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Cardiac muscle tissue is characterised by automaticity and excitability. Cardiac excitability refers to the ease with which cardiac cells undergo a series of events, including regenerative depolarisation and repolarisation during the action potential, as well as the ease with which electrical activity propagates from cell to cell. The cycle of depolarisation and repolarisation in the heart is known as the cardiac action potential and occurs approximately 60 times every minute. The heartbeat arises from a highly organised control of ionic flow through channels in the cardiac membrane, the myoplasm, and the extracellular space. The conduction system in the atria is designed to initiate atrial depolarisation and propagate the impulse toward the ventricle.
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What You'll Learn
- Cardiac muscle cells are unique from other excitable cells in that they remain permeable to potassium in the resting state
- The cardiac cell membrane generates action potentials when stimulated to the threshold
- The excitation-contraction coupling in the cardiac muscle is modulated by calcium levels
- The sinoatrial node pacemaker is dominant, but occasional spontaneous premature beats may arise
- The cardiac cell is coupled to rhythmic excitability and contracts or relaxes in phase with depolarization or repolarization
Cardiac muscle cells are unique from other excitable cells in that they remain permeable to potassium in the resting state
Cardiac muscle cells, or cardiomyocytes, are a type of excitable cell that plays a crucial role in maintaining the electrical activity necessary for the heart's rhythmic contractions. These cells exhibit distinct properties that differentiate them from other excitable cells in the body, such as neurons and skeletal muscle cells. One unique characteristic of cardiac muscle cells is their permeability to potassium ions, even when they are in a resting state.
In the resting state, the intracellular environment of a cardiac muscle cell has a specific negative charge of approximately −90 millivolts (mV) relative to the extracellular environment, which is close to 0 mV. This voltage difference is known as the resting membrane potential and is crucial for maintaining the electrochemical balance across the cell membrane. Unlike other excitable cells, cardiac muscle cells remain permeable to potassium ions, allowing a continuous exchange of these ions between the intracellular and extracellular compartments.
The permeability of cardiac muscle cells to potassium ions in the resting state is facilitated by specific ion channels, including leak channels and potassium channels like the inwardly rectifying potassium channel. This permeability ensures that the intracellular environment is responsive to changes during the depolarization phase of the cardiac action potential. Depolarization occurs when voltage-gated sodium channels open, allowing an influx of sodium ions (Na+) into the cell, which changes the membrane potential and triggers the generation of an action potential.
The ability of cardiac muscle cells to remain permeable to potassium ions in the resting state is essential for maintaining proper duration between action potentials. Potassium ions play a critical role in the repolarization phase, where they flow out of the cell, restoring the negative charge and returning the cell to its resting membrane potential. This process ensures that the cell is ready to receive another impulse and helps regulate the frequency of cardiac contractions.
In summary, cardiac muscle cells exhibit unique properties that distinguish them from other excitable cells, including their permeability to potassium ions in the resting state. This permeability is vital for maintaining the electrochemical balance across the cell membrane and facilitating the intracellular response to depolarization. By regulating the flow of potassium ions, cardiac muscle cells can maintain proper timing between action potentials, ensuring the coordinated contractions necessary for efficient cardiac output.
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The cardiac cell membrane generates action potentials when stimulated to the threshold
The cardiac cell membrane is a key component in the generation of action potentials, which are essential for the normal functioning of the heart. When stimulated beyond a certain threshold, the cardiac cell membrane initiates a sequence of events that result in the production of action potentials. This process, known as cardiac excitability, involves the regenerative depolarization and repolarization of cardiac cells, along with the propagation of electrical activity from cell to cell.
Cardiac cells have a resting state where they are fully repolarized, with an intracellular environment of approximately -90 millivolts relative to the extracellular environment of around 0 millivolts. During this resting state, voltage-sensitive channels remain closed, and the cell is ready to receive an electrical impulse. The cardiac pacemaker cells of the sinoatrial node initiate action potentials intrinsically and rhythmically, serving as an internal clock that can be adjusted by external stimuli.
When stimulated beyond the threshold, the cardiac cell membrane undergoes depolarization, a critical phase in the generation of action potentials. Depolarization involves the opening of voltage-gated sodium channels, allowing an influx of sodium ions. This influx further increases the membrane potential, creating a positive feedback loop that opens more sodium channels. The rapid increase in sodium ions drives the membrane potential towards the sodium equilibrium potential.
However, as the membrane potential becomes more positive, the sodium channels close and lock, entering an "inactivated" state. This marks the beginning of the absolute refractory period, during which the cell cannot produce another action potential. It is followed by the relative refractory period, where a stronger-than-usual stimulus is required to initiate another action potential. The absolute and relative refractory periods are essential for regulating the timing and frequency of action potentials.
The cardiac cell membrane's ability to generate action potentials when stimulated beyond the threshold is fundamental to maintaining cardiac rhythm and coordinating the contraction of the heart. This process involves the intricate interplay of ion channels, membrane potentials, and specialized conduction cells, ensuring the efficient propagation of excitation and contraction necessary for healthy cardiac output.
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The excitation-contraction coupling in the cardiac muscle is modulated by calcium levels
Cardiac excitability refers to the ability of cardiac cells to depolarize and repolarize during an action potential, as well as the ease with which electrical activity is propagated from cell to cell. The excitation-contraction (E-C) coupling process links the electric excitation of the surface membrane (action potential) to the contraction of the heart muscle cells, or cardiomyocytes. This process is modulated by calcium levels.
Cardiac contractility is regulated by changes in intracellular calcium concentration ([Ca2+]i). Normal function requires that [Ca2+]i be sufficiently high in systole and low in diastole. The calcium ions bind to troponin C, causing tropomyosin to slide off the actin filament and exposing the actin binding sites. At this point, myosin heads bind to actin, forming a cross-bridge. The myosin head pushes past the actin with a "power stroke", effectively pulling the actin and myosin filaments past one another and shortening the muscle. The myosin does this a few times with actin, binding, sliding past, reattaching, and then repeating this process and using up ATP along the way. The myosin head looks a bit like an oar in a boat—it hits the water, shifts, and comes out—and this only works in the presence of calcium ions.
The force of contraction depends on the amount of calcium bound to troponin, which is a function of both the magnitude and duration of the rise of [Ca2+]i. It is also influenced by the strength of calcium binding, which can be altered genetically and controlled by factors such as phosphorylation. The calcium needed for contraction comes primarily from the sarcoplasmic reticulum and is released through the process of calcium-induced calcium release. This process is finely tuned and depends on the relationships between the various channels and pumps involved.
The cardiac dyad, a specialized signaling nexus, is essential for excitation-contraction coupling. It consists of clusters of L-type Ca2+ channels on the sarcolemma closely apposed to clusters of ryanodine receptors (RyRs) on the sarcoplasmic reticulum membrane. The presence of T-tubules helps bring calcium deep into the cell, facilitating calcium-induced calcium release. In addition, cardiomyocytes stay physically attached to one another through proteins called desmosomes, which hold the cells together during contraction.
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The sinoatrial node pacemaker is dominant, but occasional spontaneous premature beats may arise
The cardiac conduction system is a network of nodes, specialised cells, and electrical signals that controls the heartbeat. The sinoatrial node (SA node) is a group of myocytes with pacemaker activity that acts as the heart's natural pacemaker. It generates electrical impulses that set the rhythm and rate of the heart, with a normal heart rate ranging between 60 and 100 beats per minute. The SA node is located at the junction of the crista terminalis in the upper wall of the right atrium and the opening of the superior vena cava.
The SA node is the dominant pacemaker in the heart, but it is susceptible to pathological, physiological, and pharmacological stressors. Occasionally, spontaneous premature beats may arise anywhere in the conduction system, known as premature contractions. These premature contractions can occur in the atria, node, or ventricles, resulting in premature atrial contractions, premature nodal contractions, or premature ventricular contractions, respectively. They are more frequently observed under conditions of increased excitability and impulse generation, such as physiological stress, stimulants like caffeine, or certain medications.
The SA node's pacemaker activity is inferred from the P waves of atrial depolarization on an ECG. Its function is crucial for the dependable operation of the heart, as it drives the surrounding atrial muscle without being suppressed electrotonically. The SA node also promotes antegrade propagation of excitation to the right atrium and prevents the re-entry of excitation. Additionally, it allows pacemaking to continue under diverse pathophysiological conditions.
When the SA node malfunctions, the lower segments of the conduction system, such as the atrioventricular node and the bundle of His, act as backup pacemaker cells. The atrioventricular node and bundle of His serve as crucial control points in the heart, distributing impulses to the ventricles through the right and left bundle branches. The bundle of His, along with its branches and the Purkinje fibres, resemble an upside-down tree, with the Purkinje fibres forming the canopy.
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The cardiac cell is coupled to rhythmic excitability and contracts or relaxes in phase with depolarization or repolarization
Cardiac excitability refers to the ability of cardiac cells to undergo regenerative depolarization and repolarization during the action potential. It also refers to the ease with which electrical activity propagates from one cell to another. The heartbeat is electrophysiologically generated by the highly organized control of ionic flow through channels in the cardiac membrane, the myoplasm, and the extracellular space.
Cardiac muscle tissue is characterized by automaticity and excitability. The cardiac cell is coupled to rhythmic excitability and contracts or relaxes in phase with depolarization or repolarization. Excitability is the ability of a cardiac cell to generate an action potential at its membrane in response to depolarization and to transmit an impulse along the membrane. Cardiac contractility refers to the ability of muscle tissue to contract when its thick (myosin) and thin (actin) filaments slide past each other in response to a stimulus.
The cycle of depolarization and repolarization in the heart is known as the cardiac action potential and occurs approximately 60 times every minute. In addition, cardiac muscle cells are unique from other types of excitable cells in that they remain permeable to potassium in the resting state. This facilitates the intracellular response to depolarization and, in combination with other potassium channels, ensures proper duration between and during action potentials. The conduction system in the atria is poorly defined but designed to initiate atrial depolarization and propagate the impulse toward the ventricle.
The excitability of a cardiac cell depends on many factors, including the rate and duration of pacing. The structural features of cardiac muscle are largely responsible for anisotropic activation and propagation. The propagation of excitation and contraction in the heart is necessary for efficient cardiac output. Abnormalities in the regulatory mechanisms often accompany cardiac disease.
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Frequently asked questions
Cardiac excitability refers to the ability of cardiac cells to undergo regenerative depolarization and repolarization during the action potential, as well as the ease with which electrical activity propagates from cell to cell.
The sinoatrial node, or SA node, acts as a pacemaker and is critical in generating sinus rhythm. It is composed of specialized conduction cells that generate the initiating impulse for depolarization. In healthy individuals, the heart rate is controlled by the pacemaker cells of the sinoatrial node.
Cardiac contractility refers to the ability of cardiac muscle tissue to contract when its thick (myosin) and thin (actin) filaments slide past each other in response to a stimulus. Cardiac excitability and contractility are coupled, meaning that proper cardiac excitation is necessary for optimal contraction of the cardiac tissue.
The excitability of a cardiac cell depends on several factors, including the rate and duration of pacing, as well as the presence of certain stressors such as physiological stress, stimulants like caffeine, and certain drugs.
Abnormalities in the regulatory mechanisms of cardiac excitability can lead to cardiac disease. For example, increased cellular ROS/RNS content can alter the RyR conformation, leading to a loss of excitability and causing arrhythmia. Additionally, microRNAs (miRNAs) have been implicated in the pathological processes of cardiac diseases, including cardiac hypertrophy, heart failure, and arrhythmogenesis.
