OpenStax: Chapter 19.2

Unique Properties

• Notably, it can initiate an electrical potential at a fixed rate, called autorhythmicity, which neither smooth nor skeletal muscle can do.

• Heart rate is modulated by the endocrine and nervous systems despite autorhythmicity.

Types of Cardiac Muscle Cells

• Myocardial contractile cells (99%):

  • Majority of cells in atria and ventricles.

  • Conduct impulses and pump blood.

• Myocardial conducting cells (1%):

  • Form the conduction system; smaller with fewer myofibrils.

  • Initiate and propagate action potentials, similar to neurons.

Structure of Cardiac Muscle

• Cardiomyocytes:

  • Shorter and with smaller diameters compared to skeletal muscle.

  • Exhibit striations due to myofilaments organized in sarcomeres.

• T Tubules:

  • Present only at Z discs; fewer than in skeletal muscle.

• Sarcoplasmic Reticulum:

  • Stores fewer calcium ions, leading to slower contraction onset.

• Mitochondria:

  • Abundant, providing energy for contractions.

• Cell Structure:

  • Typically single central nucleus, but may have multiple in some cells.

Intercalated Discs

• Junctions between cardiac muscle cells that support synchronized contraction.

• Made of desmosomes, tight junctions, and gap junctions for ion passage.

Cardiac Muscle Metabolism

• Undergoes aerobic respiration, primarily utilizing lipids and carbohydrates.

• Stores myoglobin, lipids, and glycogen in the cytoplasm.

• Contractile cells undergo twitch contractions with long refractory periods.

Conduction System

• Sinoatrial (SA) Node:

  • Pacemaker of the heart located in the right atrium.

  • Initiates the sinus rhythm with the highest depolarization rate.

• Atrioventricular (AV) Node:

  • Receives impulses from SA node; critical pause allows atria to contract before ventricles.

• Atrioventricular Bundle & Purkinje Fibers:

  • Conducts impulses to the ventricles for coordinated contraction.

Electrocardiogram (ECG)

• Records the electrical signal of the heart:

  • P wave: Atrial depolarization.

  • QRS complex: Ventricular depolarization.

  • T wave: Ventricular repolarization.