Figure 49.6 The Pacemaker Potential (A) The resting potential of the pacemaker cells of the sinoatrial node gradually drifts upward between action potentials. The pacemaker action potential is slow to rise and is broad. These characteristics are due to properties of Na⁺, K⁺, and two types of Ca²⁺ channels. (B) At rest, sinoatrial pacemaker cells are more permeable to Na⁺ than are neurons or other muscle cells. During rest, the K⁺ channels gradually close, but voltage-gated T-type Ca²⁺ channels open. When a threshold is reached, the L-type Ca²⁺ channels open, generating the action potential. Opening of the K⁺ channels helps the membrane potential return to its most negative value.