CARDIAC MUSCLE PHYSIOLOGY
Introduction
The physiology of cardiac muscle involves several key aspects:
- Contraction: Cardiac muscle contracts rhythmically to pump blood. It’s involuntary and controlled by electrical impulses from the sinoatrial (SA) node, spreading through the atria and ventricles.
- Autorhythmicity: Unlike skeletal muscle, cardiac muscle can initiate its own electrical impulses. This is vital for maintaining a regular heartbeat.
- Interconnected Cells: Cardiac cells are interconnected through gap junctions, allowing coordinated contraction. This ensures the heart functions as a single unit.
- Involuntary Control: Cardiac muscle is under involuntary control, primarily regulated by the autonomic nervous system (sympathetic and parasympathetic).
- Energy Demand: The heart has a high energy demand due to its constant activity. It primarily relies on aerobic metabolism and has a rich supply of mitochondria.
- Heart Valves: The heart’s valves (tricuspid, mitral, pulmonary, and aortic) prevent backflow of blood and ensure one-way flow through the heart.
- Cardiac Cycle: The cardiac cycle includes systole (contraction) and diastole (relaxation) phases for both atria and ventricles, allowing blood to be pumped efficiently.
- Frank-Starling Mechanism: The heart adjusts its force of contraction based on the volume of blood returning to it, ensuring it pumps an adequate amount of blood.
- Cardiac Output: This is the amount of blood pumped by the heart per minute. It’s a function of heart rate (beats per minute) and stroke volume (amount of blood pumped per beat).
These aspects ensure that the heart functions effectively as a pump, providing oxygenated blood to the body’s tissues and removing waste products.
Cardiac Conduction System Explained
The cardiac conduction system is a network of specialized cardiac muscle cells that initiate and propagate electrical impulses, ensuring coordinated contraction of the heart muscles. This system consists of several key components:
- Sinoatrial (SA) Node: Located in the right atrium, near the opening of the superior vena cava, the SA node is the natural pacemaker of the heart. It generates electrical impulses at a regular rate, typically around 60-100 times per minute, which triggers atrial contractions.
- Atria: The electrical impulses from the SA node spread across the atria, causing them to contract and push blood into the ventricles.
- Atrioventricular (AV) Node: Located at the base of the right atrium near the septum, the AV node serves as a relay station, slowing down the electrical impulse from the SA node before passing it to the ventricles. This delay allows the ventricles time to fill with blood from the atria.
- Bundle of His: After passing through the AV node, the electrical impulse travels to the Bundle of His, a bundle of specialized fibers that runs along the interventricular septum.
- Bundle Branches: The Bundle of His splits into right and left bundle branches, conducting the electrical impulse toward the apex of the heart.
- Purkinje Fibers: These specialized fibers spread throughout the ventricles, rapidly transmitting the electrical impulse. This leads to synchronous contraction of the ventricles, pumping blood to the lungs and the rest of the body.
The function of the cardiac conduction system is crucial for maintaining a coordinated heartbeat. It ensures that the atria contract before the ventricles, allowing for efficient blood flow, and that the heart beats at a regular rhythm. This system also responds to changes in the body’s demands by adjusting the heart rate, allowing for adaptations during exercise, stress, or other situations. The synchronized contraction provided by this system is essential for maintaining effective circulation, delivering oxygen and nutrients while removing waste products from tissues throughout the body.
Cardiac Muscle Action Potential
The cardiac muscle action potential is a crucial physiological process that allows the heart to contract rhythmically, maintaining a coordinated and efficient pumping of blood throughout the body. It involves several key components, each with specific roles in the electrical signaling of the heart.
- Resting Membrane Potential: The cardiac muscle cell starts with a resting membrane potential, typically around -90 to -95 millivolts (mV) due to the balance of ion concentrations across the cell membrane.
- Depolarization: The action potential begins with a rapid depolarization phase. Voltage-gated sodium channels open, allowing sodium ions (Na+) to rush into the cell. This influx of positive charge rapidly changes the membrane potential from the negative resting value to a more positive value, typically around +20 to +30 mV. This depolarization phase is responsible for the rising phase of the action potential.
- Early Repolarization: Shortly after depolarization, there’s a brief early repolarization phase. This is primarily caused by the inactivation of the sodium channels and a small amount of potassium efflux.
- Plateau Phase: The plateau phase is a unique feature of the cardiac action potential. During this phase, voltage-gated calcium channels open, allowing calcium ions (Ca2+) to flow into the cell. This counteracts the repolarizing effect of potassium (K+) efflux, leading to a prolonged period of relatively stable membrane potential. The plateau phase helps prevent sustained contractions and contributes to the refractory period, ensuring that the heart chambers contract in a coordinated manner.
- Repolarization: After the plateau phase, repolarization occurs. Potassium channels open, leading to an efflux of potassium ions from the cell. This return to a more negative membrane potential (around -85 mV) is the repolarization phase.
- Resting State: The action potential returns to the resting state, with the sodium and calcium channels inactivated, and the cell is ready to initiate another action potential.
This sequence of events in the cardiac action potential is essential for the coordinated contractions of the heart chambers, ensuring efficient blood circulation. It’s important to note that the action potential in cardiac muscle is longer and more extended compared to other muscle types, like skeletal muscle, which helps prevent tetanic contractions and allows the heart to function properly.
Cardiac Refractory Period & Excitation-Contraction Coupling
- Refractory Period:
The refractory period is a critical aspect of cardiac muscle function that ensures proper heart rhythm and prevents sustained contractions. It refers to the period of time during and after a cardiac muscle cell has undergone an action potential (depolarization and repolarization) when it is temporarily unresponsive to further electrical stimulation.
There are two main phases of the refractory period:
- Absolute Refractory Period: During this phase, the cardiac muscle cell cannot be restimulated, no matter how strong the stimulus. This phase is primarily due to the inactivation of voltage-gated sodium channels, which are responsible for the rapid depolarization during the action potential. This prevents premature depolarization and allows the heart to fill with blood before the next contraction.
- Relative Refractory Period: Following the absolute refractory period, there is a phase during which the cardiac muscle cell can be stimulated, but only by a stronger-than-normal stimulus. This phase is characterized by the presence of open potassium channels and the slow recovery of sodium channels from inactivation. If a strong enough stimulus is applied during this phase, it can lead to another action potential.
The refractory period plays a crucial role in preventing arrhythmias and ensuring that the heartbeats are synchronized, allowing efficient pumping of blood.
- Excitation-Contraction Coupling:
Excitation-contraction coupling is the process by which an electrical signal (excitation) triggers a mechanical response (contraction) in cardiac muscle cells. This process ensures that the heart contracts rhythmically and effectively to pump blood.
The key steps in excitation-contraction coupling in cardiac muscle are as follows:
- Action Potential Propagation: An action potential is generated in the sinoatrial (SA) node, which serves as the natural pacemaker of the heart. This electrical signal spreads through the atria, causing them to contract.
- Atrioventricular (AV) Node: The electrical signal is delayed as it passes through the AV node, allowing the ventricles time to fill with blood from the atria.
- Bundle of His and Purkinje Fibers: The action potential is rapidly conducted through the Bundle of His and Purkinje fibers, causing coordinated and powerful contractions of the ventricles.
- Calcium Release: The action potential triggers the opening of voltage-gated calcium channels in the cardiac muscle cells’ sarcolemma (cell membrane). This influx of calcium into the cell triggers the release of additional calcium from the sarcoplasmic reticulum, a specialized cellular organelle that stores calcium.
- Contraction: The increased calcium levels in the cytoplasm lead to the interaction between actin and myosin filaments, leading to muscle contraction.
- Relaxation: The removal of calcium from the cytoplasm allows the muscle to relax, which is essential for the heart to refill with blood before the next contraction.
The excitation-contraction coupling process ensures a coordinated and rhythmic contraction of the heart, allowing it to effectively pump blood throughout the body.
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