Cardiovascular Physiology PDF

CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms PROF. DR. BARLAS N. AYTAÇOĞLU CARDIOVASCULAR SURGEON GAU FACULTY OF MEDICINE  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms The heart is endowed w...

CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms PROF. DR. BARLAS N. AYTAÇOĞLU CARDIOVASCULAR SURGEON GAU FACULTY OF MEDICINE  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms The heart is endowed with a special system for – (1) generating rhythmical electrical impulses to cause rhythmical contraction of the heart muscle and – (2) conducting these impulses rapidly through the heart. When this system functions normally, the atria contract about one sixth of a second ahead of ventricular contraction, which allows filling of the ventricles before they pump the blood through the lungs and peripheral circulation.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Another special importance of the system is that it allows all portions of the ventricles to contract almost simultaneously, which is essential for most effective pressure generation in the ventricular chambers. This rhythmical and conductive system of the heart is susceptible to damage by heart disease, especially by ischemia of the heart tissues resulting from poor coronary blood flow. The effect is often a bizarre heart rhythm or abnormal sequence of contraction of the heart chambers, and the pumping effectiveness of the heart often is affected severely, even to the extent of causing death.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart Figure 1 shows the specialized excitatory and conductive system of the heart that controls cardiac contractions. FIGURE 1  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart The figure shows the sinus node (also called sinoatrial or S-A node), in which the normal rhythmical impulses are generated; the internodal pathways that conduct impulses from the sinus node to the atrioventricular (A-V) node; the A-V node, in which impulses from the atria are delayed before passing into the ventricles; the A-V bundle, which conducts impulses from the atria into the ventricles; and the left and right bundle branches of Purkinje fibers, which conduct the cardiac impulses to all parts of the ventricles.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Sinus (Sinoatrial) Node The sinus node (also called sinoatrial node [In short SA or SAN]) is a small, flattened, ellipsoid strip of specialized cardiac muscle about 3 millimeters wide, 15 millimeters long, and 1 millimeter thick. It is located in the superior posterolateral wall of the right atrium immediately below and slightly lateral to the opening of the superior vena cava.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Sinus (Sinoatrial) Node The fibers of this node have almost no contractile muscle filaments and are each only 3 to 5 micrometers in diameter, in contrast to a diameter of 10 to 15 micrometers for the surrounding atrial muscle fibers. However, the sinus nodal fibers connect directly with the atrial muscle fibers so that any action potential that begins in the sinus node spreads immediately into the atrial muscle wall.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers Some cardiac fibers have the capability of self- excitation, a process that can cause automatic rhythmical discharge and contraction. This is especially true of the fibers of the heart’s specialized conducting system, including the fibers of the sinus node. For this reason, the sinus node ordinarily controls the rate of beat of the entire heart.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers FIGURE 2 Mechanism of Sinus Nodal Rhythmicity. – Figure 2 shows action potentials recorded from inside a sinus nodal fiber for three heartbeats and, by comparison, a single ventricular muscle fiber action potential.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers FIGURE 2 Mechanism of Sinus Nodal Rhythmicity. – Note that the “resting membrane potential” of the sinus nodal fiber between discharges has a negativity of about −55 to −60 millivolts, in comparison with −85 to −90 millivolts for the ventricular muscle fiber.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers FIGURE 2 Mechanism of Sinus Nodal Rhythmicity. – The cause of this lesser negativity is that the cell membranes of the sinus fibers are naturally leaky to sodium and calcium ions, and positive charges of the entering sodium and calcium ions neutralize some of the intracellular negativity.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers Mechanism of Sinus Nodal Rhythmicity. – Opening of the fast sodium channels for a few 10,000 ths of a second is responsible for the rapid upstroke spike of the action potential observed in ventricular muscle, because of rapid influx of positive sodium ions to the interior of the fiber. – Then the “plateau” of the ventricular action potential is caused primarily by slower opening of the slow sodium- calcium channels, which lasts for about 0.3 second.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers Mechanism of Sinus Nodal Rhythmicity. – Finally, opening of potassium channels allows diffusion of large amounts of positive potassium ions in the outward direction through the fiber membrane and returns the membrane potential to its resting level.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers Mechanism of Sinus Nodal Rhythmicity. – There is a difference in the function of these channels in the sinus nodal fiber because the “resting” potential is much less negative—only −55 millivolts in the nodal fiber instead of the −90 millivolts in the ventricular muscle fiber. – At this level of −55 millivolts, the fast sodium channels mainly have already become “inactivated,” which means that they have become blocked.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers Mechanism of Sinus Nodal Rhythmicity. – The cause of this is that any time the membrane potential remains less negative than about −55 millivolts for more than a few milliseconds, the inactivation gates on the inside of the cell membrane that close the fast sodium channels become closed and remain so. – Therefore, only the slow sodium-calcium channels can open (i.e., can become “activated”) and thereby cause the action potential.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers Mechanism of Sinus Nodal Rhythmicity. – As a result, the atrial nodal action potential is slower to develop than the action potential of the ventricular muscle. – Also, after the action potential does occur, return of the potential to its negative state occurs slowly as well, rather than the abrupt return that occurs for the ventricular fiber.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers Self-Excitation of Sinus Nodal Fibers. – Because of the high sodium ion concentration in the extracellular fluid outside the nodal fiber, as well as a moderate number of already open sodium channels, positive sodium ions from outside the fibers normally tend to leak to the inside. – Therefore, between heartbeats, influx of positively charged sodium ions causes a slow rise in the resting membrane potential in the positive direction.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers Self-Excitation of Sinus Nodal Fibers. – The “resting” potential gradually rises and becomes less negative between each two heartbeats. FIGURE 2  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers Self-Excitation of Sinus Nodal Fibers. – When the potential reaches a threshold voltage of about −40 millivolts, the sodium-calcium channels become “activated,” thus causing the action potential. FIGURE 2  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers Self-Excitation of Sinus Nodal Fibers. – Therefore, basically, the inherent leakiness of the sinus nodal fibers to sodium and calcium ions causes their self-excitation.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers Self-Excitation of Sinus Nodal Fibers. – Why does this leakiness to sodium and calcium ions not cause the sinus nodal fibers to remain depolarized all the time? – The answer is that two events occur during the course of the action potential to prevent this. » First, the sodium-calcium channels become inactivated (i.e., they close) within about 100 to 150 milliseconds after opening, and » Second, at about the same time, greatly increased numbers of potassium channels open.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers Self-Excitation of Sinus Nodal Fibers. – Therefore, influx of positive calcium and sodium ions through the sodium-calcium channels ceases, while at the same time large quantities of positive potassium ions diffuse out of the fiber. – Both of these effects reduce the intracellular potential back to its negative resting level and therefore terminate the action potential.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers Self-Excitation of Sinus Nodal Fibers. – Furthermore, the potassium channels remain open for another few tenths of a second, temporarily continuing movement of positive charges out of the cell, with resultant excess negativity inside the fiber; this is called hyperpolarization. – The hyperpolarization state initially carries the “resting” membrane potential down to about −55 to −60 millivolts at the termination of the action potential.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers Self-Excitation of Sinus Nodal Fibers. – Why is this new state of hyperpolarization not maintained forever? – The reason is that during the next few tenths of a second after the action potential is over, progressively more and more potassium channels close.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers Self-Excitation of Sinus Nodal Fibers. – The inward-leaking sodium and calcium ions once again overbalance the outward flux of potassium ions, and this causes the “resting” potential to drift upward once more, finally reaching the threshold level for discharge at a potential of about −40 millivolts.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Automatic Electrical Rhythmicity of the Sinus Fibers Self-Excitation of Sinus Nodal Fibers. – Then the entire process begins again: self-excitation to cause the action potential, recovery from the action potential, hyperpolarization after the action potential is over, drift of the “resting” potential to threshold, and finally re-excitation to elicit another cycle. – This process continues indefinitely throughout a person’s life.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Internodal Pathways and Transmission of the Cardiac Impulse Through the Atria The ends of the sinus nodal fibers connect directly with surrounding atrial muscle fibers. Therefore, action potentials originating in the sinus node travel outward into these atrial muscle fibers. In this way, the action potential spreads through the entire atrial muscle mass and, eventually, to the A-V node.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Internodal Pathways and Transmission of the Cardiac Impulse Through the Atria The velocity of conduction in most atrial muscle is about 0.3 m/sec, but conduction is more rapid, about 1 m/sec, in several small bands of atrial fibers. One of these, called the anterior inter-atrial band, passes through the anterior walls of the atria to the left atrium.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and FIGURE 3 Conductive System of the Heart – Internodal Pathways and Transmission of the Cardiac Impulse Through the Atria In addition, three other small bands curve through the anterior, lateral, and posterior atrial walls and terminate in the A-V node; shown in Figure 3, these are called, respectively, the anterior, middle, and posterior internodal pathways.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Atrioventricular Node and Delay of Impulse Conduction from the Atria to the Ventricles The atrial conductive system is organized so that the cardiac impulse does not travel from the atria into the ventricles too rapidly; this delay allows time for the atria to empty their blood into the ventricles before ventricular contraction begins. It is primarily the A-V node and its adjacent conductive fibers that delay this transmission into the ventricles.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Atrioventricular Node and Delay of Impulse Conduction from the Atria to the Ventricles The A-V node is located in the posterior wall of the right atrium immediately behind the tricuspid valve, as shown in Figure 2. FIGURE 2  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and FIGURE 3 Conductive System of the Heart – Atrioventricular Node and Delay of Impulse Conduction from the Atria to the Ventricles Figure 3 shows diagrammatically the different parts of this node, plus its connections with the entering atrial internodal pathway fibers and the exiting A-V bundle.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and FIGURE 3 Conductive System of the Heart – Atrioventricular Node and Delay of Impulse Conduction from the Atria to the Ventricles The figure also shows the approximate intervals of time in fractions of a second between initial onset of the cardiac impulse in the sinus node and its subsequent appearance in the A-V nodal system.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and FIGURE 3 Conductive System of the Heart – Atrioventricular Node and Delay of Impulse Conduction from the Atria to the Ventricles Note that the impulse, after traveling through the internodal pathways, reaches the A-V node about 0.03 second after its origin in the sinus node.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and FIGURE 3 Conductive System of the Heart – Atrioventricular Node and Delay of Impulse Conduction from the Atria to the Ventricles Then there is a delay of another 0.09 second in the A-V node itself before the impulse enters the penetrating portion of the A-V bundle, where it passes into the ventricles.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and FIGURE 3 Conductive System of the Heart – Atrioventricular Node and Delay of Impulse Conduction from the Atria to the Ventricles A final delay of another 0.04 second occurs mainly in this penetrating A-V bundle, which is composed of multiple small fascicles passing through the fibrous tissue separating the atria from the ventricles.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and FIGURE 3 Conductive System of the Heart – Atrioventricular Node and Delay of Impulse Conduction from the Atria to the Ventricles Thus, the total delay in the A-V nodal and A-V bundle system is about 0.13 second.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and FIGURE 3 Conductive System of the Heart – Atrioventricular Node and Delay of Impulse Conduction from the Atria to the Ventricles This, in addition to the initial conduction delay of 0.03 second from the sinus node to the A-V node, makes a total delay of 0.16 second before the excitatory signal finally reaches the contracting muscle of the ventricles.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Atrioventricular Node and Delay of Impulse Conduction from the Atria to the Ventricles Cause of the Slow Conduction. – The slow conduction in the transitional, nodal, and penetrating A-V bundle fibers is caused mainly by diminished numbers of gap junctions between successive cells in the conducting pathways, so there is great resistance to conduction of excitatory ions from one conducting fiber to the next. – Therefore, it is easy to see why each succeeding cell is slow to be excited.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Rapid Transmission in the Ventricular Purkinje System Special Purkinje fibers lead from the A-V node through the A-V bundle into the ventricles. They are very large fibers, even muscle fibers, and they transmit action potentials at a velocity of 1.5 to 4.0 m/sec, a velocity about 6 times that in the usual ventricular muscle and 150 times that in some of the A-V nodal fibers.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Rapid Transmission in the Ventricular Purkinje System This allows almost instantaneous transmission of the cardiac impulse throughout the entire remainder of the ventricular muscle. The rapid transmission of action potentials by Purkinje fibers is believed to be caused by a very high level of permeability of the gap junctions at the intercalated discs between the successive cells that make up the Purkinje fibers.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Rapid Transmission in the Ventricular Purkinje System Therefore, ions are transmitted easily from one cell to the next, thus enhancing the velocity of transmission. The Purkinje fibers also have very few myofibrils, which means that they contract little or not at all during the course of impulse transmission.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Rapid Transmission in the Ventricular Purkinje System One-Way Conduction Through the A-V Bundle. – A special characteristic of the A-V bundle is the inability, except in abnormal states, of action potentials to travel backward from the ventricles to the atria. – This prevents re-entry of cardiac impulses by this route from the ventricles to the atria, allowing only forward conduction from the atria to the ventricles.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Rapid Transmission in the Ventricular Purkinje System One-Way Conduction Through the A-V Bundle. – Furthermore, it should be recalled that everywhere, except at the A-V bundle, the atrial muscle is separated from the ventricular muscle by a continuous fibrous barrier. – This barrier normally acts as an insulator to prevent passage of the cardiac impulse between atrial and ventricular muscle through any other route besides forward conduction through the A-V bundle itself.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Rapid Transmission in the Ventricular Purkinje System Distribution of the Purkinje Fibers FIGURE 1 in the Ventricles— The Left and Right Bundle Branches. – After penetrating the fibrous tissue between the atrial and ventricular muscle, the distal portion of the A-V bundle passes downward in the ventricular septum for 5 to 15 millimeters toward the apex of the heart, as shown in Figure 1.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Rapid Transmission in the Ventricular Purkinje System Distribution of the Purkinje Fibers FIGURE 1 in the Ventricles— The Left and Right Bundle Branches. – Then the bundle divides into left and right bundle branches that lie beneath the endocardium on the two respective sides of the ventricular septum.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Rapid Transmission in the Ventricular Purkinje System Distribution of the Purkinje Fibers FIGURE 1 in the Ventricles— The Left and Right Bundle Branches. – Each branch spreads downward toward the apex of the ventricle, progressively dividing into smaller branches. – These branches in turn course sidewise around each ventricular chamber and back toward the base of the heart.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Rapid Transmission in the Ventricular Purkinje System Distribution of the Purkinje Fibers FIGURE 1 in the Ventricles— The Left and Right Bundle Branches. – The ends of the Purkinje fibers penetrate about one third of the way into the muscle mass and finally become continuous with the cardiac muscle fibers.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Rapid Transmission in the Ventricular Purkinje System Distribution of the Purkinje Fibers in the Ventricles— The Left and Right Bundle Branches. – From the time the cardiac impulse enters the bundle branches in the ventricular septum until it reaches the terminations of the Purkinje fibers, the total elapsed time averages only 0.03 second. – Therefore, once the cardiac impulse enters the ventricular Purkinje conductive system, it spreads almost immediately to the entire ventricular muscle mass.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Transmission of the Cardiac Impulse in the Ventricular Muscle Once the impulse reaches the ends of the Purkinje fibers, it is transmitted through the ventricular muscle mass by the ventricular muscle fibers themselves. The velocity of transmission is now only 0.3 to 0.5 m/sec, one sixth that in the Purkinje fibers.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Transmission of the Cardiac Impulse in the Ventricular Muscle Because of this, transmission from the endocardial surface to the epicardial surface of the ventricle requires as much as another 0.03 second, approximately equal to the time required for transmission through the entire ventricular portion of the Purkinje system.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Transmission of the Cardiac Impulse in the Ventricular Muscle Thus, the total time for transmission of the cardiac impulse from the initial bundle branches to the last of the ventricular muscle fibers in the normal heart is about 0.06 second.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Summary of the Spread of the Cardiac Impulse Through the Heart Figure 4 shows in summary form the FIGURE 4 transmission of the cardiac impulse through the human heart. The numbers on the figure represent the intervals of time, in fractions of a second, that lapse between the origin of the cardiac impulse in the sinus node and its appearance at each respective point in the heart.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Summary of the Spread of the Cardiac Impulse Through the Heart Note that the impulse spreads at FIGURE 4 moderate velocity through the atria but is delayed more than 0.1 second in the A-V nodal region before appearing in the ventricular septal A-V bundle. Once it has entered this bundle, it spreads very rapidly through the Purkinje fibers to the entire endocardial surfaces of the ventricles.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Specialized Excitatory and Conductive System of the Heart – Summary of the Spread of the Cardiac Impulse Through the Heart It is important that the student learn in detail the course of the cardiac impulse through the heart and the precise times of its appearance in each separate part of the heart, because a thorough quantitative knowledge of this process is essential to the understanding of electrocardiography.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Sinus Node as the Pacemaker of the Heart In some abnormal conditions, impulses may not arise from the S-A node. Other parts of the heart can also exhibit intrinsic rhythmical excitation in the same way that the sinus nodal fibers do; this is particularly true of the A-V nodal and Purkinje fibers.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Sinus Node as the Pacemaker of the Heart The A-V nodal fibers, when not stimulated from some outside source, discharge at an intrinsic rhythmical rate of 40 to 60 times per minute, and the Purkinje fibers discharge at a rate somewhere between 15 and 40 times per minute. These rates are in contrast to the normal rate of the sinus node of 70 to 80 times per minute.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Sinus Node as the Pacemaker of the Heart Why then does the sinus node rather than the A-V node or the Purkinje fibers control the heart’s rhythmicity? The answer derives from the fact that the discharge rate of the sinus node is considerably faster than the natural self-excitatory discharge rate of either the A-V node or the Purkinje fibers. Each time the sinus node discharges, its impulse is conducted into both the A-V node and the Purkinje fibers, also discharging their excitable membranes.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Sinus Node as the Pacemaker of the Heart But the sinus node discharges again before either the A-V node or the Purkinje fibers can reach their own thresholds for self-excitation. Therefore, the new impulse from the sinus node discharges both the A-V node and the Purkinje fibers before self-excitation can occur in either of these.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Sinus Node as the Pacemaker of the Heart Thus, the sinus node controls the beat of the heart because its rate of rhythmical discharge is faster than that of any other part of the heart. Therefore, the sinus node is virtually always the pacemaker of the normal heart.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Sinus Node as the Pacemaker of the Heart Abnormal Pacemakers—“Ectopic” Pacemaker. – Occasionally some other part of the heart develops a rhythmical discharge rate that is more rapid than that of the sinus node. – For instance, this sometimes occurs in the A-V node or in the Purkinje fibers when one of these becomes abnormal.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Sinus Node as the Pacemaker of the Heart Abnormal Pacemakers—“Ectopic” Pacemaker. – In either case, the pacemaker of the heart shifts from the sinus node to the A-V node or to the excited Purkinje fibers. – Under rarer conditions, a place in the atrial or ventricular muscle develops excessive excitability and becomes the pacemaker.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Sinus Node as the Pacemaker of the Heart Abnormal Pacemakers—“Ectopic” Pacemaker. – A pacemaker elsewhere than the sinus node is called an “ectopic” pacemaker. – An ectopic pacemaker causes an abnormal sequence of contraction of the different parts of the heart and can cause significant debility of heart pumping.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Sinus Node as the Pacemaker of the Heart Abnormal Pacemakers—“Ectopic” Pacemaker. – Another cause of shift of the pacemaker is blockage of transmission of the cardiac impulse from the sinus node to the other parts of the heart. – The new pacemaker then occurs most frequently at the A-V node or in the penetrating portion of the A-V bundle on the way to the ventricles.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Sinus Node as the Pacemaker of the Heart Abnormal Pacemakers—“Ectopic” Pacemaker. – When A-V block occurs—that is, when the cardiac impulse fails to pass from the atria into the ventricles through the A-V nodal and bundle system—the atria continue to beat at the normal rate of rhythm of the sinus node, while a new pacemaker usually develops in the Purkinje system of the ventricles and drives the ventricular muscle at a new rate somewhere between 15 and 40 beats per minute.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Sinus Node as the Pacemaker of the Heart Abnormal Pacemakers—“Ectopic” Pacemaker. – After sudden A-V bundle block, the Purkinje system does not begin to emit its intrinsic rhythmical impulses until 5 to 20 seconds later because, before the blockage, the Purkinje fibers had been “over-driven” by the rapid sinus impulses and, consequently, are in a suppressed state.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Sinus Node as the Pacemaker of the Heart Abnormal Pacemakers—“Ectopic” Pacemaker. – During these 5 to 20 seconds, the ventricles fail to pump blood, and the person faints after the first 4 to 5 seconds because of lack of blood flow to the brain. – This delayed pickup of the heartbeat is called Stokes-Adams syndrome. If the delay period is too long, it can lead to death.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Role of the Purkinje System in Causing Synchronous Contraction of the Ventricular Muscle Sinus Node as the Pacemaker of the Heart The cardiac impulse arrives at almost all portions of the ventricles within a narrow span of time, exciting the first ventricular muscle fiber only 0.03 to 0.06 second ahead of excitation of the last ventricular muscle fiber. This causes all portions of the ventricular muscle in both ventricles to begin contracting at almost the same time and then to continue contracting for about another 0.3 second.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Control of Heart Rhythmicity and Impulse Conduction by the Cardiac Nerves: Sympathetic and Parasympathetic Nerves The heart is supplied with both sympathetic and parasympathetic nerves, as shown in Figure 5. FIGURE 5  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Control of Heart Rhythmicity and Impulse Conduction by the Cardiac Nerves: Sympathetic and Parasympathetic Nerves The parasympathetic nerves (the vagi) are distributed mainly to the S-A and A-V nodes, to a lesser extent to the muscle of the two atria, and very little directly to the ventricular muscle. The sympathetic nerves, conversely, are distributed to all parts of the heart, with strong representation to the ventricular muscle, as well as to all the other areas.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Control of Heart Rhythmicity and Impulse Conduction by the Cardiac Nerves: Sympathetic and Parasympathetic Nerves Parasympathetic (Vagal) Stimulation Can Slow or Even Block Cardiac Rhythm and Conduction —“Ventricular Escape.” – Stimulation of the parasympathetic nerves to the heart (the vagi) causes the hormone acetylcholine to be released at the vagal endings. – This hormone has two major effects on the heart.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Control of Heart Rhythmicity and Impulse Conduction by the Cardiac Nerves: Sympathetic and Parasympathetic Nerves Parasympathetic (Vagal) Stimulation Can Slow or Even Block Cardiac Rhythm and Conduction —“Ventricular Escape.” – First, it decreases the rate of rhythm of the sinus node, – Second, it decreases the excitability of the A-V junctional fibers between the atrial musculature and the A-V node, thereby slowing transmission of the cardiac impulse into the ventricles.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Control of Heart Rhythmicity and Impulse Conduction by the Cardiac Nerves: Sympathetic and Parasympathetic Nerves Parasympathetic (Vagal) Stimulation Can Slow or Even Block Cardiac Rhythm and Conduction —“Ventricular Escape.” – Weak to moderate vagal stimulation slows the rate of heart pumping, often to as little as one-half normal. – Strong stimulation of the vagi can stop completely the rhythmical excitation by the sinus node or block completely transmission of the cardiac impulse from the atria into the ventricles through the A-V mode.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Control of Heart Rhythmicity and Impulse Conduction by the Cardiac Nerves: Sympathetic and Parasympathetic Nerves Parasympathetic (Vagal) Stimulation Can Slow or Even Block Cardiac Rhythm and Conduction —“Ventricular Escape.” – In either case, rhythmical excitatory signals are no longer transmitted into the ventricles. – The ventricles stop beating for 5 to 20 seconds, but then some small area in the Purkinje fibers, usually in the ventricular septal portion of the A-V bundle, develops a rhythm of its own and causes ventricular contraction at a rate of 15 to 40 beats per minute.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Control of Heart Rhythmicity and Impulse Conduction by the Cardiac Nerves: Sympathetic and Parasympathetic Nerves Parasympathetic (Vagal) Stimulation Can Slow or Even Block Cardiac Rhythm and Conduction—“Ventricular Escape.” – This phenomenon is called ventricular escape.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Control of Heart Rhythmicity and Impulse Conduction by the Cardiac Nerves: Sympathetic and Parasympathetic Nerves Mechanism of the Vagal Effects. – The acetylcholine released at the vagal nerve endings greatly increases the permeability of the fiber membranes to potassium ions, which allows rapid leakage of potassium out of the conductive fibers. – This causes increased negativity inside the fibers, an effect called hyperpolarization, which makes this excitable tissue much less excitable.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Control of Heart Rhythmicity and Impulse Conduction by the Cardiac Nerves: Sympathetic and Parasympathetic Nerves Mechanism of the Vagal Effects. – In the sinus node, the state of hyperpolarization decreases the “resting” membrane potential of the sinus nodal fibers to a level considerably more negative than usual, to −65 to −75 millivolts rather than the normal level of −55 to −60 millivolts. – Therefore, the initial rise of the sinus nodal membrane potential caused by inward sodium and calcium leakage requires much longer to reach the threshold potential for excitation.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Control of Heart Rhythmicity and Impulse Conduction by the Cardiac Nerves: Sympathetic and Parasympathetic Nerves Mechanism of the Vagal Effects. – This greatly slows the rate of rhythmicity of these nodal fibers. – If the vagal stimulation is strong enough, it is possible to stop entirely the rhythmical self-excitation of this node.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Control of Heart Rhythmicity and Impulse Conduction by the Cardiac Nerves: Sympathetic and Parasympathetic Nerves – Mechanism of the Vagal Effects. – In the A-V node, a state of hyperpolarization caused by vagal stimulation makes it difficult for the small atrial fibers entering the node to generate enough electricity to excite the nodal fibers. – Therefore, the safety factor for transmission of the cardiac impulse through the transitional fibers into the A-V nodal fibers decreases. – A moderate decrease simply delays conduction of the impulse, but a large decrease blocks conduction entirely.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Control of Heart Rhythmicity and Impulse Conduction by the Cardiac Nerves: Sympathetic and Parasympathetic Nerves Effect of Sympathetic Stimulation on Cardiac Rhythm and Conduction. – Sympathetic stimulation causes essentially the opposite effects on the heart to those caused by vagal stimulation, as follows: » First, it increases the rate of sinus nodal discharge. » Second, it increases the rate of conduction, as well as the level of excitability in all portions of the heart. » Third, it increases greatly the force of contraction of all the cardiac musculature, both atrial and ventricular  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Control of Heart Rhythmicity and Impulse Conduction by the Cardiac Nerves: Sympathetic and Parasympathetic Nerves Effect of Sympathetic Stimulation on Cardiac Rhythm and Conduction. – In short, sympathetic stimulation increases the overall activity of the heart. Maximal stimulation can almost triple the frequency of heartbeat and can increase the strength of heart contraction as much as twofold.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Control of Heart Rhythmicity and Impulse Conduction by the Cardiac Nerves: Sympathetic and Parasympathetic Nerves Effect of Sympathetic Stimulation on Cardiac Rhythm and Conduction. – Mechanism of the Sympathetic Effect. » Stimulation of the sympathetic nerves releases the hormone norepinephrine at the sympathetic nerve endings. » Norepinephrine in turn stimulates beta-1 adrenergic receptors, which mediate the effects on heart rate. » The precise mechanism by which beta-1 adrenergic stimulation acts on cardiac muscle fibers is somewhat unclear, but the belief is that it increases the permeability of the fiber membrane to sodium and calcium ions.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Control of Heart Rhythmicity and Impulse Conduction by the Cardiac Nerves: Sympathetic and Parasympathetic Nerves Effect of Sympathetic Stimulation on Cardiac Rhythm and Conduction. – Mechanism of the Sympathetic Effect. » In the sinus node, an increase of sodium-calcium permeability causes a more positive resting potential and also causes increased rate of upward drift of the diastolic membrane potential toward the threshold level for self- excitation, thus accelerating self-excitation and, therefore, increasing the heart rate.  CARDIOVASCULAR PHYSIOLOGY Rhythmical Excitation of the Heart And Abnormal Rythms Control of Excitation and Conduction in the Heart – Control of Heart Rhythmicity and Impulse Conduction by the Cardiac Nerves: Sympathetic and Parasympathetic Nerves Effect of Sympathetic Stimulation on Cardiac Rhythm and Conduction. – Mechanism of the Sympathetic Effect. » In the A-V node and A-V bundles, increased sodium- calcium permeability makes it easier for the action potential to excite each succeeding portion of the conducting fiber bundles, thereby decreasing the conduction time from the atria to the ventricles. » The increase in permeability to calcium ions is at least partially responsible for the increase in contractile strength of the cardiac muscle under the influence of sympathetic stimulation, because calcium ions play a powerful role in exciting the contractile process of the myofibrils.