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).