AB

Week 2: Cardiac Muscle

Body Membranes

  • Serous: double-layered; pleura, peritoneum, pericardium.
  • Mucous: line tracts open to exterior (e.g., digestive).
  • Synovial: inside joints; secrete lubricating synovial fluid.
  • Cutaneous: skin.

Heart Location & Protective Sac

  • Sits in mediastinum, apex angled left.
  • Pericardium = fibrous outer layer + serous inner layer.
    • Parietal layer lines fibrous sac.
    • Visceral layer (epicardium) adheres to heart surface.
    • Pericardial cavity holds lubricating fluid; cushions, prevents over-expansion.
  • Disorders:
    • Pericarditis – inflamed membranes ➔ friction rub, chest pain.
    • Cardiac tamponade – fluid build-up compresses heart, ↓ output.

Heart Wall

  • Epicardium: connective tissue, vessels, makes pericardial fluid.
  • Myocardium: cardiac muscle; thickness ∝ workload (left ventricle thickest).
  • Endocardium: endothelium + CT; lines chambers/valves, smooth flow.

Chambers & Septa

  • Atria (receiving): right = deoxygenated from body, left = oxygenated from lungs; separated by interatrial septum; auricles add volume; right atrium shows pectinate muscles.
  • Ventricles (pumping): right ➔ lungs, left ➔ systemic organs; separated by interventricular septum; trabeculae carneae prevent wall adhesion.

Valves (One-Way Flow)

  • Atrioventricular (AV): right = tricuspid, left = bicuspid/mitral; cusps anchored by chordae tendineae to papillary muscles to stop prolapse.
  • Semilunar: pulmonary (right ventricle ➔ pulmonary trunk), aortic (left ventricle ➔ aorta); prevent backflow into ventricles.

Circulatory Circuits

  • Pulmonary: right ventricle ➔ lungs ➔ left atrium.
  • Systemic: left ventricle ➔ body ➔ right atrium.

Coronary Circulation

  • Arterial:
    • Left coronary ➔ anterior interventricular (LAD) & circumflex.
    • Right coronary ➔ right marginal & posterior interventricular; supplies SA node.
  • Venous: great, middle, left marginal veins ➔ coronary sinus ➔ right atrium.

Cardiac Muscle Tissue

  • Striated, involuntary, branched cardiomyocytes.
  • Intercalated discs: gap junctions (rapid impulse) + desmosomes (strength).

Electrical Activity

Pacemaker (SA-node) Cells

  • Unstable potential starts at -60\ \text{mV}, slow Na^+ leak.
  • Threshold -40\ \text{mV} ➔ Ca^{2+} influx.
  • Peak > 0\ \text{mV} ➔ K^+ efflux ➔ repolarize; cycle sets rhythm.

Ventricular Contractile Cells

  • Rest -90\ \text{mV} (stable).
  • Stimulus ➔ rapid Na^+ entry to ≈ +30\ \text{mV}.
  • Plateau 200\text{–}250\ \text{ms}: Ca^{2+} influx sustains depolarization.
  • Repolarization: K^+ exit.

Refractory Periods

  • Absolute (≈ 250\ \text{ms}): no new AP possible; prevents tetanus, ensures fill time.
  • Relative: stronger-than-normal stimulus can trigger AP; protects rhythm.

Key Numbers & Facts

  • 4 chambers; 2 circuits.
  • Left ventricle wall thickest; right ventricle thinner; atria thinnest.
  • Pericardial cavity fluid minimizes friction; NOT for oxygen/nutrient delivery.
  • Chordae tendineae anchor AV valve cusps; do not open/close valves (pressure does).