a&p II

EKG

1.An electrocardiogram is a recording of the electrical changes that occur during a cardiac cycle. 2.The first wave, the P wave, corresponds to the depolarization of the atria. 3.The QRS complex corresponds to the depolarization of ventricles and hides the repolarization of atria. 4.The T waves ends the ECG pattern and corresponds to ventricular repolarization

Cardiac Conduction System

1.Specialized cardiac muscle tissue conducts impulses throughout the myocardium and comprises the cardiac conduction system. 2.A self-exciting mass of specialized cardiac muscle called the sinoatrial node (S-A node or pacemaker), located on the posterior right atrium, and generates the impulses for the heartbeat. 3.Impulses spread next to the atrial syncytium, it contracts, and impulses travel to the junctional fibers leading to the atrioventricular node (A-V node) located in the septum. a.Junctional fibers are small, allowing the atria to contract before the impulse spreads rapidly over the ventricles. 4.Branches of the A-V bundle give rise to Purkinje fibers leading to papillary muscles; these fibers stimulate contraction of the papillary muscles at the same time the ventricles contract.

Regulation of the Cardiac Cycle

1.The amount of blood pumped at any one time must adjust to the current needs of the body (more is needed during strenuous exercise). 2.The S-A node is innervated by branches of the sympathetic and parasympathetic divisions, so the CNS controls heart rate. a.Sympathetic impulses speed up and parasympathetic impulses slow down heart rate. 3.The cardiac control center of the medulla oblongata maintains a balance between the sympathetic and parasympathetic divisions of the nervous system in response to messages from baroreceptors, which detect changes in blood pressure. 4.Impulses from cerebrum or hypothalamus may also influence heart rate, as do body temperature and the concentrations of certain ions.