Wiktor Wise
The cardiac cycle is a series of pressure changes within the heart, which result in blood movement through different chambers of the heart and the body as a whole. These pressure changes originate as conductive electrochemical changes within the myocardium that result in the concentric contraction of cardiac muscle. The timing and strength of each heartbeat are signalled by arterial baroreceptors, which are stretch sensors that lie in concentrated patches within the aortic arch and at the bifurcation of the carotid arteries. The electrical impulse travels from the sinus node to the atrioventricular node (also called the AV node). So, which electrical event immediately precedes atrial systole?
| Characteristics | Values |
|---|---|
| Electrical event immediately preceding atrial systole | An electrical stimulus is generated by the sinus node (sinoatrial node, or SA node) |
| The sinus node is a small mass of specialized tissue located in the right upper chamber (atria) of the heart | |
| The electrical impulse travels from the sinus node to the atrioventricular node (AV node) | |
| The AV node slows down the electrical impulse for a very short period before continuing down the conduction pathway via the bundle of His into the ventricles | |
| The atria are then activated, and the electrical stimulus causes the ventricles to contract and pump out blood | |
| The cardiac cycle is a series of pressure changes within the heart | |
| These pressure changes originate as conductive electrochemical changes within the myocardium that result in the concentric contraction of cardiac muscle |
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What You'll Learn
Electrical stimulation from the sinus node
The heart is a pump made of muscle tissue. Its pumping action is regulated by electrical impulses. An electrical stimulus is generated by the sinus node (also called the sinoatrial node or SA node). This is a small mass of specialised tissue located in the right upper chamber (atria) of the heart. The sinus node acts as the heart's natural pacemaker.
The SA node represents a crescent-like cluster of myocytes divided by connective tissue, spreading over a few square millimetres. These cells can spontaneously generate an electrical impulse. The integrated activity of these so-called pacemaker cells forms the SA node. This electrical impulse is then transmitted by perinodal cells, or transitional (T) cells, to the right atrium and then through the rest of the heart’s electrical conduction system, eventually resulting in myocardial contraction and blood distribution to the rest of the body.
The sinus node continuously generates electrical impulses, setting a healthy heart's normal rhythm and rate. The sinus node generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions. The atria are then activated. The electrical stimulus travels down through the conduction pathways and causes the heart's ventricles to contract and pump out blood. The 2 upper chambers of the heart (atria) are stimulated first and contract for a short period of time before the 2 lower chambers of the heart (ventricles).
The electrical impulse travels from the sinus node to the atrioventricular node (also called the AV node). There, impulses are slowed down for a very short period, then continue down the conduction pathway via the bundle of His into the ventricles. The bundle of His divides into right and left pathways, called bundle branches, to stimulate the right and left ventricles.
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Atrial systole begins at the P wave deflection
The cardiac cycle refers to the performance of the human heart from the beginning of one heartbeat to the beginning of the next. It involves two periods: the diastole and the systole. During the diastole, the heart muscle relaxes and refills with blood. This is followed by the systole, a period of robust contraction and pumping of blood.
The cardiac cycle begins with the diastole, during which the heart relaxes and expands to receive an influx of blood returning from the lungs and other systems of the body. The atria and ventricles are relaxed, and blood flows into the atria from the veins, which are higher in pressure. As blood flows into the atria, the pressure rises, causing blood to move passively from the atria into the ventricles.
Atrial systole marks the end of the diastole and the beginning of the systole. It is the contraction of the cardiac muscle cells of both atria following electrical stimulation and conduction of electrical currents across the atrial chambers. Atrial systole begins at the P wave deflection, which represents the electrical depolarization of the atria. This electrical stimulus is generated by the sinus node (also called the sinoatrial node or SA node), a small mass of specialized tissue located in the right upper chamber (atria) of the heart. The P wave deflection occurs when the electrical impulse travels from the sinus node to the atrioventricular node (AV node), where it is slowed down briefly before continuing down the conduction pathway into the ventricles.
The P wave is typically biphasic, with the first part originating from the right atrium and the latter part from the left atrium. The deflection of the P wave is influenced by the forces in the right and left atria, with anterior forces in the right atrium resulting in an initial positive deflection and posterior forces in the left atrium causing a later negative deflection. A large negative deflection can indicate a left atrial abnormality, such as hypertrophy or dilation.
In summary, atrial systole begins at the P wave deflection, which represents the electrical depolarization and contraction of the atria, triggered by the electrical stimulus generated by the sinus node. This initiates the systole phase of the cardiac cycle, during which the heart undergoes contraction and pumps blood into circulation.
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The role of the AV node
The atrioventricular node (AV node) is a crucial component of the heart's electrical conduction system. It is a small structure, measuring approximately 1 x 3 x 5 mm, located in the lower back section of the interatrial septum near the coronary sinus. The AV node connects the electrical systems of the atria and ventricles, ensuring the coordination of their contractions.
The AV node plays a critical role in maintaining the proper timing of heartbeats. It receives electrical impulses from the sinoatrial (SA) node, which acts as the primary pacemaker of the heart. The AV node then slightly delays these impulses by about 0.09 seconds or approximately 40 milliseconds. This delay is of utmost importance as it allows the atria to eject blood into the ventricles before the ventricles contract. This delay also protects the ventricles from excessively fast rate responses to atrial arrhythmias.
The AV node's ability to delay impulses is due to its unique property of decremental conduction. This means that the more frequently the AV node is stimulated, the slower it conducts the electrical signals. This property is vital for preventing rapid conduction to the ventricles in cases of rapid atrial rhythms, such as atrial fibrillation or atrial flutter. Additionally, the AV node can independently produce electrical impulses, acting as a secondary pacemaker when necessary. This intrinsic firing rate of the AV node is typically between 40 and 60 times per minute.
Furthermore, the AV node is associated with certain pathologic conditions. For example, in heart block, the AV node may become increasingly refractory to impulses, resulting in dropped beats. Accelerated junctional rhythm and junctional tachycardia are also related to the AV node, where it generates electrical impulses at a faster rate than normal. Understanding the anatomy and function of the AV node is crucial for comprehending these pathologic conditions and their clinical implications.
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Bundle of His and Purkinje fibres
The bundle of His and Purkinje fibres are both components of the cardiac conduction system, playing critical roles in the transmission of electrical signals through the heart. The bundle of His, discovered by Swiss cardiologist Wilhelm His Jr. in 1893, is a specialised muscle fibre located between the atrium and ventricular chambers of the heart. It connects the auricular and ventricular chambers and is composed of various types of cells, including slender and broad transitional cells, Purkinje-like cells, and pacemaker cells. The bundle of His divides into right and left bundle branches, which stimulate the right and left ventricles, respectively.
The Purkinje fibres, discovered by Czech anatomist Jan Evangelista Purkyně in 1839, are located in the ventricular walls of the heart, below the endocardium in a space called the subendocardium. They are composed of fibres capable of transmitting electrical impulses quickly and efficiently through the heart muscles. The Purkinje fibres receive electrical impulses from the bundle of His and transmit them to the heart's ventricles, resulting in ventricular contraction and the ejection of blood from the ventricles into circulation.
The electrical conduction system of the heart, including the bundle of His and Purkinje fibres, ensures the rhythmic and synchronised function of the heart. The bundle of His helps maintain the sinus rhythm and prevents irregular heartbeats or arrhythmias. It also maintains the atrioventricular delay, allowing for the complete ejection of blood from the ventricles.
Problems with the normal pumping function of the heart can be due to disorders in the nodal or network conduction system. Changes in ECG patterns related to the bundle of His and Purkinje fibres aid in the diagnosis of conduction abnormalities. Implantable devices such as pacemakers can target these structures to optimise cardiac function in conduction disorders.
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Ventricular systole follows
The heart is a pump made up of muscle tissue. Its pumping action is regulated by an electrical conduction system that coordinates the contraction of the various chambers of the heart. An electrical stimulus is generated by the sinus node, a small mass of specialised tissue located in the right upper chamber (atria) of the heart. The electrical impulse travels from the sinus node to the atrioventricular node (AV node). There, the impulses are slowed down for a very short period, then continue down the conduction pathway via the bundle of His into the ventricles.
Initially, as the muscles in the ventricles contract, the pressure of the blood within the chamber rises, but it is not yet high enough to open the semilunar valves (pulmonary and aortic) and be ejected from the heart. However, blood pressure quickly rises above that of the atria, which are now relaxed and in diastole. This increase in pressure causes blood to flow back toward the atria, closing the tricuspid and mitral valves.
In the second phase of ventricular systole, oxygenated blood is pumped from the left ventricle through the aortic valve and aorta to all body systems. Simultaneously, oxygen-poor blood is pumped from the right ventricle through the pulmonic valve and pulmonary artery to the lungs.
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Frequently asked questions
The electrical event that immediately precedes atrial systole is the depolarization of the sinoatrial node in the upper right atrium.
The sinoatrial node, often referred to as the SA node or the cardiac pacemaker, is a small mass of specialized tissue located in the right upper chamber (atria) of the heart.
The sinoatrial node generates an electrical stimulus that travels through the conduction pathways, causing the contraction of the heart's ventricles and the pumping of blood.
Atrial systole is the contraction of the cardiac muscle cells of both atria, which completes the diastole and initiates ventricular systole.
An ECG (electrocardiogram) records the electrical activity of the heart. Atrial systole is represented by the P wave deflection of a steady signal on the ECG, indicating the contraction of the atria.
