PACEMAKER AND NONPACEMAKER CELLS -Heart has primarily two kinds of myocyte cells- Pacemaker cells (Those that can spontaneously initiate action potentials) and Nonpacemaker cells. -Automaticity is the special feature of Pacemaker cells. It is nothing but the ability to depolarize above a threshold voltage in a rhythmic fashion which results in action potentials. These cells are mainly found in Sinoatrial node, Atrioventricular node, bundle of his and Purkinje fibers. -The Nonpacemaker cells can be mainly seen in Atrial and ventricular myocytes. Majority if cardiac contraction is due to Nonpacemaker cells of the heart. CARDIAC ACTION POTENTIALS -As in other muscle cells, cardiac membrane potentials are also dependent on the chemical and electrical gradients that are established due to the movement of various ions into and outside the cell. The driving force for the movement of ions into and outside the cell is the difference between the Nernst potential and cells membrane potential. This ions movement of is carried out with the help of various ion channels that are present in the cell membrane. -Unlike the action potentials of skeletal muscle or nerves, Cardiac action potentials are longer; this is to facilitate the sustained depolarization and contraction required to empty the blood from heart's chamber. -The action potentials of various cardiac cells are as follows: -Each type of cardiac cell fires according to a specific cycle which consists of different phases. 1. SA Nodal Cells- Phase 4: Represents pacemaker current, due to inward Na+. Phase 0: Denotes rapid depolarization facilitated by voltage gates Ca2+ channels. Phase 3: Is rapid repolarisation phase, due to delayed rectifier outward K+ current. 2. Ventricular Myocytes- Phase 4: Resting membrane potential; in this phase the cells are prepared for next depolarization. The inward and outward currents are equal. Phase 0: Rapid depolarization; Results in action potential, inward Na+ current through voltage gated sodium channels. Phase 1: Early phase of repolarization, Sodium current is decreased and K+ ion efflux slowly increases. Phase 2: Plateau; Denotes ventricular action potential, It is a result of fine balance between an inward Ca2+ current, and K+ outward current. Phase 3: Rapid repolarization (Late phase); the inward Ca2+ current decreases and large increase in K+ current is seen thus repolarising the cell. ECG -In general the overall electrical activity of heart is measured in the form of ECG than measuring the changes in individual cell. -A normal ECG contains three main wave forms they are as follows: P wave- Represents atrial depolarization. QRS complex- Represents ventricular depolarization T wave- Ventricular repolarization. Apart from these other significant aspects of ECG are the PR interval, QT interval and ST segment. The PR interval is the time between the initial depolarization of atria to initial depolarization of ventricles. Hence PR interval varies with conduction velocity through AV node. QT interval represents cycle of ventricular depolarization and repolarisation. ST segment represents period during which ventricles are depolarized (plateau phase of action potential). FIRING RATE OF CELLS Usually various pacemaker cells possess different innate firing rates, so the cells which have fastest rate of firing sets the heart rate. The SA node has the fastest firing rate (60-100 times/minute) and hence they are said to be the primary pacemaker cells of heart. The AV node has 50-60 times per minute, and the cells of the purkinje have the slowest rate of 30-40 times per minute, hence are known as latent pacemakers. THIS BLOG IS FREE OF PLAGIARISM Reference: Principles of Pharmacology: The Pathophysiologic Basis of Drug Therapy, 2nd Edition by David E. Golan. Published by Wolter Kluwer/Lippincott Williams & Wilkins, Pg: 308-312.