Cardiac + action potentials

AP of contractile cells

threshold -(-70), RMP- (-90)

1. depolarization- fast V-gated Na+ channels open activating negative feed back cycle, inactivating Na+ channel while opening Ca+ and K+ channels (K+ leaks out, Ca+ leaks in)

2. plateau phase- (absolute refractory period)- Caused by Ca+ slowly enters through Ca+ channels which keeps the cell depolarized as most K+ channels become closed.

3. repolarization- inactivation of Ca+ channels, opening of voltage gated K+ channels

AP of Pacemaker cells

1. pacemaker potential slow depolarization caused by the opening of Na+ channels while closing K+ channels

2. depolarization- action potential beings when pacemaker potential hits -40MV, this opens V-gated calicum channels, depolarization is caused by the entrance of Ca+ and closed K+ channels.

3. repolarization (get more negative)- happens at 10mV opens K+ channels by calcium channels inactivating, K+ brings the membrane potential to negative through K+ entering and stops Ca+ channels

purposes of plateau

1. ensures sustained contraction for complete blood ejection

2. ensures a long refractory period to prevent tetanic contractions

sequence of depolarization

modulated by the ANS

sinoatrial node→ right atrial wall- hearts pacemaker is called sinus rhythm, 75x/min

Atrioventricular node→ junction of atria and ventricles; short delay because atria is completing contraction, wave moves to ventriculus without the sinoatrial node 50x/min

atrioventricular bundle→ bundle of HIS splits into R and L bundle branches and around into the subendocardial network