Cardiac Muscle Contraction

Heart Function and Electrical Conduction

• Heart relaxation occurs when ventricles fill with blood.
• Contraction of the heart occurs from the apex to the base.

Cardiac Muscle Cells

• Contractile cells make up the bulk of atrial and ventricular walls and produce powerful contractions to propel blood.
• Ca2+ binding to troponin triggers muscle contraction.
• Autorhythmic cells (pacemakers) control and coordinate contractile cells.

Action Potentials

• The first action potential occurs in the SA node, resulting in depolarization.
• The second action potential occurs in the myocardium, resulting in contraction.
• Cardiac muscle cells can generate action potentials spontaneously without nervous stimulation.

Ion Channels and Stimulation

• Funny channels (If) are more permeable to sodium than potassium, allowing depolarization.
• Sympathetic stimulation alters permeability to different ions, exciting beta 1 receptors and increasing heart rate.
• Catecholamines (e.g., adrenaline) stimulate the heart.
• Parasympathetic stimulation increases pacemaker potential, hyperpolarizing SA node cells and slowing the heart rate.

Depolarization and Repolarization

• Depolarization of the SA node involves calcium.
• Sodium is involved in myocardium depolarization.
• The cardiac action potential has three stages: rapid depolarization, plateau, and repolarization.

Calcium-Induced Calcium Release and Muscle Contraction

• Calcium-induced calcium release triggers muscle contraction.
• Cardiac muscle contraction can be graded, with calcium levels determining the force of contraction.
• G-proteins open calcium channels, increasing calcium release and muscle contraction.

Drugs and the Heart

• Digitalis blocks the sodium-potassium channel, increasing intracellular sodium and calcium, and slowing the heart rate.

Electrical Conduction in the Heart

• The SA node, internodal pathways, AV node, AV bundle, bundle branches, and Purkinje fibers coordinate contraction in the heart.
• The atria contract while the ventricles relax, and vice versa.

Electrocardiogram (EKG)

• An EKG represents the summed electrical activity of all cells recorded from the surface of the body.
• Waves, segments, and intervals are used to analyze the EKG.
• The axis of the EKG represents the general movement of the waveform.

EKG Components

• The P wave represents atrial depolarization and systole.
• The PR interval is the time from the onset of the P wave to the beginning of the QRS complex.
• The QRS complex represents ventricular depolarization.
• The ST segment represents the time at which the entire ventricle is depolarized.
• The Q-T interval represents ventricular repolarization.

Abnormal Heart Rhythms

• Tachycardia: a heart rate faster than normal.
• Bradycardia: a heart rate slower than normal.
• Arrhythmia: an irregular heartbeat.

Heart Pathologies

• Ischemia: insufficient blood flow to the heart.
• Hypoxia: low levels of oxygen.
• Anoxia: absence of oxygen.
• Myocardial infarction: abrupt reduction in coronary blood flow to a segment of myocardium.
• Stenosis: restricted blood flow.