Cardiovascular System: Cardiac Muscle, Conduction, and Electrical Activity

Cardiovascular System

Objectives

The cardiovascular system is essential for transporting nutrients, gases, and wastes throughout the body. This section focuses on the heart's anatomy, its electrical activity as seen in the ECG, the cardiac cycle, and the regulation of cardiac output.

• Basic cardiac anatomy

• Correlate electrical activity with ECG

• Cardiac cycle

• Cardiac output and regulation

Cardiac Muscle

Types of Myocytes in the Myocardium

The myocardium contains two main types of cardiac muscle cells, each with distinct functions in heart physiology.

• Contractile cells: Responsible for the contraction of the heart, generating the force needed to pump blood.

• Pacemaker cells: Noncontractile cells that spontaneously depolarize and initiate depolarization of the entire heart. These cells do not need nervous system stimulation, unlike skeletal muscle fibers.

Additional info: Pacemaker cells are primarily located in the sinoatrial (SA) node and atrioventricular (AV) node.

Intrinsic Cardiac Conduction System

Pathway of Electrical Impulses

The heart's intrinsic conduction system coordinates the rhythmic contraction of cardiac muscle through a sequence of electrical events.

• Sinoatrial (SA) node: The primary pacemaker, located in the right atrium, initiates impulses.

• Atrioventricular (AV) node: Impulses pause here (~0.1 s), allowing atria to contract before ventricles.

• AV bundle (Bundle of His): Connects atria to ventricles and transmits impulses.

• Bundle branches: Conduct impulses through the interventricular septum.

• Subendocardial conducting network (Purkinje fibers): Depolarizes contractile cells of both ventricles.

Additional info: This system ensures coordinated contraction and efficient blood flow.

Action Potential in Cardiac vs. Skeletal Muscle

Comparison of Electrical and Mechanical Events

Cardiac muscle action potentials differ significantly from those in skeletal muscle, affecting contraction and refractory periods.

• Cardiac muscle: Has a longer action potential and contraction duration, with a pronounced plateau phase due to calcium influx.

• Skeletal muscle: Shorter action potential and contraction, allowing for rapid, repeated contractions.

Key Equation:

$\text{Action Potential Duration (Cardiac)} \gg \text{Action Potential Duration (Skeletal)}$

Additional info: The plateau phase in cardiac muscle prevents premature contractions.

Tetanic Contractions and Refractory Period

Prevention of Tetany in Cardiac Muscle

Cardiac muscle fibers have a longer absolute refractory period than skeletal muscle fibers, which is crucial for heart function.

• Sustained contraction: Ensures efficient ejection of blood from the heart.

• Longer refractory period: Prevents tetanic contractions, allowing the heart to relax and refill between beats.

Additional info: Tetanic contractions would impair the heart's ability to pump blood effectively.