Cardiac Muscle Physiology Quiz

Cardiac Muscle Contraction

• Contractile cells within the heart are connected through intercalated disks, which contain gap junctions and allow for force and graded potential transmission between cells.
• Cardiac contractile cells have long-lasting action potentials, keeping the cell depolarized for a longer period.
• Cardiac muscle's long refractory period prevents summation of force.
• Cardiac muscle contraction shares features with both skeletal and smooth muscle contractions.

Two Types of Cardiac Muscle Cells

• Autorhythmic cells initiate action potentials and set the heart’s rhythm.
• Contractile cells produce force to pump blood.
• Intercalated disks connect contractile cells.

Action Potential: Autorhythmic Cells

• Autorhythmic cells have unstable membrane potentials, which means they have no resting potential and continually depolarize, reaching threshold to trigger action potentials.
• The influx of sodium (Na+) through "if" channels initiates depolarization in autorhythmic cells.
• Depolarization is further accelerated by the opening of calcium (Ca2+) channels.
• Repolarization occurs when Ca2+ channels close, and potassium (K+) channels open.
• The sodium-potassium pump, Na+-K+-ATPase, maintains the sodium gradient necessary for the “if” channel to allow depolarization.

Electrical Conduction in the Heart

• The sinoatrial (SA) node initiates the heartbeat (pacemaker).
• The SA node depolarizes, sending electrical activity to the atrioventricular (AV) node through internodal pathways.
• The depolarization spreads slowly across the atria, allowing for atrial contraction.
• Conduction slows through the AV node.
• Depolarization moves rapidly through the ventricular conducting system, reaching the apex of the heart.
• The Purkinje fibers rapidly conduct the signal across the ventricles, causing ventricular contraction..

Myocardial Contractile Cells: Structure

• Cardiac muscle cells have sarcomeres.
• Intercalated disks connect cardiac muscle cells, providing structural support and allowing for synchronized contraction.
• Each cardiac muscle cell contains a nucleus, mitochondria, and other organelles.

Myocardial Contractile Cells: Action Potentials

• Cardiac muscle cells have a long action potential, which includes a plateau phase.
• The influx of sodium (Na+) initiates depolarization.
• Sodium channels close, and calcium (Ca2+) channels open, contributing to the plateau phase.
• The plateau phase is sustained by slow potassium (K+) channels, preventing rapid repolarization.
• The action potential ends when Ca2+ channels close, and slow K+ channels open fully.
• No hyperpolarization occurs in cardiac muscle action potentials.

Refractory Period: Skeletal Muscle

• The refractory period is the timeframe after an action potential when a cell is unable to generate another action potential.
• Skeletal muscle has a short refractory period, allowing for summation of force.

Refractory Period: Cardiac Muscle

• Cardiac muscle has a long refractory period, preventing summation of force and ensuring coordinated contractions.

Cardiac Muscle Contraction

• The action potential propagates from one cardiomyocyte to another through gap junctions, ensuring synchronized contraction.
• The calcium (Ca2+) that enters the cardiomyocyte during the plateau phase of the action potential triggers the release of calcium from the sarcoplasmic reticulum.
• Calcium binds to troponin, exposing myosin-binding sites on actin.
• Myosin binds to actin and initiates the cross-bridge cycle, leading to muscle contraction.
• Calcium is then pumped back into the sarcoplasmic reticulum, reducing the concentration of calcium in the cytosol.
• Calcium is exchanged with sodium (Na+) by the NCX antiporter.
• The sodium gradient is maintained by the Na+-K+-ATPase.
• These processes lead to muscle relaxation.

Pressure-Volume Relationship during Cardiac Cycle

• The cardiac cycle includes phases of filling, contraction, and ejection of blood.
• The pressure-volume relationship describes the changes in pressure and volume within the ventricles during the cardiac cycle.
• First heart sound (S1) occurs during the closing of the AV valves.
• Second heart sound (S2) occurs during the closing of the semilunar valves..

Heart Sounds

• The first heart sound (S1) corresponds to the closing of the atrioventricular (AV) valves.
• The second heart sound (S2) corresponds to the closing of the semilunar valves.

Stroke Volume

• The volume of blood ejected from the heart with each beat is known asthe stroke volume (SV).
• Stroke volume is dependent on the end diastolic volume (EDV) and the end systolic volume (ESV).
• The difference between end diastolic volume (EDV) and end systolic volume (ESV) is the stroke volume, which is the volume of blood ejected by the heart during each heartbeat.

Cardiovascular System

• It includes the heart, the systemic and pulmonary circuits.

Blood Flow

• Changes in volume cause changes in pressure, which drives blood flow through the circulatory system.
• Unidirectional blood flow in the heart is made possible by the opening and closing of heart valves.