Cardiac Muscle Anatomy and Physiology

Cardiac Muscle Characteristics

• Cardiac muscle is striated, short, fat, branched, and interconnected
• Connective tissue endomysium acts as both tendon and insertion
• Intercalated discs anchor cardiac cells together and allow free passage of ions
• Heart muscle behaves as a functional syncytium

Properties of Heart Muscle

• Heart muscle is stimulated by nerves and is self-excitable (automaticity)
• Contracts as a unit
• Has a long (250 ms) absolute refractory period
• Contraction is similar to skeletal muscle contraction

Intrinsic Conduction System

• Autorhythmic cells initiate action potentials
• Unstable resting potentials called pacemaker potentials
• Use calcium influx for rising phase of the action potential

SA Node and AV Node

• Sinoatrial (SA) node generates impulses about 75 times/minute
• Atrioventricular (AV) node delays the impulse approximately 0.1 second

Electrical Conduction

• Impulse passes from atria to ventricles via the atrioventricular bundle (bundle of His)
• AV bundle splits into two pathways in the interventricular septum (bundle branches)
• Bundle branches carry the impulse toward the apex of the heart
• Purkinje fibers carry the impulse to the heart apex and ventricular walls

Regulation of Heart Rate

• Heart is stimulated by the sympathetic cardioacceleratory center
• Heart is inhibited by the parasympathetic cardioinhibitory center

Electrocardiogram (ECG)

• Electrical activity is recorded by electrocardiogram (ECG)
• P wave corresponds to depolarization of SA node
• QRS complex corresponds to ventricular depolarization
• T wave corresponds to ventricular repolarization
• Atrial repolarization record is masked by the larger QRS complex