Bundle of His

The Cardiac Cycle. The heart (Eig. lb) performs its function as a pump as a result of a rhythmical spread of a wave of excitation (depolarization) that excites the atrial and ventricular muscle masses to contract sequentially. Maximum pump efficiency occurs when the atrial or ventricular muscle masses contract synchronously (see Eig. 1). The wave of excitation begins with the generation of electrical impulses within the SA node and spreads through the atria. The SA node is referred to as the pacemaker of the heart and exhibits automaticity, ie, it depolarizes and repolarizes spontaneously. The wave then excites sequentially the AV node the bundle of His, ie, the penetrating portion of the AV node the bundle branches, ie, the branching portions of the AV node the terminal Purkinje fibers and finally the ventricular myocardium. After the wave of excitation depolarizes these various stmetures of the heart, repolarization occurs so that each of the stmetures is ready for the next wave of excitation. Until repolarization occurs the stmetures are said to be refractory to excitation. During repolarization of the atria and ventricles, the muscles relax, allowing the chambers of the heart to fill with blood that is to be expelled with the next wave of excitation and resultant contraction. This process repeats itself 60—100 times or beats per minute... 

The cardiotonics affect the transmission of electrical impulses along the pathway of the conduction system of tiie heart. The conduction system of die heart is a group of specialized nerve fibers consisting of die SA node, die AV node, the bundle of His, and die branches of Purkinje (Fig. 39-2). Each heartbeat (or contraction of tiie ventricles) is tiie result of an electrical impulse tiiat normally starts in tiie SA node, is tiien received by die AV node, and travels down die bundle of His and through tiie Purkinje fibers (see Fig. 39-2). The heartbeat can be felt as a pulse at the wrist and otiier areas of die body where an artery is close to the surface or lies near a bone When the electrical impulse reaches the... 

Figure 39-2. The conducting system of the heart. Impulses originating in the node are transmitted through the atria to the AV node down the bundle of His and the bundle branches through the Flirkinje fi bers to the ventrides.

First-degree AV nodal blockade occurs due to inhibition of conduction within the upper portion of the node.15 Mobitz type I second-degree AV nodal blockade occurs as a result of inhibition of conduction further down within the node.12,15 Mobitz type II second-degree AV nodal blockade is caused by inhibition of conduction within or below the level of the bundle of His.12,15 Third-degree AV nodal blockade maybe a result of inhibition of conduction either within the AV node or within the bundle of His or the His-Purkinje system.12,15 AV block may occur as a result of age-related AV node degeneration. 

From the AV node, the electrical impulse spreads through the AV bundle or the bundle of His. This portion of the conduction system penetrates the fibrous tissue separating the atria from the ventricles and enters the interventricular septum where it divides into the left and right bundle branches. The bundle branches travel down the septum toward the apex of the heart and then reverse direction, traveling back toward the atria along the outer ventricle walls. This route of conduction of the impulse facilitates ejection of blood from the ventricles. If the impulse were to be conducted directly from the atria to the ventricles, the ventricular contraction would begin at the top of the chambers and proceed downward toward the apex. This would trap the blood at the bottom of the chambers. Instead, the wave of ventricular electrical stimulation and, therefore, contraction moves from the apex of the heart toward the top of the chambers where the semilunar valves are located and ejection takes place. 

Increased rate of conduction through the bundle of His and the Purkinje fibers... 

Buna process, 22 111 Bundle branches, 5 80 Bundle of His, 5 80 Bundles scrap, 21 409 Bundpacker, 7 691 B nnagel formula, 23 473 Bunsen burners, 7 455—451, 458 Bunsenite, color, 7 331 Bunsen-Roscoe law, 19 108 Bunyaviruses, 3 138 Buoyancy... 

Pharmacology Procainamide, a class lA antiarrhythmic, increases the effective refractory period of the atria, and to a lesser extent the bundle of His-Purkinje system and ventricles of the heart. 

FIGURE 23-2 Schematic representation of the conduction system of the heart. Conduction normally follows the pathways indicated by the dashed lines. Impulses originate in the sinoatrial node and are transmitted to the atrioventricular node. Impulses are then conducted from the atrioventricular node to the ventricles by the bundle of His and bundle branches. 

Q4 The cardiac impulse arises in the pacemaker tissue of the heart, the SA node. The nodal tissues of the heart-the SA and AV nodes in the right atrium - are spontaneously rhythmic. The impulse generated by the SA node spreads, rather like ripples on a pond, over the atria and reaches the AV node to excite it. From the AV node the impulse travels via the bundle of His along the Purkinje fibres, which are enlarged muscle cells with a high conduction velocity, to the ventricles. The cardiac impulse reaches the apex of the heart first and then spreads over the muscle of the two ventricles. 

The cardiac impulse starts in the SA node in the right atrium, spreads over the atria to excite the AV node and down the bundle of His to excite the ventricles. 

In a healthy heart, the primary rhythm is generated by the sinuatrial node. If the latter is damaged or disconnected from the subsequent parts of conducting system, lower centers such as the atrioventricular node or the bundle of His can take over and supply a somewhat slower rhtythm. 

The sinoatrial node (SA), consisting of spindle-shaped cells, initiates the electrical activity of the heart. From its location in the right atrium in proximity to the superior vena cava, the electrical activity spreads to the atria whose cells are larger than those of the SA. The pulse from the atria spreads to the atrioventricular node (AV), the gateway to the ventricles. The atria and the ventricles are electrically isolated. The AV node also slows down the electrical activity giving the atria time to fill. The bundle of His is the upper end of the electrical path, which through the Purkinje fibers allows the electrical signal to activate the ventricles and thus to pump the blood. 

Cardiovascular system. Atropine reduces vagal tone thus increasing the heart rate, and enhancing conduction in the bundle of His, effects that are less marked in the elderly in whom vagal tone is low. Full atropinisation may increase rate by 30 beats/min in the young, but has little effect in the old. 

The normal orderly sequence of events in cardiac contraction is initiated by a primary pacemaker, the sinoatrial (SA) node,21 which is located near the surface at the junction of the right atrium and the superior vena cava. One of its properties is automaticity. The normal firing frequency is 60-100 impulses/minute. The established rhythm is conducted to the atrioventricular (AV) node. This node serves to slow the beat somewhat so that atrial contraction can occur before the ventricle is stimulated. The AV node is in the septum (dividing wall) between the atria. The impulse is conducted from the AV node to a common bundle of fibers (Bundle of His) that cross the right atrium to the left ventricle. From there a division of fibers directs impulses along the septum dividing the ventricles, down to the lateral walls and the apex of the heart. The branching of the common bundle leads into the Purkinje fibers that innervate the heart musculature of the ventricles. 

Ventricular arrhythmias arise most often from cells in the bundle of His and Purkinje s fibers. These ectopic foci can cause premature contraction of ventricular cells, resulting in no beneficial... 

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