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Electrical Activity of the Heart: Action Potential, Automaticity, and Conduction 1 & 2 Clive M

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Electrical Activity of the Heart: Action Potential, Automaticity, and Conduction 1 & 2 Clive M Electrical Activity of the Heart: Action Potential, Automaticity, and Conduction 1 & 2 Clive M. Baumgarten, Ph.D. OBJECTIVES: 1. Describe the basic characteristics of cardiac electrical activity and the spread of the action potential through the heart 2. Compare the characteristics of action potentials in different parts of the heart 3. Describe how serum K modulates resting potential 4. Describe the ionic basis for the cardiac action potential and changes in ion currents during each phase of the action potential 5. Identify differences in electrical activity across the tissues of the heart 6. Describe the basis for normal automaticity 7. Describe the basis for excitability 8. Describe the basis for conduction of the cardiac action potential 9. Describe how the responsiveness relationship and the Na+ channel cycle modulate cardiac electrical activity I. BASIC ELECTROPHYSIOLOGIC CHARACTERISTICS OF CARDIAC MUSCLE A. Electrical activity is myogenic, i.e., it originates in the heart. The heart is an electrical syncitium (i.e., behaves as if one cell). The action potential spreads from cell-to-cell initiating contraction. Cardiac electrical activity is modulated by the autonomic nervous system. B. Cardiac cells are electrically coupled by low resistance conducting pathways gap junctions located at the intercalated disc, at the ends of cells, and at nexus, points of side-to-side contact. The low resistance pathways (wide channels) are formed by connexins. Connexins permit the flow of current and the spread of the action potential from cell-to-cell. C. Action potentials are much longer in duration in cardiac muscle (up to 400 msec) than in nerve or skeletal muscle (~5 msec). Action potential characteristics vary in different portions of the heart. II. ELECTRICAL ANATOMY OF THE HEART A. The pacemaker of the heart is the sino-atrial node (SAN). The SAN is located under the endocardium in the right atrium at the superior vena cava. Action potentials arise in the SAN spontaneously and propagate to the rest of the heart with a specific sequence and specific timing. B. The rate of spontaneous firing of the SAN determines normal heart rate. Parasympathetic (vagus n.; acetylcholine) and sympathetic (norepinephrine) nerves enervate the SAN and modulate heart rate. Slowing of the HR (parasympathetic) is termed a negative chronotropic effect, and increasing HR (sympathetic) is termed a positive chronotropic effect. C. The SAN action potential spreads into atria in all directions. The atrial internodal tracts are preferential conduction pathways from SAN to AVN and LA (of little physiological significance). Internodal tracts are also called the atrial specialized conducting system. Conduction velocity is ~1 m/sec. D. The AV node is the electrical link between the atria and ventricles. It is located under the endocardium in the floor of the right atrium at the septum. The speed of conduction of the action potential slows dramatically (~20 to l00-fold) in the AV node. Normally, AV node conduction velocity is 0.01 to 0.05 m/sec. This gives rise to the AV delay. The AV delay allows for the time needed to complete filling of the ventricle before ventricular contraction begins. Conduction velocity in AVN is modulated by autonomic tone. Sympathetics speed conduction velocity, termed a positive dromotropic effect, and parasympathetics slow conduction velocity, a negative dromotropic effect. E. After passing through AV node, excitation enters the ventricular specialized conducting system (His-Purkinje system) at the Bundle of His (Common Bundle), proceeds through right and left bundle branches, their divisions, and terminal Purkinje fibers to ventricular muscle. Conduction velocity is 2-4 m/sec in the His-Purkinje system, the fastest conduction velocity in the heart. The His- Purkinje system rapidly distributes the cardiac impulse; this allows contraction to proceed from apex to base, as needed for efficient ejection of blood. F. In the ventricular muscle, the action potential spreads at ~1 m/sec. (Details of sequence of ventricular activation are covered in the ECG lecture.) G. Normal sinus rhythm (NSR) is the normal rhythm of the heart. Normal sinus rhythm requires that: 1. the impulse that initiates the beat must arise in the SAN; 2. the rate must be 60 to 100 bpm and regular; 3. excitation must occur in a normal sequence (given above) with appropriate timing of excitation of various portions of the heart. III. THE ACTION POTENTIAL A. Phases of the Action Potential Phase 0 – upstroke Phase 1 − initial repolarization Phase 2 – plateau Phase 3 – repolarization Phase 4 − electrical diastole B. Resting Potential In atria, ventricle and Purkinje fibers, the membrane potential is normally constant during phase 4 and is termed the resting potential (Em). Em is largely + + determined by the transmembrane K gradient and approaches EK, the K equilibrium potential. Changes in serum K+ over the clinically observed range significantly alter Em. Em of SA and AV nodes is essentially insensitive to serum K+ (below ~20 mM). In SAN and AVN, where phase 4 is not constant, the most negative potential attained is termed the maximum diastolic potential (MDP) rather than Em. (Note: Alterations in serum K+ must be electroneutral, e.g., opposite changes in K+ and Na+ or matching changes in K+ and Cl-; effects of + altered serum K on Em are due to changes in EK. Students sometimes erroneously assume that the altered serum K+ changes simply increases the number of positive charges outside the cell.) C. Action Potential Duration Action potential duration (APD) varies inversely with rate. As heart rate increases, the APD shortens. D. Characteristics of the action potential vary in different portions of the heart. 1. Em is about −85 mV (inside neg.), except at SAN and AVN, Em is −60 mV. 2. APD: 150 to 400 msec PKJ > Vent > Atria > AVN ≈ SAN 3. Upstroke is rapid except in SA and AV nodes. 4. Some tissues can spontaneously initiate action potentials. They are said to exhibit automaticity. 5. Phases 2 and 3 run together in SAN and AVN and phase 1 is absent. IV. IONIC BASIS FOR THE CARDIAC ACTION POTENTIAL A. The movement of charge across the membrane (current flow) causes changes in membrane potential. Inward movement of positive charge (inward current; e.g., influx of Na+ and Ca2+) causes depolarization. Outward movement of positive charge (outward current; e.g., efflux of K+ or influx of Cl-) causes hyperpolarization. A constant membrane potential means inward and outward currents are equal. B. The magnitude of current carried by an ion (I) is equal to the product of the conductance (g) of the membrane for that ion and the driving force (i.e., how far from electrochemical equilibrium is the distribution of the ion). + For K : IK = gK (Em − EK) + For Na : INa = gNa (Em − ENa) 1. Conductance is the reciprocal of electrical resistance; it is related to but not identical to permeability and reflects the ability of ions to cross the membrane. High conductance implies that it is easy for ions to cross the membrane. 2. Driving force is the difference between Em and the Nernst equilibrium potential for the ion. Driving force is the electrochemical gradient, the sum of the electrical and concentration (chemical) gradients, expressed in terms of volts. The electrochemical gradient drives diffusion of ions across the membrane. 3. Direction of current. By definition: (1) the direction of current flow is the direction of flow of cations (K+ efflux = outward current; Cl– efflux = inward current); (2) positive current is outward current (repolarizing or hyperpolarizing) and negative current is inward current (depolarizing). • The sign and direction of current are set by the sign of the driving force. + • K : Em always is positive to EK. This means driving force (Em − EK) is + + positive, and K current (IK) is positive (outward), corresponding to K efflux. + • Na : Em always is negative to ENa. This means the driving force + (Em − ENa) is negative, and Na current (INa) is negative (inward), corresponding to Na+ influx. 4. Nernst equilibrium potential represents the membrane potential at which the electrical and chemical gradients are equal and opposite. At that potential, influx and efflux of an ion are equal, i.e., there is no net flux and no current. + If Em were positive to EK, the electrical gradient drawing K in would be less than the concentration gradient pushing K+ out, and a net K+ efflux (positive current) would be observed. The Nernst equation is: Equilibrium Potential = −2.303 RT/zF log [C]i/[C]o = −61/z log [C]i/[C]o where R is the Gas Constant, T the temperature (°K), F Faraday's constant, and z the valance (e.g., Na+ = +1, K+ = +1, Cl- = −1, Ca2+ = +2). The approximate values of the equilibrium potentials for K+, Na+ and Ca2+ and how changes in intra or extracellular concentrations affects the equilibrium potentials are important. + + EK = −61 log [K ]i/[K ]o = −61 log (150 mM / 4 mM) ≈ −100 mV + + ENa = −61 log [Na ]i/[Na ]o = −61 log (15 mM / 150 mM) ≈ +60 mV 2+ 2+ -7 -3 ECa = −61/2 log [Ca ]i/[Ca ]o = −61/2 log (10 M / 2×10 M) ≈ +110 mV C. Membrane Conductances. Electrical activity is controlled by the voltage and time- dependence of membrane conductance. Several types of channels with differing selective ionic permeabilities have been identified; each channel type is a different protein molecule. Each channel type has its own conductance term. Current Effect on Em Current Conductance Ion Equilib. Pot. During AP During AP + INa gNa Na +60 mV Inward Depolarizing + IK gK K −100 mV Outward Repolarizing 2+ ICa gCa Ca +100 mV Inward Depolarizing + IK1 gK1 K −100 mV Outward Repolarizing + Ito gto K −100 mV Outward Repolarizing _______________________________________________________________ + K Current Nomenclature. Some texts use 'IK' to include IK and IK1, but + separate membrane proteins are responsible.
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