Cardiac Muscle: Structure, Intercalated Discs & Autorhythmicity

Cardiac muscle (myocardium) is a striated, involuntary muscle specialized for continuous, rhythmic contraction.

Unique Structural Features

• Cardiomyocytes: Branching, mononucleated (usually), centrally placed nucleus.
• Intercalated discs: End-to-end junctions with:
  • Desmosomes — mechanical adhesion (resist shear).
  • Fascia adherens — anchor actin filaments.
  • Gap junctions (connexin 43) — low-resistance electrical coupling → synchronized contraction.
• T-tubules: At Z-lines (not A-I junction); larger, fewer than skeletal.
• Sarcoplasmic reticulum: Less developed; relies more on trans-sarcolemmal Ca²⁺ influx (L-type channels).
• Mitochondria: 30–35% volume — high aerobic capacity.

Functional Specializations

• Autorhythmicity: SA node pacemaker cells (If funny current) → spontaneous depolarization → wave of excitation.
• Long action potential (plateau phase via L-type Ca²⁺) → prolonged refractory period → prevents tetanus.
• Frank-Starling mechanism: Stretch → ↑ Ca²⁺ sensitivity → ↑ force (intrinsic regulation).
• Autonomic modulation: Sympathetic (β₁: ↑HR, ↑contractility, ↑conduction) / Parasympathetic (M₂: ↓HR, ↓AV conduction).

Clinical Relevance

• Ischemia → ATP depletion → contractile failure, arrhythmia.
• Hypertrophy (pressure/volume overload) → initial compensation → eventual failure.
• Limited regeneration → fibrosis post-MI.