19.2 Cardiac Muscle and Electrical Activity
5 min read•june 18, 2024
Cardiac muscle tissue is the powerhouse of the heart, enabling its continuous pumping action. Its unique structure, including and branching cells, allows for coordinated contractions. High mitochondrial content fuels the heart's relentless energy demands.
The heart's electrical system orchestrates its rhythm through specialized structures like the SA node and conduction pathways. Understanding cardiac electrical activity is crucial for interpreting ECGs and diagnosing abnormalities that can disrupt the heart's vital function.
Cardiac Muscle Tissue
Structural features of cardiac muscle
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- Intercalated discs connect adjacent allowing rapid transmission of electrical impulses and coordinated contraction
- Contain that enable electrical coupling between cells
- provide mechanical strength to withstand the force of contraction
- anchor actin filaments to the cell membrane for efficient force transmission
- enables the heart to generate its own electrical impulses without external stimulation
- (SA and AV nodes) possess unstable resting membrane potentials that spontaneously depolarize
- Allows the heart to maintain a regular rhythm even in the absence of neural input
- Branching structure of cardiomyocytes facilitates efficient contraction and pumping of blood
- Shorter and wider than skeletal muscle cells (ventricular cardiomyocytes ~100 μm long and 10-25 μm wide)
- Branching morphology increases the surface area for intercellular connections and force generation
- High mitochondrial content supports the heart's continuous energy demands
- Mitochondria occupy ~30-40% of cardiomyocyte volume
- Produce ATP through oxidative phosphorylation to fuel the constant contraction and relaxation cycle
- The , composed of cardiac muscle tissue, forms the middle and thickest layer of the heart wall
- Contains sarcomeres, the basic functional units of muscle contraction, arranged in a highly organized structure
Electrical Activity of the Heart
Components of cardiac conduction system
- acts as the primary initiating electrical impulses
- Located in the right atrium near the opening of the superior vena cava
- Exhibits the highest intrinsic rate of (~60-100 bpm)
- receives impulses from the SA node and delays their transmission to the ventricles
- Positioned in the interatrial septum near the tricuspid valve
- Allows time for atrial contraction to complete before ventricular contraction begins
- rapidly conducts impulses from the AV node to the ventricles
- Divides into left and right bundle branches within the interventricular septum
- Ensures synchronized activation of the ventricular
- are the terminal branches of the bundle branches
- Spread electrical impulses throughout the ventricular walls
- Enable rapid and coordinated ventricular contraction from apex to base
Ion movements in cardiac cells
- of cardiomyocytes is maintained at around -90 mV
- Primarily determined by the concentration gradient and permeability of K+ ions
- High intracellular K+ concentration and open K+ leak channels contribute to the negative potential
- occurs when voltage-gated Na+ channels open allowing rapid influx of Na+ ions
- Raises the membrane potential towards the Na+ equilibrium potential (+30 mV)
- Reaches a threshold potential triggering an
- Plateau phase is characterized by a slow influx of Ca2+ ions through L-type
- Prolongs duration (200-400 ms) compared to skeletal muscle
- Sustains contraction by maintaining a depolarized state
- restores the negative
- Efflux of K+ ions through delayed rectifier K+ channels
- Closure of Ca2+ channels and activation of Na+/K+ ATPase pump
- The , during which the cardiac muscle cannot be re-excited, prevents tetanic contractions and ensures coordinated pumping
ECG interpretation and cardiac cycle
- represents atrial depolarization preceding atrial contraction
- Typically <0.12 s in duration and <2.5 mm in amplitude
- Absence may indicate or sinoatrial block
- corresponds to ventricular depolarization and contraction (systole)
- Normally <0.12 s in duration and 5-30 mm in amplitude
- Widened QRS suggests or ventricular hypertrophy
- reflects ventricular during relaxation (diastole)
- Follows the and is typically upright in most leads
- Inverted or flattened T waves may signify myocardial ischemia or electrolyte imbalances
- measures the time from the start of atrial depolarization to ventricular depolarization
- Normally 0.12-0.20 s in duration
- Prolonged PR interval indicates an AV conduction delay (1st-degree AV block)
- encompasses the duration of ventricular depolarization and repolarization
- Varies with and is typically corrected (QTc) using Bazett's formula:
- Prolonged QTc (>450 ms in men, >460 ms in women) increases the risk of ventricular arrhythmias
- The , consisting of systole and diastole, is regulated by the , which relates ventricular stretch to contractile force
Cardiac conduction abnormalities
- is a slow heart rate (<60 bpm) originating from the SA node
- May result from increased vagal tone, medications (beta-blockers), or hypothyroidism
- Generally benign but can cause symptoms like fatigue or dizziness if severe
- refers to a rapid heart rate (>100 bpm) initiated by the SA node
- Occurs in response to physiological stressors (exercise, fever) or pathological conditions (anemia, hyperthyroidism)
- Rarely requires treatment unless underlying cause is identified
- involves impaired conduction between the atria and ventricles
- First-degree: Prolonged PR interval (>0.20 s) but all impulses are conducted
- Second-degree: Intermittent conduction with some P waves not followed by QRS complexes (Mobitz I or II)
- Third-degree: Complete dissociation between atrial and ventricular activity, requiring pacemaker implantation
- Bundle branch block refers to impaired conduction in either the left or right bundle branch
- Leads to abnormal ventricular activation and a widened QRS complex (>0.12 s)
- Left bundle branch block (LBBB) may indicate underlying heart disease or cardiomyopathy
- Atrial fibrillation is characterized by rapid, irregular, and disorganized atrial activation
- Multiple ectopic foci in the atria fire at rates up to 300-600 bpm
- Increases the risk of thromboembolic events (stroke) due to ineffective atrial contraction and stasis of blood
Historical context
- developed the first practical in the early 20th century, revolutionizing the study of cardiac electrical activity and laying the foundation for modern cardiology
Key Terms to Review (67)
Action Potential: An action potential is a rapid, transient electrical signal that travels along the cell membrane of excitable cells, such as neurons and muscle cells. It is the fundamental unit of communication in the nervous system, enabling the transmission of information between different parts of the body.
Artificial pacemaker: An artificial pacemaker is a small electronic device implanted in the chest to regulate the heartbeat. It sends electrical impulses to the heart muscle, ensuring the heart maintains an adequate rate and rhythm.
Atrial Fibrillation: Atrial fibrillation is an irregular and often rapid heart rate that can increase the risk of stroke, heart failure, and other heart-related complications. It occurs when the upper chambers of the heart (the atria) experience chaotic electrical signals, causing them to quiver (fibrillate) instead of beating normally.
Atrioventricular (AV) node: The atrioventricular (AV) node is a cluster of cells situated in the heart between the atria and ventricles that plays a crucial role in regulating the timing of heart muscle contractions by delaying electrical signals. This delay ensures that the atria have finished contracting to fill the ventricles with blood before they themselves contract.
Atrioventricular Block: Atrioventricular (AV) block is a conduction disorder in the heart where the electrical impulses generated in the atria are partially or completely blocked from reaching the ventricles. This disruption in the normal electrical pathway can lead to various degrees of heart rhythm disturbances.
Atrioventricular bundle: The atrioventricular bundle, also known as the bundle of His, is a collection of heart muscle cells specialized for electrical conduction that transmits the electrical impulses from the atria to the ventricles. This ensures that the heart chambers contract in a coordinated manner, allowing efficient pumping of blood throughout the body.
Atrioventricular bundle branches: Atrioventricular bundle branches are specialized cardiac muscle fibers that conduct electrical impulses from the atrioventricular node down through the heart, ensuring coordinated contraction of the ventricles. They are part of the heart's electrical conduction system that controls the heart rate and rhythm.
Atrioventricular Node: The atrioventricular (AV) node is a group of specialized cardiac muscle cells located in the right atrium of the heart. It serves as a critical junction in the electrical conduction system, coordinating the contraction of the atria and ventricles to ensure efficient blood flow throughout the cardiovascular system.
Autorhythmicity: Autorhythmicity is the ability of cardiac muscle cells to initiate their own electrical impulses without needing an external stimulus. This intrinsic property ensures a consistent heart rate and rhythm necessary for efficient blood circulation.
Autorhythmicity: Autorhythmicity refers to the intrinsic ability of certain tissues, particularly cardiac muscle, to generate and maintain rhythmic electrical impulses without the need for external stimulation. This self-generated, spontaneous electrical activity is a fundamental characteristic that allows these tissues to function as natural pacemakers, regulating the rhythmic contraction and relaxation of the heart.
Bachmann’s bundle: Bachmann’s bundle is a band of cardiac muscle fibers that conducts electrical impulses from the atria to the ventricles, specifically facilitating the conduction from the right atrium to the left atrium in the heart. It plays a crucial role in ensuring synchronized contraction of both atria during the cardiac cycle.
Bundle Branch Block: A bundle branch block is a disruption in the electrical conduction system of the heart, specifically in the bundle of His or its branches, which can lead to an abnormal heart rhythm and impaired cardiac function. This term is crucial in understanding the topics of 19.2 Cardiac Muscle and Electrical Activity, as it directly relates to the heart's ability to effectively coordinate and distribute electrical signals throughout the myocardium.
Bundle of His: The Bundle of His is a collection of heart muscle cells specialized for electrical conduction that transmits the electrical impulses from the AV node to the point of the apex in the heart's ventricles. This key structure ensures the coordinated contraction of the ventricles, essential for maintaining a rhythmic and efficient heartbeat.
Bundle of His: The bundle of His is a specialized conduction system in the heart that transmits electrical impulses from the atrioventricular (AV) node to the ventricles, allowing for the coordinated contraction of the heart. It is a critical component of the cardiac electrical activity and plays a vital role in the overall functioning of the cardiovascular system.
Calcium Channels: Calcium channels are specialized pore-forming proteins that allow the controlled passage of calcium ions (Ca2+) across cell membranes. They play a crucial role in regulating various physiological processes, including muscle contraction, neurotransmitter release, and intracellular signaling.
Cardiac cycle: The cardiac cycle is the sequence of events that occurs in the heart during one complete heartbeat, encompassing both the contraction (systole) and relaxation (diastole) of both atria and ventricles. It ensures blood is pumped effectively throughout the body.
Cardiac Cycle: The cardiac cycle refers to the sequence of events that occur during a single heartbeat, including the contraction and relaxation of the heart's chambers and the flow of blood through the cardiovascular system. This cycle is essential for maintaining the heart's pumping function and ensuring the continuous circulation of blood throughout the body.
Cardiac Muscle and Electrical Activity: Cardiac muscle is the specialized type of striated muscle found in the heart that is responsible for the rhythmic contractions that pump blood throughout the body. The electrical activity that coordinates these contractions is a critical component of cardiac function and overall cardiovascular health.
Cardiomyocytes: Cardiomyocytes, also known as cardiac muscle cells, are the contractile cells that make up the majority of the heart muscle. These specialized cells are responsible for the rhythmic contraction and relaxation of the heart, enabling it to pump blood throughout the body effectively.
Depolarization: Depolarization is the process by which the electrical potential across a cell membrane, typically a neuron or cardiac muscle cell, becomes less negative. This change in membrane potential is a crucial step in the generation and propagation of electrical signals within the body's nervous and cardiovascular systems.
Desmosomes: Desmosomes are specialized cell-cell junctions that mechanically connect adjacent epithelial cells, forming a strong adhesive bond. They are crucial for maintaining the structural integrity and barrier function of epithelial tissues, such as the skin and cardiac muscle.
Electrocardiogram: An electrocardiogram (ECG or EKG) is a diagnostic test that records the electrical activity of the heart. It is a non-invasive way to assess the heart's function and identify any abnormalities in the heart's rhythm or structure.
Electrocardiogram (ECG): An electrocardiogram (ECG) is a medical test that records the electrical activity of the heart over a period of time using electrodes placed on the skin. It is essential for diagnosing and monitoring heart conditions by capturing the timing and duration of each electrical phase in your heartbeat.
Fascia Adherens: Fascia adherens is a type of cell-cell junction found in cardiac muscle tissue that helps connect adjacent cardiomyocytes (heart muscle cells) and facilitate the coordinated contraction of the heart. It is a crucial structural and functional component in the context of cardiac muscle tissue and electrical activity.
Frank-Starling Law: The Frank-Starling law, also known as the Frank-Starling mechanism, is a fundamental principle that describes the relationship between the volume of blood entering the heart (preload) and the force of contraction of the heart's ventricles. It explains how the heart can automatically adjust its output to meet the body's changing demands for blood flow.
Gap Junctions: Gap junctions are specialized intercellular connections that allow for the direct transfer of ions, small molecules, and electrical signals between adjacent cells. These specialized membrane channels facilitate communication and synchronization between connected cells, playing crucial roles in various tissues and physiological processes.
Heart block: Heart block is a condition where the electrical signals that control heartbeats are partially or completely blocked from reaching certain areas of the heart. This can cause irregular heart rhythms or slower heart rates.
Heart Rate: Heart rate is the number of times the heart beats per minute, providing a measure of the heart's pumping activity and overall cardiovascular function. It is a crucial indicator of cardiac health and physical fitness in the context of cardiac muscle and electrical activity.
Interatrial band: The interatrial band, also known as the Bachmann's bundle, is a band of cardiac muscle fibers that connects the right and left atria in the heart. It plays a crucial role in coordinating the electrical conduction between these two chambers, ensuring that they contract simultaneously during the cardiac cycle.
Intercalated Discs: Intercalated discs are specialized structures found in cardiac muscle cells that allow for the efficient transmission of electrical signals and mechanical forces between adjacent cells. These discs act as the communication hubs that coordinate the synchronized contraction of the heart, ensuring its ability to function as a unified pump.
Internodal pathways: Internodal pathways are specialized muscle fibers in the heart that conduct electrical impulses from the sinoatrial (SA) node to the atrioventricular (AV) node. They ensure rapid and coordinated transmission of electrical activity necessary for synchronized heart contractions.
Myocardial conducting cells: Myocardial conducting cells are specialized cardiac muscle cells responsible for generating and transmitting electrical impulses that regulate the heart's beating. These cells coordinate the heart's contractions to ensure efficient blood pumping throughout the body.
Myocardial contractile cells: Myocardial contractile cells are specialized muscle cells in the heart responsible for generating the force needed to pump blood throughout the body. These cells make up the majority of the heart muscle and function through coordinated contractions.
Myocardium: The myocardium is the thick, muscular layer of the heart wall, composed of cardiac muscle cells that allow it to contract and pump blood throughout the body. It is situated between the outer epicardium and the inner endocardium layers of the heart.
Myocardium: The myocardium is the middle and thickest layer of the heart wall, composed of cardiac muscle tissue. It is responsible for the rhythmic contraction and pumping action of the heart, playing a crucial role in the cardiovascular system's ability to circulate blood throughout the body.
P wave: The P wave is the first wave in an electrocardiogram (ECG or EKG), representing the depolarization of the atria in response to an electrical impulse from the sinoatrial node. This process initiates atrial contraction, contributing to blood flow into the ventricles.
Pacemaker: A pacemaker is a specialized group of cardiac muscle cells in the heart that generates and regulates the electrical impulses responsible for initiating the heartbeat. These cells ensure the heart maintains a regular rhythm, coordinating the contractions of the heart chambers.
Pacemaker Cells: Pacemaker cells, also known as cardiac pacemaker cells, are specialized cells located in the heart that generate and propagate the electrical impulses that control the rhythmic contraction of the cardiac muscle. They are responsible for initiating and maintaining the heart's natural pacemaker function, ensuring the coordinated and efficient pumping of blood throughout the body.
Potassium Channels: Potassium channels are specialized membrane proteins that allow the selective passage of potassium ions (K+) across the cell membrane. They play a crucial role in regulating the electrical activity of cells, particularly in the context of action potentials and cardiac muscle function.
PR interval: The PR interval is the time period on an electrocardiogram (ECG) that measures the duration from the onset of atrial depolarization to the onset of ventricular depolarization. It reflects the electrical conduction time through the atria and the atrioventricular (AV) node, serving as an important indicator of cardiac health and function. Understanding the PR interval is crucial for evaluating various cardiac conditions, as abnormalities can suggest issues with electrical conduction within the heart.
Prepotential depolarization: Prepotential depolarization is the gradual increase in the electrical charge of cardiac pacemaker cells, which brings them to the threshold required to generate a heartbeat. This process ensures the heart beats at a regular pace without external stimuli.
Purkinje fibers: Purkinje fibers are specialized muscle fibers located in the walls of the heart's ventricles, responsible for conducting electrical impulses that trigger heart contractions. They ensure the simultaneous contraction of the ventricles, allowing efficient blood pumping throughout the body.
Purkinje Fibers: Purkinje fibers are specialized cardiac conduction fibers that rapidly transmit electrical impulses from the atrioventricular (AV) node to the ventricular myocardium, playing a crucial role in the electrical activity and coordinated contraction of the heart.
QRS complex: The QRS complex is a segment of an electrocardiogram (ECG) that represents the electrical impulse as it spreads through the ventricles, causing them to contract. This rapid depolarization of the heart's ventricular fibers is crucial for pumping blood out of the heart.
QRS Complex: The QRS complex is a prominent feature in the electrocardiogram (ECG) that represents the depolarization of the ventricles, the main pumping chambers of the heart. It is a critical component in understanding the electrical activity and function of the cardiac muscle.
QT Interval: The QT interval is a measure of the time it takes for the ventricles of the heart to depolarize and then repolarize, representing the duration of the ventricular electrical activity. It is an important indicator of cardiac health and is closely associated with the electrical activity and muscle contraction of the heart.
Refractory Period: The refractory period is a crucial concept in various physiological processes, including the function of nervous tissue, cardiac muscle, and the propagation of action potentials. It refers to the time interval during which a cell or tissue is unable to respond to a new stimulus or generate another action potential, even if the necessary stimulus is applied.
Relative refractory period: The relative refractory period is the phase following an action potential during which a neuron can be stimulated to initiate another action potential, but only by a stronger-than-usual stimulus. It occurs right after the absolute refractory period and represents a time of decreased sensitivity to new stimuli.
Repolarization: Repolarization is the process during an action potential when a neuron's membrane potential returns to its resting negative state after depolarization. It occurs due to the efflux of potassium ions (K+) through channels in the neuron's membrane.
Repolarization: Repolarization is the process by which the resting membrane potential of an excitable cell, such as a neuron or a cardiac muscle cell, is restored after an action potential has been generated. This crucial phase of the action potential cycle allows the cell to regain its ability to respond to subsequent stimuli.
Resting membrane potential: Resting membrane potential is the electrical charge difference across the neuronal membrane when the neuron is not actively transmitting a signal. It is typically negative, indicating that the inside of the neuron is more negatively charged compared to the outside.
Resting Membrane Potential: The resting membrane potential is the electrical charge difference across the cell membrane when the cell is not actively transmitting an electrical signal. It is a crucial concept in understanding the function of nervous tissue, the action potential, and the electrical activity of cardiac muscle.
Sarcomere: A sarcomere is the basic contractile unit of muscle fiber in skeletal muscle, made up of long protein filaments including actin and myosin that slide past each other to produce a muscle contraction. It is bounded by Z lines to which the actin filaments are attached.
Sarcomere: The sarcomere is the fundamental contractile unit of skeletal, cardiac, and smooth muscle fibers. It is the basic structural and functional unit responsible for the contraction and relaxation of muscle tissue, and is a key component in the overall process of muscle motion and activity.
Sinoatrial (SA) node: The sinoatrial (SA) node is a cluster of cells located in the right atrium of the heart that initiates the electrical impulses responsible for each heartbeat. It acts as the natural pacemaker of the heart, setting the rhythm and pace for cardiac activity.
Sinoatrial Node: The sinoatrial (SA) node is a specialized group of cells located in the right atrium of the heart that acts as the heart's natural pacemaker. It generates the electrical impulses that coordinate the rhythmic contractions of the heart's chambers, initiating the cardiac cycle.
Sinus bradycardia: Sinus bradycardia is a condition characterized by a slower than normal heart rate, specifically fewer than 60 beats per minute, originating from the sinoatrial node. This term highlights the role of the heart's natural pacemaker, which controls the rhythm and rate of the heartbeat, and indicates potential implications for cardiac function, electrical conduction, and overall health.
Sinus rhythm: Sinus rhythm is the normal heartbeat initiated by the sinoatrial node, characterized by a regular rhythm with a rate of 60 to 100 beats per minute. It indicates proper electrical activity and functioning of the heart in generating pulses.
Sinus Tachycardia: Sinus tachycardia is a type of cardiac arrhythmia characterized by an abnormally rapid heart rate originating from the sinus node, the heart's natural pacemaker. This condition is closely linked to the electrical activity of the heart and the functioning of cardiac muscle.
Sodium Channels: Sodium channels are specialized membrane proteins found in the cell membranes of many cell types, including cardiac muscle cells. They are responsible for the rapid influx of sodium ions into the cell, which is a crucial step in the initiation and propagation of the electrical impulse that drives the contraction of the heart.
Spontaneous depolarization: Spontaneous depolarization is the automatic and gradual change in membrane potential that occurs in certain cardiac muscle cells, leading them to reach the threshold potential and generate an action potential without external stimulation. It is crucial for initiating and regulating the heart's rhythm.
Stroke Volume: Stroke volume is the amount of blood pumped from the ventricles of the heart with each contraction. It is a crucial factor in determining cardiac output, which is the total volume of blood pumped by the heart per minute. Stroke volume is an important concept in understanding the heart's ability to effectively circulate blood throughout the body.
Stroke volume (SV): Stroke volume is the amount of blood pumped by the left ventricle of the heart in one contraction. It varies depending on bodily demands, such as during exercise or rest.
T wave: The T wave represents the repolarization of the ventricles in an electrocardiogram (ECG) during a cardiac cycle. It is an essential component in diagnosing heart rhythm and function.
T Wave: The T wave is an important component of the electrocardiogram (ECG) that represents the electrical repolarization of the ventricles, the lower chambers of the heart. It is a crucial indicator of cardiac function and is closely related to the topics of cardiac muscle and electrical activity.
The Action Potential: An action potential is a rapid, temporary change in the electrical membrane potential of a neuron or muscle cell, allowing it to transmit a signal. It involves an influx of sodium ions into the cell followed by an efflux of potassium ions, restoring the original electrical condition.
Willem Einthoven: Willem Einthoven was a Dutch physiologist who is credited with the invention of the electrocardiogram (ECG or EKG), a diagnostic tool that records the electrical activity of the heart. His contributions were instrumental in understanding the electrical properties of cardiac muscle and its role in the heart's function.