Answer
**Cardiocyte Resting Potential:**
The resting membrane potential of a cardiomyocyte (cardiac muscle cell) is typically around -90 to -95 millivolts (mV), which means the inside of the cell is more negatively charged compared to the outside.
**Actions of Gated Sodium, Calcium, and Potassium Channels:**
- **Sodium Channels:** During depolarization, voltage-gated sodium channels open, allowing an influx of sodium ions (Na+). This depolarizes the membrane, leading to the rising phase of the action potential.
- **Calcium Channels:** Calcium channels open more slowly than sodium channels. They contribute to a sustained influx of calcium ions (Ca2+) during the plateau phase of the action potential, which is crucial for contraction.
- **Potassium Channels:** Potassium channels play a role in repolarization. They open during the falling phase of the action potential, allowing efflux of potassium ions (K+), which restores the cell's negative resting potential.
**Differences in Myocardial Action Potentials vs. Neuronal Action Potentials:**
- **Myocardial Action Potentials:** Myocardial action potentials have a longer duration compared to neuronal action potentials. The distinctive feature of cardiac action potentials is the presence of a plateau phase, which is absent in neurons. The plateau phase is caused by the sustained influx of calcium ions through slow calcium channels.
- **Neuronal Action Potentials:** Neurons have rapid and brief action potentials. They have a clear distinction between depolarization and repolarization phases.
**Plateau and Unusually Long Refractory Period:**
- **Plateau Phase:** The plateau phase in myocardial action potentials is characterized by a prolonged depolarization due to the balance between slow calcium influx and potassium efflux. This phase prolongs the refractory period, preventing rapid re-excitation and allowing sufficient time for ventricular contraction.
- **Long Refractory Period:** The unusually long refractory period in cardiac muscle ensures that the muscle cannot be re-excited until the action potential is completed. This is essential to prevent tetanic contractions and allow the heart adequate time to relax and fill with blood before the next contraction.
**Supporting Pumping Effectiveness of the Heart:**
The plateau phase and the long refractory period are crucial for the pumping effectiveness of the heart:
- The plateau maintains a sustained contraction by preventing rapid repolarization and allowing the heart muscle to fully contract before relaxing.
- The long refractory period prevents tetanic contractions and allows for complete relaxation of the heart between contractions, facilitating efficient filling of the chambers.
In summary, the myocardial action potential involves the actions of sodium, calcium, and potassium channels. The presence of a plateau phase, the distinct shape of the action potential, and the long refractory period contribute to the coordinated and effective pumping action of the heart by ensuring proper contraction, relaxation, and filling of the cardiac chambers.
Work Step by Step
**Cardiocyte Resting Potential:**
The resting membrane potential of a cardiomyocyte (cardiac muscle cell) is typically around -90 to -95 millivolts (mV), which means the inside of the cell is more negatively charged compared to the outside.
**Actions of Gated Sodium, Calcium, and Potassium Channels:**
- **Sodium Channels:** During depolarization, voltage-gated sodium channels open, allowing an influx of sodium ions (Na+). This depolarizes the membrane, leading to the rising phase of the action potential.
- **Calcium Channels:** Calcium channels open more slowly than sodium channels. They contribute to a sustained influx of calcium ions (Ca2+) during the plateau phase of the action potential, which is crucial for contraction.
- **Potassium Channels:** Potassium channels play a role in repolarization. They open during the falling phase of the action potential, allowing efflux of potassium ions (K+), which restores the cell's negative resting potential.
**Differences in Myocardial Action Potentials vs. Neuronal Action Potentials:**
- **Myocardial Action Potentials:** Myocardial action potentials have a longer duration compared to neuronal action potentials. The distinctive feature of cardiac action potentials is the presence of a plateau phase, which is absent in neurons. The plateau phase is caused by the sustained influx of calcium ions through slow calcium channels.
- **Neuronal Action Potentials:** Neurons have rapid and brief action potentials. They have a clear distinction between depolarization and repolarization phases.
**Plateau and Unusually Long Refractory Period:**
- **Plateau Phase:** The plateau phase in myocardial action potentials is characterized by a prolonged depolarization due to the balance between slow calcium influx and potassium efflux. This phase prolongs the refractory period, preventing rapid re-excitation and allowing sufficient time for ventricular contraction.
- **Long Refractory Period:** The unusually long refractory period in cardiac muscle ensures that the muscle cannot be re-excited until the action potential is completed. This is essential to prevent tetanic contractions and allow the heart adequate time to relax and fill with blood before the next contraction.
**Supporting Pumping Effectiveness of the Heart:**
The plateau phase and the long refractory period are crucial for the pumping effectiveness of the heart:
- The plateau maintains a sustained contraction by preventing rapid repolarization and allowing the heart muscle to fully contract before relaxing.
- The long refractory period prevents tetanic contractions and allows for complete relaxation of the heart between contractions, facilitating efficient filling of the chambers.
In summary, the myocardial action potential involves the actions of sodium, calcium, and potassium channels. The presence of a plateau phase, the distinct shape of the action potential, and the long refractory period contribute to the coordinated and effective pumping action of the heart by ensuring proper contraction, relaxation, and filling of the cardiac chambers.
