Sinoatrial (SA) node Action Potential
Why are 'if' channels funny?
Na channels are "funny" - they open when membrane is relatively polarized or at rest (unusual !)
Phase 4
Pacemaker Potential Repolarization opens the Na+ if channels Na+ if channels open and membrane starts to SPONTANEOUSLY depolarize again
Describe the pacemaker potential in Phase 4?
Pacemaker Potential UNSTABLE
Phase 0
Rapid Depolarization When depolarization reaches threshold (-40 mV) L-type Ca2+ channels open. if channels are now closing. As Ca2+ channels start closing, K+ channels start opening in response to the depolarization
Phase 3
Repolarization due to Closure of Ca2+ channels Opening of K+ channels As the membrane becomes more and more repolarized the Na+ if channels start to open again
Characteristic Features of SA node AP?
Unstable resting potential No plateau phase Upstroke of action potential (phase 0) is due to inward Ca2+ current (not Na+). Upstroke slower than in ventricular AP
Phase 4
if channels open inward Na+ current Depolarization Open T-type Ca2+ channels T= transient iCa More depolarization