Contractile Cell Action Potentials + Excitation-Contraction Coupling

Contractile Cells

Make up 99% of cardiac muscle cells; Action potentials are similar to those of skeletal muscle

Contractile Cell Action Potentials

Pacemaker cell potentials are transmitted via intercalated discs to contractile cells

Rapid Depolarization Phase

Voltage-gated sodium ion channels in the sarcolemma are activated; sodium ions flow in and depolarize the membrane; much faster than pacemaker cell depolarization because calcium ion channels activate more slowly

Initial Repolarization Phase

Due to the abrupt inactivation of the sodium ion channels and a very small outflow of potassium ions through selected potassium ion channels that are open briefly

Plateau Phase

Depolarization is sustained at about 0 mV, a period known as the Plateau Phase; this is due to the slow opening of calcium ion channels which flow in as potassium ions leave, resulting in little net change in membrane potential

Repolarization Phase

Both sodium and calcium ion channels return to their resting states and most potassium ion channels open; the membrane potential returns to its resting value of about −85 mV

Contractile Cell vs. Skeletal Muscle Action Potentials

Contractile cell action potentials resemble those of skeletal muscle fibers with the exception of the plateau phase; compare the timing of action potentials in skeletal muscle fibers and cardiac contractile cells

Importance of the Plateau Phase

If cardiac action potentials lasted only about 1–5 msec, our resting heart rate would be 15 times faster than it should be; plateau phase lengthens the action potential to about 200–300 msec which slows the heart rate and allows the contraction to develop more force

Effective Refractory Period

Time during which an excitable cell cannot be stimulated to contract again; This prevents the cells from entering a state of tetany (sustained contraction) and allows the heart to relax and the ventricles to fill with blood

Excitation-Contraction Coupling

Occurs by the sliding-filament mechanism; Depolarization propagates through the sarcolemma into the T-tubules, which causes the sarcoplasmic reticulum to release calcium ions that bind to troponin, allowing actin and myosin to bind and a cross-bridge cycle to begin

Calcium Ion-Induced Calcium Ion Release

Cardiac muscle cells need additional calcium ions from the ECF due to a less extensive sarcoplasmic reticulum; calcium from the ECF stimulates more calcium ion release from the sarcoplasmic reticulum in a phenomenon known as Calcium Ion-Induced Calcium Ion Release