Cardiac myocytes

Vagus input hyperpolarizes SA node - slow heart rate even with constant slope.
     
autonomic input affects both slope of phase 4 AND slope of phase 0 in SA node;  chronotropic and dromotropic effects (rate and conduction speed)

Parasympathetic (vagal) activation, which releases acetylcholine (ACh) onto the SA node, ALSO decreases pacemaker rate by increasing gK⁺ and decreasing slow inward gCa⁺⁺ and gNa⁺; the pacemaker current (I_f) is suppressed.  These ionic conductance changes decrease the slope of phase 4 of the action potential, thereby increasing the time required to reach threshold. Vagal activity also hyperpolarizes the pacemaker cell during Phase 4, which results in a longer time to reach threshold voltage.

total conduction time = 210 msec. = systole  at hr of 60 bpm there is one beat per sec.  so, diastole (filling) lasts 790 msec.  importance = higher heart rates reduce time of diastole (systole does not change).  danger of tachycardia in patients = reduced filling.

FIGURES TO SUPPLEMENT LECTURE