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Cardiac Physiology Lecture – Vocabulary Review

Learning Philosophy & Class Expectations

  • Instructor stresses understanding before memorizing
    • Rewrite notes only after concepts are clear.
    • Students are encouraged to stop the lecture and ask questions.

Review: Core Characteristics of Muscle Tissue

  • Contractility – ability to shorten/produce tension ("they move").
  • Extensibility – ability to stretch/lengthen.
  • Elasticity – passive recoil after being stretched ("rubber-band" property).
  • Excitability – ability to respond to electrical/chemical stimulus.
  • Autorhythmicity (a.k.a. self-excitability)
    • Unique to cardiac muscle and select smooth muscle (e.g., uterus, GI tract).
    • Cells spontaneously depolarize at a regular rhythm without neural input.

Myocardium: Cell Types & Percentages

  • Heart mass ≈ 65 % muscle (myocardium).
  • Two functional cell classes:
    1. Contractile Myocytes (≈ 99 %)
    • Full actin–myosin apparatus; their job: squeeze blood.
    1. Conductile / Autorhythmic Myocytes (≈ 1 %)
    • Modified muscle cells; sparse sarcomeres.
    • Do NOT contribute to force; instead conduct electrical impulses to trigger the 99 %.

Intrinsic Conductive (Electrical) Pathway

  1. SA (Sino-Atrial) Node – natural pacemaker

    • Location: roof of right atrium near former sinus venosus.
    • Autorhythmic rate ≈ 100\;\text{dep. min}^{-1} (≈ 100 bpm) in absence of autonomic or hormonal influence.
    • Fastest self-depolarizing tissue → sets overall heart rhythm.
    • Depolarization spreads through internodal pathways over both atria simultaneously → atrial contraction delivers final ~30 % of ventricular fill.

  2. Annulus Fibrosus (Ring of Fibrous Tissue)

    • Dense connective tissue rings surrounding AV, aortic & pulmonary valve orifices.
    • Provides rigid, non-stretch frame so cusps open/close consistently.
    • Electrical insulator → impulses cannot jump directly from atria to ventricles.

  3. AV (Atrioventricular) Node

    • Location: floor of right atrium (just above tricuspid annulus).
    • Three functional roles:
    1. Receives inflowing atrial impulse.
    2. Delays it (traffic-jam effect) – conduction velocity 0.03\;\text{m\,s}^{-1} (10× slower than ordinary myocardium).
      • Purpose: provides time for ventricles to complete filling before they are stimulated.
    3. Passes impulse into ventricles via specialized bundle.

  4. AV Bundle / Bundle of His

    • Only normal electrical bridge through the annulus fibrosus.
    • Descends through upper interventricular septum.

  5. Bundle Branches

    • Left & Right subdivisions of Bundle of His within septum.
    • Ensure simultaneous stimulation of both ventricles.

  6. Purkinje Fibers (a.k.a. Sub-Endocardial Conductive Network)

    • Tree-like network faning from apex upward along ventricular walls & papillary muscles.
    • Conduction velocity 3\;\text{m\,s}^{-1} (10× faster than ordinary myocardium).
    • Rapidly deliver impulse to apex first, so contraction proceeds bottom → up (tooth-paste analogy) and blood is ejected toward semilunar valves.

Conduction Velocity Cheat-Sheet

  • Ordinary myocardium: 0.3\;\text{m\,s}^{-1} (≈ 1 ft s⁻¹).
  • AV node: 10× slower ⇒ 0.03\;\text{m\,s}^{-1}.
  • Purkinje system (bundles + fibers): 10× faster ⇒ 3\;\text{m\,s}^{-1}.

Sequence Summary

  1. SA node fires → atrial depolarization/contract.
  2. Impulse halted at annulus fibrosus → must enter AV node.
  3. AV node delays