What happens during the depolarization phase of cardiac muscle cells?

What happens during the depolarization phase of cardiac muscle cells?

• A. Slow calcium influx through L-type channels
• B. Fast voltage-gated Na+ channels open, sodium enters, changing from -90 mV to +30 mV
• C. Potassium outflow dominates
• D. Chloride channels open

Answer: (B)

Depolarization is driven by a rapid inward current through fast voltage-gated sodium channels in cardiac muscle. When the resting membrane potential is around -90 mV, these channels open quickly in response to a stimulus, allowing Na+ to flood into the cell. This rapid influx pushes the membrane potential from negative toward positive, producing a quick upstroke to about +30 mV. This swift, sizable change is the hallmark of depolarization. Calcium entering through L-type channels mainly supports the later plateau phase, where Ca2+ influx helps balance K+ efflux to sustain a longer depolarized state. Potassium moving out of the cell dominates during repolarization, driving the membrane potential back toward its resting negative value. Chloride channels aren’t the primary drivers of depolarization in this context.

Depolarization is driven by a rapid inward current through fast voltage-gated sodium channels in cardiac muscle. When the resting membrane potential is around -90 mV, these channels open quickly in response to a stimulus, allowing Na+ to flood into the cell. This rapid influx pushes the membrane potential from negative toward positive, producing a quick upstroke to about +30 mV. This swift, sizable change is the hallmark of depolarization.

Calcium entering through L-type channels mainly supports the later plateau phase, where Ca2+ influx helps balance K+ efflux to sustain a longer depolarized state. Potassium moving out of the cell dominates during repolarization, driving the membrane potential back toward its resting negative value. Chloride channels aren’t the primary drivers of depolarization in this context.