19.7 b Electrical and Mechanical Events of Cardiac Muscle Cells

Electrical events involving propagation of the action potential at the sarcolemma of a cardiac muscle cell

1. Depolarization. An action potential propagated through the heart's conduction system (and via gap junctions) triggers the opening of fast voltage-gated Nat channels in the sarcolemma of a cardiac muscle cell.

Sufficient Nat enters the cardiac muscle cell to change the membrane potential from -90 mV to +30 mV. Voltage-gated Nat channels close to the inactivated state (see L section 12.6a). This change from a relatively negative membrane potential to a relatively positive membrane potential is depolarization.

2. Plateau. Depolarization triggers the opening of voltage-gated Kt channels, and Kt exits the cardiac muscle cells. There is a slight change in the membrane potential. Almost immediately, slow voltage-gated Caft channels in the sarcolemma also open, and Cat from the interstitial fluid enters the cardiac muscle cell. The entering of Caft stimulates the sarcoplasmic reticulum (SR) to release more Ca2+ (greater than 80% of the Caft is released from the SR). The exit of positively charged Kt from the cytosol into the interstitial fluid and the simultaneous entrance of positively charged Caft into the cytosol (from the interstitial fluid and SR) result in no voltage change at the sarcolemma. Thus, the sarcolemma of the cardiac muscle cell remains in a depolarized state. This "leveling off" is referred to as the plateau.

3. Repolarization. Voltage-gated Caft channels then close, and Kt channels remain open to complete repolarization. Sufficient Kt exits the cardiac muscle cell to change the membrane potential from being relatively positive back to -90 mV (the RMP). This reversal of charge is called repolarization. It is repolarization that allows a cardiac muscle cell to propagate a new action potential along its sarcolemma when the cardiac muscle is stimulated again.

Depolarization occurs with

Na+ inflow into the cardiac muscle cells

The plateau(which delays repolarization is due to

both the outflow of K+ from the cytosol to the interstitial fluid and the simultaneous inflow of Ca2+ into the cytosol(from the interstitial fluid and SR)

Repolarization is due

only to the outflow of K+ from the cytosol to the interstitial fluid.

these 3 events of an action potential are

repeated along the length of sarcolemma as the voltage-gated channels are triggered to open sequentially.

Conductivity

The propagation of the electrical signal along the plasma membrane

Conductivity serves to

functionally connect the plasma membrane (sarcolemma) of the cardiac muscle cell (where stimulation occurs) to the interior of the cardiac muscle cell (where contraction occurs)

Summary of the repeating steps of cardiac muscle contraction involving sarcomeres (called cross bridge cycling) is as follows

• Crossbridge formation: Myosin heads attached to actin form a crossbridge between the thick and thin filaments.

• Powerstroke: The myosin head swivels (the powerstroke), which pulls the thin filament past the thick filament a short distance, which decreases the width of asarcomere.

• Release of myosin head: ATP binds to the myosin head to release the myosin head from actin.

• Reset of myosin head: ATP is split by myosin ATPase, providing the energy to reset the myosin head.

• Cardiac muscle relaxation is initiated with the closing of voltage-gated Ca2+ channels (as described in step 3 of L figure 19.19). The continuous removal of Calt from the cytosol occurs by (a) plasma membrane Calt pumps, which move Caft out of the cardiac muscle cell to the interstitial fluid, and (b) sarcoplasmic reticulum Cat pumps, which move Cat into the sarcoplasmic reticulum. This removal decreases calcium levels within the cytosol. Calcium is released from troponin with the subsequent decrease in crossbridges between the thin and thick filaments. Sarcomeres return to their resting length as the cardiac muscle cell now relaxes (see L' section 10.3d).

Cardiac muscle cells cannot exhibit

tetany

Tetany

A sustained muscle contraction without relaxation.

The distinction of tenancy is ___ for the heart to be able to function as a____ to move blood through the cardiovascular system.

critical; mechanical pump

Muscle tension

The red line on each group, includes muscle contraction and muscle relaxation.

These mechanical events are due to sarcomeres within these muscle cells first shortening during contraction and then lengthening during relaxation.

an Action potential

shown as a blue line on the graph includes both depolarization and depolarization.

These electrical events are occurring at the sarcolemmas of these muscles cells

Refractory period

The shaded green region on each graph, represents the time when the muscle cannot be restimulated to contract.

Is dependent upon how quickly depolarization and depolarization occurs at the sarcolemma; the faster that depolarization occurs, the shorter the refractory period.

Muscle contraction and relaxation associated with sarcomeres within cardiac muscle cells occur over approximately

250 milliseconds

Repolarization at their sarcolemmas does not occur immediately following depolarization which results in a refractory period that is

relatively long; almost 250 milliseconds

The relatively long refractory period is due

the plateau event at the sarcolemma, which delays repolarization.

while sarcomeres are still contracting and relaxing within cardiac muscle,

the depolarization of the sarcolemmas has not occurred to allowed for new stimulation

Because of the relatively long refractory period, cardiac muscle has an

extended period of time in which it cannot be restimulated.

This delay in restimualtion allows time for

sarcomeres of cardiac muscles cells within the heart chamber wall to fully contract and relax before being stimulated again.

Cardiac muscle composing the myocardium of the heart chamber walls do not exhibit ___ but continue to both ___ and ___ following each stimulation (an essential feature for the heart to pump the blood)

tetany; contract; relax