BIOL 244 Chapter 18: Heart Part 3Lecture 11 (September 25) Electrocardiogram- graphic record of heart activity; composite of all AP generated by nodal and contractile cells at a given time P Wave Small, lasts about 0.08 seconds & results from movement of depolarization frwave from SA node through atria Approx. 0.1 seconds after P wave begins, the atria contracts QRS wave complex Results from ventricular depolarization & precedes ventricular contraction Complicated shape b/c paths of depolarization waves through the ventricular walls change continuously, producing corresponding changes in current direction Time required for each ventricle to depolarize depends on its size relative to other ventricle Average duration is 0.08 seconds T waves Caused by ventricular repolarization Typically lasts about 0.16 seconds More spread out with lower amplitude than QRS complex b/c repolarization is slower than depolarization Heart Sound (Sound of Korotkoff) Often described as lub-dup (associated with heart valves closing) 1st sound= AV valves closing Signifies point when ventrictular pressure rises above atrial pressure Louder, longer, & more resonant 2nd sound= SL valves snap shut at beginning of ventricular relaxation (diastole) Short, sharp sound Regulation of heart rate Autonomic nervous system Sympathetic nerve fibers enhance contractility & speeds relaxation Enhances ca2+ movements in contractile cells, lowers ESV Parasympathetic- initiated cardiac responses are mediated by acetycholine, which hyperpolarizes membranes of its effector cells by opening K+ channels Little to no effect on cardiac contractility  Heart has vagal tone because dominant influence is inhibitory Atrial reflex- autonomic reflex initiated by increased venous return & increased atrial filling Stretching atrial walls increases heart rate by stimulating SA node & atrial stretch receptors Chemical regulation of heart rate Hormones Epinephrine- enhances heart rate & contractility  Thyroxine- increases metabolic rate & production of body heat; causes sustained increase in heart rate- Enhances effects of epinephrine & norephinephrine  Ions- normal levels needed Other factors: Age Gender Exercise Body temperature Heart problems Bradycardia Heart rate below 60 beats/minute May result from low body temperature, certain drugs, or parasympathetic nervouse activation Known and desirable consequences of endurance training Tachycardia Abnormally fast heart rate that may result from elevated body temperature, stress, certain drugs, or heart disease Heart rate over 100 beats/minute Prolonged tachycardia may progress to fibrillation (condition of rapid uncoordinated heart contractions which makes the heart useless as a pump) Coronary Artery Disease Atherosclerosis  Essentially fatty buildup that clogs the coronary arteries, impairs blood & oxygen delivery to cardiac cells Heart becomes increasingly hypoxic & begins to contract ineffectively  Most serious consequences are hypertensive heart disease & coronary artery occlusion Diet= single most important contributor - Consume less animal fat, cholesterol,& salt Myocardial Infarction (MI) aka heart attack Caused by prolonged coronary blockage Cells die; dead tissue replaced with noncontractile scar tissue Survival depends on extent/location of damage Congestive Heart Failure Heart is such an inefficient pump that blood circulation is inadequate to meet tissue