Mechanism of Contraction of Contractile Cardiac Muscle Fibers

1. Na+ influx from extracellular space, causes positive feedback opening of voltage-gated Na+ channels;membrane potential quickly depolarizes (-90 to +30); Na+ channels close within 3 ms of opening.
2. Depolarization causes release of Ca++ from sarcoplasmic reticulum (as in skeletal muscle), allowing sliding actin and myosin to proceed.
   
3. Depolarization ALSO causes opening of slow Ca++ channels on the membrane (special to cardiac muscle), further increasing Ca++ influx and activation of filaments. This causes more prolonged depolarization than in skeletal muscle, resulting in a plateau action potential, rather than a "spiked" action potential (as in skeletal muscle cells).

Differences Between Skeletal & Cardiac MUSCLE Contraction

1. All-or-None Law - Gap junctions allow all cardiac muscle cells to be linked electrochemically, so that activation of a small group of cells spreads like a wave throughout the entire heart. This is essential for "synchronistic" contraction of the heart as opposed to skeletal muscle.
   
2. Automicity (Autorhythmicity) - some skeletal muscle cells are "self-excitable" (see below, allowing for rhythmic waves of contraction to adjacent cells throughout the heart. Skeletal muscle cells must be stimulated by independent motor neurons as part of a motor unit.
   
3. Length of Absolute Refractory Period - The absolute refractory period of cardiac muscle cells is much longer than skeletal muscle cells (250 ms vs. 2-3 ms), preventing wave summation and tetanic contractions which would cause the heart to stop pumping rhythmically.

Category: Physiology Notes