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