Phase 0 in pacemaker cells is slower than Phase 0 in ventricular myocytes.

Phase 0 in pacemaker cells is slower than Phase 0 in ventricular myocytes.

• A. True
• B. False
• C. Not determinable
• D. Partially true

Answer: (A)

Phase 0 rapid depolarization is defined by the inward current that drives the membrane potential upward. In ventricular myocytes, this upstroke is powered mainly by a large fast inward Na+ current through voltage-gated sodium channels, producing a very rapid and steep rise and, hence, fast conduction. Pacemaker cells in the SA and AV nodes rely on calcium entry for their phase 0, primarily through L-type calcium channels (with some contribution from T-type). Calcium channels activate more slowly and carry a smaller current than the fast sodium channels, so the rate of rise of the membrane potential is slower. This makes phase 0 in pacemaker cells slower than in ventricular myocytes, which fits their function of slower, automatic pacing and conduction through nodal tissue.

Phase 0 rapid depolarization is defined by the inward current that drives the membrane potential upward. In ventricular myocytes, this upstroke is powered mainly by a large fast inward Na+ current through voltage-gated sodium channels, producing a very rapid and steep rise and, hence, fast conduction. Pacemaker cells in the SA and AV nodes rely on calcium entry for their phase 0, primarily through L-type calcium channels (with some contribution from T-type). Calcium channels activate more slowly and carry a smaller current than the fast sodium channels, so the rate of rise of the membrane potential is slower. This makes phase 0 in pacemaker cells slower than in ventricular myocytes, which fits their function of slower, automatic pacing and conduction through nodal tissue.