Overview SA Node Action Potential; also AV Node Phase 0, upstroke action potential opens L-type voltage-gated Ca2+ channels secondary to T-type voltage-gated Ca2+ channels further depolarizing cell membrane enough to reach threshold for L-type votage-gated Ca2+ activation → upstroke slope of upstroke is slower than other cardiac myocytes login to view 2 more bullets Phase 1 and 2 are absent in SA node action potential Phase 3: repolarization voltage-gated K+ channels open → ↑ K+ conductance → outward K+ current → repolarization Phase 4: diastolic depolarization accounts for automaticity of SA node cells to generate an action potential spontaneously without neural input hyperpolarization-activated cyclic nucleotide-gated (HCN) channel generates "funny" current (If) repolarization activates HCN channels non-selective monovalent ion channel login to view 1 more bullet If slow depolarization activates T-type voltage-gated calcium channels reduced outward K+ current contributes to depolarization Heart rate set by the rate of depolarization in phase 4 ↑ rate of phase 4 depolarization → reach threshold faster → ↑ action potential per unit time by SA node cells → ↑ heart rate pharmacology correlate sympathetic nerveous system stimulation → ↑ heart rate login to view 2 more bullets parasympathetic nervous system stimulation → ↓ heart rate login to view 3 more bullets