1.3 Cardiac Conduction System and Electrical Activity

The sinoatrial node, often referred to as the SA node, is a small mass of specialized cardiac muscle cells located in the right atrium of the heart. It functions as the primary pacemaker, generating electrical impulses that initiate each heartbeat and regulate the rhythm of the heart. This essential role connects the SA node to the overall functioning of both systemic and pulmonary circulation, as it influences how effectively blood is pumped through these pathways.

Atrioventricular Node: A cluster of cells located at the junction between the atria and ventricles that receives impulses from the SA node and delays them slightly before passing them on to the ventricles.

Heart Rate: The number of times the heart beats in one minute, influenced by signals from the SA node and other parts of the cardiac conduction system.

Cardiac Cycle: The sequence of events that occur during one complete heartbeat, including contraction and relaxation phases driven by electrical impulses from the SA node.

Purkinje fibers are specialized muscle fibers located in the inner ventricular walls of the heart, playing a crucial role in the heart's conduction system. They are responsible for rapidly conducting electrical impulses from the atrioventricular (AV) node to the ventricles, ensuring synchronized contraction of the heart muscle. Their unique structure allows for efficient propagation of signals, coordinating the heart's rhythm and contributing to effective systemic and pulmonary circulation.

Atrioventricular Node: The AV node is a cluster of cells located between the atria and ventricles that acts as a gatekeeper for electrical signals, slowing them down before they pass into the ventricles.

Bundle of His: Also known as the atrioventricular bundle, this structure conducts impulses from the AV node to the right and left bundle branches, leading to the Purkinje fibers.

Cardiac Cycle: The cardiac cycle refers to the sequence of events in one complete heartbeat, including contraction (systole) and relaxation (diastole) phases that affect both systemic and pulmonary circulation.

The bundle of His is a collection of heart muscle cells responsible for transmitting electrical impulses from the atrioventricular (AV) node to the ventricles, coordinating the heartbeat. It plays a critical role in the cardiac conduction system, ensuring that the heart beats efficiently and effectively by allowing the ventricles to contract in sync with the atria. This structure is vital for maintaining proper systemic and pulmonary circulation by ensuring that blood is pumped through both circuits correctly.

Atrioventricular Node (AV Node): A specialized cluster of cells located between the atria and ventricles that serves as a critical relay point in the cardiac conduction system, slowing down electrical impulses before they reach the ventricles.

Purkinje Fibers: Modified muscle fibers that conduct electrical impulses throughout the ventricles, ensuring coordinated contraction and effective pumping of blood.

Cardiac Cycle: The sequence of events that occurs during one complete heartbeat, including contraction (systole) and relaxation (diastole) of the heart chambers.

Depolarization is a change in a cell's membrane potential, making it more positive compared to its resting state. This process is crucial for the generation of action potentials in excitable cells, such as neurons and muscle fibers, allowing them to transmit signals and contract. During depolarization, sodium ions flood into the cell through voltage-gated channels, leading to a rapid rise in membrane potential and initiating various physiological responses.

Action Potential: A rapid rise and fall in membrane potential that occurs when a neuron or muscle cell is activated, characterized by depolarization followed by repolarization.

Resting Membrane Potential: The electrical potential difference across the membrane of a resting cell, typically around -70 mV, maintained by the distribution of ions inside and outside the cell.

Repolarization: The process following depolarization where the membrane potential returns to its resting state, primarily due to the efflux of potassium ions from the cell.

Sympathetic stimulation refers to the activation of the sympathetic nervous system, which prepares the body for 'fight or flight' responses during stressful situations. This process increases heart rate, enhances blood flow to muscles, and boosts energy availability, impacting the cardiac conduction system and electrical activity of the heart by promoting increased excitability and conduction velocity in cardiac tissues.

Parasympathetic Nervous System: The part of the autonomic nervous system that conserves energy by slowing down the heart rate and promoting digestion.

Cardiac Output: The volume of blood the heart pumps per minute, which is influenced by heart rate and stroke volume.

Norepinephrine: A neurotransmitter released during sympathetic stimulation that increases heart rate and force of contraction in cardiac muscle.

Repolarization is the process during which a cell's membrane potential returns to its resting state after depolarization, primarily involving the movement of ions across the cell membrane. In the context of cardiac muscle cells, repolarization is crucial for the heart's electrical cycle, ensuring that the heart can relax and refill with blood after contraction. This phase is essential for maintaining the rhythm and function of the heart, as it allows for proper recovery before the next heartbeat.

Depolarization: The phase in which the membrane potential of a cell becomes less negative (more positive) due to the influx of sodium ions, leading to the initiation of an action potential.

Action Potential: A rapid change in the membrane potential of a cell that results in a temporary reversal of electrical charge, allowing for signal transmission in neurons and contraction in muscle cells.

Refractory Period: The time following an action potential during which a cell cannot be stimulated to fire another action potential, ensuring proper timing and coordination of muscle contractions.

Systole is the phase of the cardiac cycle during which the heart muscles contract, pumping blood out of the chambers. This contraction is crucial for maintaining blood flow throughout the body and is directly linked to the function of the cardiac chambers and valves, as well as the heart's electrical activity. Understanding systole is key to comprehending how blood is circulated through both systemic and pulmonary pathways.

Diastole: The phase of the cardiac cycle when the heart muscles relax, allowing the chambers to fill with blood.

Cardiac Output: The volume of blood pumped by the heart per minute, which is influenced by heart rate and stroke volume during systole.

Electrocardiogram (ECG): A test that measures the electrical activity of the heart, showing the timing of systole and diastole.

Diastole refers to the phase of the cardiac cycle when the heart muscles relax, allowing the chambers of the heart to fill with blood. During this period, both the atria and ventricles undergo relaxation, which is crucial for efficient blood flow and maintaining proper circulation throughout the body. The timing and coordination of diastole are essential for optimal function of the heart's chambers and valves, affecting systemic and pulmonary circulation.

Systole: Systole is the phase of the cardiac cycle when the heart muscles contract, pumping blood out of the chambers into the arteries.

Atrioventricular Valves: These are valves located between the atria and ventricles, ensuring one-way blood flow from the atria to the ventricles during diastole.

Stroke Volume: Stroke volume is the amount of blood ejected by the ventricles during each contraction, influenced by how well the heart fills with blood during diastole.

An electrocardiogram (ECG or EKG) is a test that records the electrical activity of the heart over a period of time using electrodes placed on the skin. This diagnostic tool helps in understanding the heart's rhythm, size, and any potential damage to the heart muscle, linking closely to the cardiac cycle and the heart's conduction system. By visualizing the electrical impulses that trigger each heartbeat, an ECG provides vital information about the heart’s function and health.

Cardiac Cycle: The sequence of events in one complete heartbeat, including contraction (systole) and relaxation (diastole) of the heart chambers.

Arrhythmia: An irregular heartbeat that can be identified through changes in the patterns observed in an electrocardiogram.

P Wave: The part of the ECG that represents atrial depolarization, which is the first electrical impulse leading to heart contraction.

An ECG, or electrocardiogram, is a medical test that records the electrical activity of the heart over a period of time. This test is crucial in understanding how well the heart is functioning, as it provides insights into heart rhythms, size, and the presence of any structural issues. By capturing the electrical impulses that trigger each heartbeat, an ECG is instrumental in diagnosing various cardiac conditions and monitoring the heart's health.

P Wave: The P Wave represents the depolarization of the atria, which is the first phase of the heart's electrical cycle.

QRS Complex: The QRS Complex signifies the depolarization of the ventricles, reflecting a key phase in the heart's pumping action.

T Wave: The T Wave corresponds to the repolarization of the ventricles, indicating the recovery phase after a heartbeat.