neurotransmitters are released to adjacent target cells.

  • Characteristics: Ion channels involved typically open/close rapidly for short durations.

C. Endocrine Communication

  • Involves the release of hormones into the bloodstream, reaching distant target cells through the circulatory system.