Phase 0 in pacemaker cells occurs faster than Phase 0 in cardiac cells.

Phase 0 in pacemaker cells occurs faster than Phase 0 in cardiac cells.

• A. True
• B. False
• C. Cannot be determined
• D. Not applicable.

Answer: (B)

Phase 0 is the rapid depolarization phase of the cardiac action potential. In ventricular and atrial myocytes, this upstroke is very fast because it’s driven by the rapid influx of sodium through fast voltage-gated Na channels, giving a steep rise in membrane potential. Pacemaker cells in the SA and AV nodes, however, lack these fast Na channels for the upstroke. Instead, their phase 0 depolarization is mainly due to calcium entry through L-type calcium channels, which produces a slower, less steep upstroke. This slower depolarization means phase 0 in pacemaker cells is not faster than in working cardiac cells; it is slower. Hence the statement is false. The slower upstroke in pacemaker cells is why conduction from the nodal tissue is intrinsically slower than conduction through the working myocardium.

Phase 0 is the rapid depolarization phase of the cardiac action potential. In ventricular and atrial myocytes, this upstroke is very fast because it’s driven by the rapid influx of sodium through fast voltage-gated Na channels, giving a steep rise in membrane potential. Pacemaker cells in the SA and AV nodes, however, lack these fast Na channels for the upstroke. Instead, their phase 0 depolarization is mainly due to calcium entry through L-type calcium channels, which produces a slower, less steep upstroke. This slower depolarization means phase 0 in pacemaker cells is not faster than in working cardiac cells; it is slower. Hence the statement is false. The slower upstroke in pacemaker cells is why conduction from the nodal tissue is intrinsically slower than conduction through the working myocardium.