Haseeb Javaid

PHYSIO ASSIGNMENT HASEEB JAVAID Roll no : 78 ANS NO 1 : Properties of myocardium : Myocardial cells have several different electrophysiologic properties : automaticity , excitability, Conductivity , contractility, rhythmicity, and refractoriness. ANS NO 2 : Purkinje fibers also have the ability of firing at arate of 15-40 beats per minute if upstream conduction or pacemaking ability is compromised. In contrast, the SA node in normal state can fire at 60-100 beats per minute. ANS NO 3 : To increase heart rate, the autonomic nervous system increases sympatheticoutflow to the SA node, with concurrent inhibition of vagal tone. Inhibition of vagal tone is necessary for the sympathetic nerves to increase heart rate because vagal influences inhibit the action of sympathetic nerve activity at the SA node. ANS NO 4 : Phase 0 (rapid upstroke): primarily Na+ channel opening Phase 1 (early rapid repolarization): inactivation of Na+ current, opening of K+ channels Phase 2 (plateau phase): balance between K+ and Ca2+ currents Phase 3 (final rapid repolarizations): activation of Ca2+ channels Phase 4 (diastolic depolarization): balance between Na+ and K+currents Ca2+, Calcium; K+, potassium; Na+,sodium. Phase 1: Early Rapid Repolarization The early rapid repolarization phase of the action potential, which follows immediately after phase 0, results both from rapid inactivation of the majority of the Na+current and from activation of a transient outward current (ITO), carried mainly by K+ions. Phases 2 and 3: Plateau Phase and Final Rapid Repolarization The action potential plateau and final rapid repolarization are mediated by a balance between the slow inward current and outward, predominantly K+, current. During the plateau phase, membrane conductanceto all ions falls, and very little current flows. Phase 3, regenerative rapid repolarization, results from time-dependent inactivation of L-type Ca2+ current and increasing outward current through delayed rectifier K+channels. The net membrane currentbecomes outward, and the cell repolarizes. Phase 4: Diastolic Depolarization and Pacemaker Current Phase 4 diastolic depolarization, or normal automaticity, is a normal feature of cardiac cells in the sinus and atrioventricular nodes(AVN), but subsidiary pacemaker activity is also observed in the His-Purkinje systemand in some specialized atrial and ventricular myocardial cells. Pacemaker discharge from the sinus node normally predominates because the rate of diastolic depolarization in the sinoatrial node is faster than in other pacemaker tissues. Molecular Biology of Ion Channels The preceding sections focus on the electrical events that underlie cardiac electrical excitability and on the identification of cardiac ionic currents on the basis of their biophysical properties. This section reviews the molecular structures behind these electrical phenomena. The first step in understanding the molecular physiology of cardiac electrical excitability is to identify the ion channel proteins responsible for the ionic currents.