Myocardium: Structure, Function, and Clinical Significance

The Cardiac Muscle: Engine of Circulation

The myocardium is the thick middle layer of the heart wall, composed of specialized striated muscle cells (cardiomyocytes) arranged in complex spiral architecture.

Histologic Organization

• Cardiomyocytes: Branching, mononucleated, interconnected by intercalated discs
• Intercalated discs: Desmosomes (mechanical), fascia adherens, gap junctions (electrical — connexin 43)
• Sarcomeres: Contractile units (Z-line to Z-line); α-actinin, titin, myosin, actin
• T-tubules / SR: Excitation-contraction coupling (dyad: L-type Ca²⁺ channel + RyR2)
• Mitochondria: 30-40% volume — high oxidative capacity

Seven Essential Functions

Function	Mechanism	Clinical Correlate
1. Contraction/Relaxation	Cross-bridge cycling; Ca²⁺ transient	Systolic/diastolic heart failure
2. Force Generation	Frank-Starling: preload → sarcomere stretch → force	Volume overload, diastolic dysfunction
3. Electrical Propagation	Gap junctions → syncytial conduction; Purkinje network	Arrhythmias, conduction blocks
4. Valve Regulation	Pressure gradients from contraction close valves	MR, AR, MS, AS from myocardial dysfunction
5. Structural Integrity	Collagen matrix (types I, III) + titin elasticity	Dilated cardiomyopathy, rupture post-MI
6. Blood Pressure Maintenance	CO × SVR; CO = HR × SV (myocardial)	Cardiogenic shock, hypertensive heart disease
7. O₂/Nutrient Delivery	Coronary perfusion (diastole) → systemic delivery	Ischemia, infarction, hibernating myocardium

Metabolic Uniqueness

• Obligate aerobic: Fatty acids (60-90%), glucose, lactate, ketones
• No glycogen reserves (minimal)
• High O₂ extraction (70-80%) → flow-dependent supply
• Ischemia → rapid ATP depletion → contractile failure in seconds

Pathology Highlights

• Infarction: Coagulative necrosis → inflammation → scar (fibrosis)
• Cardiomyopathies: Dilated, hypertrophic, restrictive, ARVC
• Myocarditis: Viral, autoimmune, toxic — inflammatory infiltrate
• Hypertrophy: Pressure (concentric) vs. volume (eccentric) overload

Assessment Modalities

• Echo: Wall motion, thickness, strain, EF
• CMR: Tissue characterization (LGE, T1/T2 mapping), gold standard mass/volume
• PET: Viability (FDG), perfusion, inflammation
• Biomarkers: Troponin (injury), BNP/NT-proBNP (stretch), Galectin-3, ST2 (fibrosis)

The myocardium is not merely a pump — it's a living, electrically active, metabolically demanding, self-regulating organ whose failure cascades through every system.