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• The heart functions as two separate pumps, the right side pumping blood to the lungs and the left side pumping blood to the systemic circulation.
• Each side consists of two chambers: an atrium and a ventricle.
• The atrium acts as a weak primer pump for the ventricle, assisting with blood flow into it.
• The ventricles provide the main pumping force to propel blood into the pulmonary and systemic circulations.
• A two-layered sac called the pericardium surrounds and protects the heart, holding it in place.
• Special mechanisms in the heart facilitate continuous rhythmic contractions, enabling the heart to act as a reliable pump.
• The heart is composed of three major types of muscle: atrial muscle, ventricular muscle, and specialized excitatory and conductive muscle fibers.
• Atrial and ventricular muscles contract similarly to skeletal muscle, but with a longer contraction duration.
• Specialized excitatory and conductive fibers have limited contractile ability and facilitate electrical excitation in the heart.

Syncytial Nature of Cardiac Muscle

• Cardiac muscle fibers connect in a latticework, dividing, recombining, and spreading throughout the heart.
• Cardiac muscle functions as a syncytium, where cells interconnect, allowing action potentials to spread from one cell to another across the network.
• The heart consists of two syncytia: the atrial syncytium and the ventricular syncytium.
• The atria are separated from the ventricles by fibrous tissue, which prevents direct action potential transmission. Instead, signals pass through a specialized conductive system called the atrioventricular (AV) bundle.
• The heart is divided into two syncytia, the atria contract slightly ahead of the ventricles ensuring optimal pump efficiency.

Histology of Cardiac muscle

• Cardiac muscle fibers arrange in a way that the fibers divide, recombine, and separate.
• Cardiac muscle is striated similar to skeletal muscle.
• The muscles contain actin and myosin filaments.
• Transverse dark bands called intercalated discs interconnect cardiac muscle fibers, providing cell-to-cell adhesion.
• At intercalated discs, cell membranes fuse to form gap junctions, allowing ions to flow freely, facilitating action potential spread.
• The syncytial nature of cardiac muscle tissue causes an action potential to spread to all cells through the interconnecting network.

Ventricular Rotation

• The ventricle consists of layered muscular fibers that allow it to contract in a twisting motion during systole.
• Epicardial layers twist leftward, and endocardial layers twist rightward.
• The apex of the ventricle moves counterclockwise, and the base moves clockwise that allows it to eject and relax blood.

Cardiac Action Potential

• The recorded action potential in a ventricular muscle fiber averages about 105 millivolts.
• The membrane depolarizes for approximately 0.2 seconds, creating a plateau.
• The plateau extends the action potential's duration.
• Ventricular muscle contraction lasts up to 15 times longer than skeletal muscle contraction.
• In cardiac muscle, channels for calcium ions are slow to activate extending the action potential.
• The cardiac muscle action potential arises from the combination of fast sodium channels and slow L-type calcium channels.
• The L-type calcium channels open more slowly and remain open for a longer duration than fast sodium channels, thus allowing considerable amounts of both calcium and sodium ions to flow through.
• Calcium ions activate the muscle contractile process.

Differences Between Skeletal Muscle and Cardiac Muscle Action Potentials

• A primary difference between skeletal and cardiac muscle is that after the initial stimulus of an action potential permeability of cardiac membrane to potassium ions decreases.
• The reduced potassium permeability is likely a result of the influx of calcium ions through the calcium channels.

Phases of Cardiac action potential

• Phase 0 (Depolarization): Fast sodium channels open when the cardiac muscle cell is stimulated.
• Phase 1 (Initial Repolarization): Fast sodium channels close.
• Phase 2 (Plateau): Increased calcium ion permeability and decreased potassium ion permeability occur. After 0.2 - 0.3 seconds the calcium and sodium channels close and potassium channels open which terminates the plateau.
• Phase 3 (Rapid Repolarization): Calcium channels close and potassium channels open.
• Phase 4 (Resting Membrane Potential): On average is -80 to -90 millivolts.

Refractory Period of Cardiac Muscle

• The heart is resistant to restimulation during its action potential.
• The refractory period of heart tissue is the interval during which restimulation cannot occur and lasts approximately 0.25 to 0.30 seconds.
• An additional .05 seconds of relative refraction where the heart can be stimulated by a strong signal.

Excitation-Contraction Coupling

• Transverse tubules transmit action potentials throughout the heart.
• T-tubules activate calcium channels facilitating the release of calcium ions.
• Calcium ions trigger the sliding of actin and myosin filaments during the contraction.

Calcium and Contraction

• Additional calcium ions also enter the sarcoplasm during the action potential via the T tubules.
• Calcium initiates the chemical reactions promoting actin and myosin filament sliding, leading to muscle contraction.
• These additional ions activate other calcium release channels in the sarcoplasmic reticulum. The process is called calcium-induced calcium release.

Sarcoplasmic Reticulum

• Calcium ions are then released within the sarcoplasm.
• Followed by the calcium ions reacting with troponin and causing the cross-bridges to form.
• The process of heart contractions occurs.

Importance of Extracellular Calcium Ion Concentration

• In extracellular fluid, calcium ions are stored to prepare for cellular muscle contractions.
• The ability to have atrial contractions is dependent on these extracellular ions.

Contraction Duration

• A few thousands of a second following after potential.
• Ends a few thousands of second following a potential.
• The duration typically depends on the potential length that includes a plateau in 0.2 seconds and 0.3 seconds in ventricular muscles.

Cardiac Cycle

• The cardiac cycle is the period from the beginning of one heartbeat to the start of the next.
• The sinus node creates its action potential.
• Electrical pulses are sent throughout the heart with slight delays that enable atrial and ventricular contraction.
• The atrioventricular node allows the atria to contract before the ventricles, while the ventricles prime the heart.

Atrial Function as a primer for ventricles with blood flow

• Blood flows from the great veins into the atria, and 80% blood into the ventricles before they contract.
• This contraction pumps the remaining 20%.
• The atria increase pumping effectiveness.
• A resting heart is able to fully function as the heart can increase in output by 300 and 400 percent.
• Atrial pressure contributes small amounts to the force with its a, c, and v.

Contraction

• A wave is the first wave which originates from the atria during contraction.
• During atrioventricular contraction the right side reads 4 to