Action Potentials - Part 4: Myocardial Contractile Cells

Summary

This video explores the action potentials in myocardial contractile cells, highlighting how tetany (sustained contraction) is prevented to avoid life-threatening conditions. It explains the sequence of events during the action potential, including depolarization, sodium and potassium ion movements, and the plateau phase where calcium influx balances repolarization. A longer refractory period ensures the cells cannot be re-excited too soon, preventing abnormal contractions. The video also compares these processes with neurons and skeletal muscle cells, emphasizing the unique properties of myocardial action potentials that protect the heart's rhythm.

Takeaways

• 😀 Tetany is prevented in cardiac muscle cells by a longer refractory period that prevents re-excitation before muscle contraction is over.
• 😀 The resting potential of myocardial contractile cells is around -90 millivolts, mainly maintained by potassium ion efflux through delayed rectifier potassium channels.
• 😀 Action potentials in myocardial contractile cells are not triggered by neurotransmitters but by electrochemical stimulation from neighboring cells.
• 😀 Depolarization occurs when positive ions move into myocardial contractile cells through gap junctions, causing the membrane potential to increase.
• 😀 Sodium channels open at around -65 millivolts, allowing rapid influx of sodium ions, leading to rapid depolarization.
• 😀 The maximum attainable membrane potential in myocardial contractile cells is approximately +20 millivolts during depolarization.
• 😀 Partial repolarization happens initially in myocardial contractile cells, involving potassium efflux and chloride influx.
• 😀 The plateau phase in repolarization lasts around 200 to 400 milliseconds and is due to a balance between repolarizing currents and a slow calcium influx.
• 😀 L-type calcium channels, activated at -40 millivolts, play a key role in the plateau phase of myocardial action potentials.
• 😀 After the plateau phase, the repolarizing ion currents dominate, leading to final repolarization through potassium and chloride channels.
• 😀 Myocardial contractile cells do not undergo afterhyperpolarization like skeletal muscle cells, but instead experience a long absolute refractory period to ensure proper cell functioning.

Q & A

What prevents tetany from occurring in myocardial contractile cells?

Tetany in myocardial contractile cells is prevented by a long refractory period, which ensures that the muscle cannot be re-excited before it has fully relaxed, thus preventing sustained contraction.

What is the resting potential of myocardial contractile cells, and how is it maintained?

The resting potential of myocardial contractile cells is around -90 millivolts, and it is primarily maintained by potassium ion efflux through delayed rectifier potassium channels.

How is the action potential in myocardial contractile cells initiated?

The action potential in myocardial contractile cells is initiated by electrochemical stimulation from a neighboring cell. This leads to the influx of positive ions through gap junctions, which causes depolarization.

What role do voltage-gated sodium channels play during the action potential?

Voltage-gated sodium channels open when the membrane potential reaches about -65 millivolts, allowing a rapid influx of sodium ions, which causes the rapid depolarization of the myocardial contractile cell.

What happens after the sodium channels close in myocardial contractile cells?

Once the sodium channels close, the rapid depolarization stops, and the cell begins to partially repolarize. This is accompanied by the efflux of potassium ions and the influx of chloride ions.

What is the plateau phase, and why is it important?

The plateau phase occurs after partial repolarization, where the membrane potential is maintained around 0 millivolts. It is important because it results from a balance between potassium and chloride efflux and calcium influx, which prolongs the action potential and prevents tetany.

How do L-type calcium channels contribute to the action potential?

L-type calcium channels open when the membrane potential exceeds -40 millivolts but with a delay. The influx of calcium ions through these channels during the plateau phase counteracts the repolarizing ion currents and helps maintain the membrane potential.

What occurs once the calcium channels close during the action potential?

Once the calcium channels close, potassium efflux dominates, which leads to the final repolarization of the myocardial contractile cell.

How does the refractory period ensure the correct functioning of myocardial contractile cells?

The long refractory period prevents the myocardial contractile cell from being re-excited during contraction. This ensures that the cell completes its contraction and relaxation phases, contributing to proper heart function.

What is the difference between the refractory period and afterhyperpolarization in myocardial contractile cells?

Unlike skeletal muscle cells, myocardial contractile cells do not experience afterhyperpolarization. Instead, they have a long refractory period, during which the sodium channels are inactive, ensuring that the cell is not re-excited until its resting potential is restored.