Human Physiology Chapter 6: Communication, Integration, and Homeostasis

Cell Communication Fundamentals

  • Physiological Signals: Electrical signals involve changes in membrane potential; chemical signals (ligands) are secreted into extracellular fluid (ECF).
  • Local Communication: Includes gap junctions (direct cytoplasmic bridges), contact-dependent signals (cell adhesion molecules), and autocrine/paracrine signals (diffusion through ECF).
  • Long-Distance Communication: Facilitated by the endocrine system (hormones) and the nervous system (neurotransmitters, neuromodulators, or neurohormones).
  • Cytokines: Peptides that function as both local and long-distance signals; unlike hormones, they are produced on demand by any nucleated cell rather than specialized glands.

Signal Transduction Pathways

  • Receptor Locations: Lipophilic ligands bind to intracellular receptors (cytosol or nucleus) to alter gene activity; lipophobic ligands bind to cell membrane receptors.
  • Pathway Steps: First messenger (external signal) $\rightarrow$ Receptor protein $\rightarrow$ Intracellular signal molecules (second messengers) $\rightarrow$ Target proteins $\rightarrow$ Cellular response.
  • Membrane Receptor Categories: Includes receptor-channels, G protein-coupled receptors (GPCR), receptor-enzymes (e.g., tyrosine kinase), and integrin receptors.
  • Transduction and Amplification: Signal transduction converts an extracellular signal to an intracellular one; amplification uses amplifier enzymes to create many second messenger molecules from a single ligand.
  • Key Second Messengers:
    • cAMP: Activated by adenylyl cyclase via GPCR.
    • DAG and IP3IP_3: Produced via phospholipase C (PLC) activity.
    • Ion Flow: Rapid responses caused by changing permeability to Na+Na^+, K+K^+, or Cl−Cl^-.

Novel Signal Molecules

  • Calcium (Ca2+Ca^{2+}): An essential intracellular messenger; binds to calmodulin or other proteins to trigger exocytosis or movement.
  • Soluble Gases: Nitric oxide (NO), Carbon monoxide (CO), and Hydrogen sulfide (H2SH_2S) act as short-lived paracrine or autocrine signals.
  • Arachidonic Acid Cascade: Produces eicosanoids such as leukotrienes and prostanoids (prostaglandins and thromboxanes) which mediate inflammation and pain.

Modulation and Control Systems

  • Receptor Dynamics: Exhibit specificity, competition, and saturation. Agonists mimic ligands; antagonists block receptor activity.
  • Response Regulation: Cells adjust sensitivity via up-regulation (increasing receptors) or down-regulation (decreasing receptors or binding affinity).
  • Cannon's Postulates: Concepts of tonic control (continuous signal modulation) and antagonistic control (opposing signals like insulin and glucagon) maintain homeostasis.
  • Reflex Pathway Steps: Stimulus $\rightarrow$ Sensor $\rightarrow$ Input Signal (afferent) $\rightarrow$ Integrating Center $\rightarrow$ Output Signal (efferent) $\rightarrow$ Target $\rightarrow$ Response.

Neural versus Endocrine Regulation

  • Specificity: Neural is highly specific (direct connections); endocrine is widespread (blood distribution).
  • Nature of Signal: Neural uses electrical and chemical (neurotransmitters); endocrine uses only chemical (hormones).
  • Speed and Duration: Neural is fast and short-lived; endocrine is slower but longer-lasting.
  • Coding: Neural intensity is coded by signal frequency; endocrine intensity is coded by the amount of hormone secreted.