Cardiac Action Potential: Depolarization

Study Notes

Cardiac Action Potential

Depolarization

The process of depolarization in cardiac cells is triggered by an electrical impulse from the sinoatrial (SA) node.
The rapid depolarization phase is mediated by the opening of fast sodium channels, allowing an influx of sodium ions into the cell.
This rapid influx of positive charge causes the membrane potential to rapidly become less negative, reaching a threshold of around -70mV.
Depolarization is further facilitated by the opening of calcium channels, which also allow an influx of positively charged calcium ions.

Repolarization

The repolarization phase is characterized by the closing of sodium channels and the opening of potassium channels.
The efflux of potassium ions out of the cell causes the membrane potential to become more negative, returning to the resting state.
The slow repolarization phase is mediated by the opening of slow potassium channels, which allow a gradual efflux of potassium ions.
Repolarization is complete when the membrane potential returns to its resting state, around -90mV.

Refractory Period

The refractory period is the time during which the cardiac cell is unable to generate another action potential.
There are two types of refractory periods:
- Absolute refractory period: The period during which the cardiac cell is completely unresponsive to stimuli, lasting around 200-250ms.
- Relative refractory period: The period during which the cardiac cell is partially responsive to stimuli, lasting around 50-100ms.
The refractory period is essential for allowing the cardiac cell to recover and prepare for the next action potential.

Ion Channels

Ion channels are integral membrane proteins that regulate the flow of ions across the cell membrane.
There are several types of ion channels involved in the cardiac action potential:
- Fast sodium channels: Responsible for the rapid depolarization phase.
- Calcium channels: Involved in the depolarization phase and contraction of the cardiac muscle.
- Potassium channels: Responsible for the repolarization phase.
- Slow potassium channels: Involved in the slow repolarization phase.
Ion channels are gated, meaning they can open and close in response to changes in the membrane potential.

Cardiac Action Potential

Depolarization

Triggered by an electrical impulse from the sinoatrial (SA) node
Rapid depolarization phase mediated by opening of fast sodium channels, allowing influx of sodium ions
Membrane potential rapidly becomes less negative, reaching a threshold of around -70mV
Depolarization facilitated by opening of calcium channels, allowing influx of positively charged calcium ions

Repolarization

Characterized by closing of sodium channels and opening of potassium channels
Efflux of potassium ions out of the cell causes membrane potential to become more negative
Membrane potential returns to resting state, around -90mV
Slow repolarization phase mediated by opening of slow potassium channels

Refractory Period

Time during which the cardiac cell is unable to generate another action potential
Two types:
- Absolute refractory period: Cardiac cell completely unresponsive to stimuli, lasting around 200-250ms
- Relative refractory period: Cardiac cell partially responsive to stimuli, lasting around 50-100ms
Essential for allowing the cardiac cell to recover and prepare for the next action potential

Ion Channels

Integral membrane proteins regulating flow of ions across the cell membrane
Several types involved in the cardiac action potential:
- Fast sodium channels: Responsible for rapid depolarization phase
- Calcium channels: Involved in depolarization phase and contraction of the cardiac muscle
- Potassium channels: Responsible for repolarization phase
- Slow potassium channels: Involved in slow repolarization phase
Ion channels are gated, opening and closing in response to changes in the membrane potential

Description

This quiz covers the process of depolarization in cardiac cells, triggered by the sinoatrial node, and the role of sodium channels in rapid depolarization. Learn about the membrane potential and threshold levels.