CARDIAC PHYSIOLOGY Amelyn U. Ramos-Rafael,M.D. Functional Anatomy of the Heart 1
Functional Anatomy of The Heart The Atria relatively thin walled The Ventricles ventricular walls thicker than atrial walls Ventricular muscle fibers in spiral pattern The Cardiac valves Located between the atria and ventricle Tricuspid Mitral Valvular insufficiency / Valvular stenosis Pulmonary and aortic valves relatively small and light, cup-like, quite strong Cardiac Muscle 3 major types of cardiac muscle: atrial muscle ventricular muscle specialized excitatory and conductive muscle fibers 2
Arranged in latticework Striated Contain actin and myosin filaments 2 Syncytiums: atrial syncytium ventricular syncytium The Specialized Cardiac Tissues Specialized Excitatory and Conductive System of the Heart Sinus node (SA node) in which the normal rhythmical impulse is generated Internodal pathways that conduct the impulse from the SA node to the AV node AV node in which the impulse from the atria is delayed before passing into the ventricles AV Bundle which conducts the impulse from the atria into the ventricles The Left and Right Bundles of Purkinje fibers which conduct the cardiac impulse to all parts of the ventricles 3
The Specialized Cardiac Tissues Nodal Tissues SA node lies in the sulcus between the SVC and RA made up of slender spindle-shaped cells primary pacemaker AV node located in the subendocardial region of the RA The Specialized Cardiac Tissues The internodal and interatrial pathways: the anterior internodal tract the middle internodal tract (Wenckebach s tract) the posterior internodal tract (Thorel s tract) Velocity of conduction through these pathways is more rapid than through the ordinary contracting atrial muscle. Functionally, the AV node is made up of 3 zones: atrionodal (AN) nodal (N) nodal His (NH) 4
The Bundle of His and Its Branches The Purkinje fibers The velocity of conduction in the Purkinje fiber is fastest among cardiac tissues. The Electrophysiology of the Heart RMP follows the same principles described on nerve and muscle Normal cardiac muscle -85 to 95 mv Purkinje fibers -90 to 100 mv The Action Potential Threshold potential or firing level = -60 to 70 mv Phases of the Action Potential: Phase 4 the resting phase Phase 0- rapid depolarization Phase 1 begins at the peak of the AP the 1st stage of the recovery of the membrane Phase 2 characterized by a relatively long period of very little changes in the membrane potential plateau Phase 3 rapid return of the membrane potential to resting level the beginning and end of phase 3 are not clearly demarcated 5
Why the long AP and the Plateau? The AP is caused by opening 2 types of channels: fast sodium channels slow calcium channels (calcium sodium channels) prolonged period of depolarization causing the plateau The permeability of cardiac muscle membrane to potassium decreases maybe caused by excess calcium influx through the calcium channels this greatly decreases the outflux of potassium ions during the AP plateau and thereby prevents early return of the potential to its resting level. Action Potentials in Various Cardiac Cells SA node Atrial and ventricular muscles Bundle of His and Purkinje fibers 6
Action Potentials in Various Cardiac Cells SA node phase 4 is not constant (unstable) Gradual reduction in the membrane potential (slow diastolic depolarization) APs generated even in the absence of external stimuli (automatic or pacemaker cells) Other features: a) slow phase 0 depolarization b) phase 1 is not clear c) separation between phase 2 and phase 3 is not clearly delineated Mechanism of Sinus Nodal Rhythmicity : SA node RMP = -55mV Fast sodium channels inactivated AP is slower to develop than that of ventricular muscle, and it recovers with a slow decrement of the potential Self-Excitation of Sinus Nodal Fiber: Inherent leakiness Why does this leakiness to sodium ions not cause the sinus nodal fibers to remain depolarized all the time? The Ca-Na channels become inactivated within about 100-150msec after opening At about the same time, greatly increased numbers of potassium channels open Why is the new state of hyperpolarization not maintained forever? 7
Action Potentials in Various Cardiac Cells Atrial and ventricular muscles the phase of the AP are more distinct phase 0 depolarization is rapid phase 4 remains constant no diastolic depolarization hence they do not spontaneously generate AP Action Potentials in Various Cardiac Cells Bundle of His and Purkinje fibers AP show distinct phases low degree of automaticity hence phase 4 is relatively constant their rhythmicity is less than the SA node and so they remain only latent pacemakers. 8
The Absolute and Relative Refractory Period Absolute RP complete loss of excitability, NO response even if intensity of stimulus is markedly increased duration extends from phase 0, through phase 1 and 2, and part of 3 Relative RP stronger stimulus is used Compared with AP elicited early in this period, an AP elicited later in the recovery period would require weaker stimulus to evoke, would be of greater magnitude and would show faster phase 0 depolarizaton THANK YOU! For Not Listening! 9