Electrical Conduction in Myocardial Cells - Pacemaker cells spontaneously depolarize by leaking sodium - When it reaches threshold, it triggers an action potential which travels along the atria Through gap junctions - Sympathetic activation causes epinephrine to increase the rate - Parasympathetic activation causes Acetylcholine to decrease the rate Heartbeat and Rhythm - Rhythmic electrical activity keeps the heart beating - Rhythm comes from the heart itself through pacemakers Regulation of heart rate - Hormones – Epinephrine and Norepinephrine and Acetylcholine - Cations – Sodium positive increases heart rate - Elevated Calcium two positive speeds heart rate - Increased temperature causes the SA node to depolarize more rapidly to increased Heart rate - Tachycardia – Elevated heart rate – above 100 - Bradycardia – Low heart rate – below 60 Electrocardiogram - EKG or ECG - Recording of the electrical signals generated by the action potentials produced by heart Muscle fibers during each heartbeat - Can help to determine - If conducting pathway is abnormal - If the heart is enlarged - If parts of the heart are damaged - 3 recognizable waves Different Waveforms produced by an ECG - P-wave - A small upward deflection – represents atrial depolarization - QRS Complex - Large, upright, triangular wave flanked by two smaller downward waves – represents Rapid ventricular depolarization - T-wave - Small, wider upward deflection – represents ventricular repolarization Cardiac Cycle - Atrial systole - Depolarization of the atria follows depolarization of the SA node - Forces blood from the left and right atria into the left and right ventricles - Conduction slows at the AV node giving the atria time to contract - Atrial diastole - Follows atrial repolarization - Happens concurrently with ventricular systole - Ventricular systole - Follows ventricular depolarization - Begins shortly after QRS appears and continues through S-T segment - Ventricular diastole - Begins shortly after the T-wave begins Cardiac Output - The heart must increase or decrease based on a metabolic need - Cardiac output is the volume of blood ejected from the left or right ventricles into the aorta Or pulmonary trunk each minute - Stroke volume (SV): Volume of blood ejected by the ventricle during each contraction - Cardiac output (CO): Stroke volume times heart rate