CARDIAC MUSCLE / EKG

Types of Muscle in the Body

  • Skeletal Muscle

    • Characteristics:

    • Giant fibers

    • Multinucleated

    • Striated

  • Smooth Muscle

    • Characteristics:

    • Much smaller cells

    • Not striated

  • Cardiac Muscle

    • Characteristics:

    • Striated

    • Larger than smooth muscle, smaller than skeletal muscle

    • Branched fibers

Intercalated Discs in Cardiac Muscle

  • Structure of Intercalated Discs

    • Composed of two structures:

    • Desmosomes (also known as anchoring junctions)

      • Function: Connect cell membranes, holding cells together under strain during contraction.

      • Nature: Localized and strong connection; do not block movement of material between cells.

    • Gap Junctions

      • Function: Allow rapid communication of electrical signals between neighboring cardiac muscle cells.

Mechanism of Tension Generation in Cardiac Muscle

  • Action Potential & Membrane Potential

    • Required for tension generation in cardiac muscle.

    • Action Potential Process:

    • Travels along the surface of cardiac muscle and goes down T tubules.

    • T Tubules: Less extensive than in skeletal muscle due to smaller fiber size.

    • Calcium Influx:

    • Action potential triggers voltage-gated calcium channels to open.

    • Extracellular calcium enters cardiac muscle cell.

    • Some calcium binds to troponin to initiate contraction.

    • Some interacts with receptors on smooth endoplasmic reticulum, triggering further calcium release (this process is known as calcium-mediated calcium release).

Muscle Contraction Mechanism

  • Similar to skeletal muscle:

    • Calcium binds to troponin, displacing tropomyosin.

    • Activated myosin forms cross bridges with actin, leading to muscle contraction and tension generation.

  • Energy Production:

    • Unlike skeletal muscle, cardiac muscle relies predominantly on aerobic metabolism.

    • Primarily oxidizes fatty acids for energy.

    • Requires excellent vascular supply for adequate oxygenation, emphasizing the importance of coronary circulation.

Vascular Supply and Ischemia

  • Arteriogram: Used to assess heart vessels and identify potential ischemia, which can lead to myocardial infarction (heart attack).

  • Identification: Arteries shown with contrast dye highlighting areas of stenosis (narrowing) that may impair blood supply, potentially causing chest pain during exertion.

Cardiac Action Potentials

  • Cardiac muscle cells exhibit unique action potentials differing from neurons and skeletal muscle.

  • Two types of cardiac cells:

    • Contractile Cells: Make up 99% of cardiac muscle; create tension.

    • Autorhythmic Cells: Generate spontaneous action potentials, responsible for heart rhythm.

Contractile Cell Action Potential

  • Phases Breakdown:

    • Phase 0: Rapid depolarization due to opening of voltage-gated sodium channels (Na+ influx).

    • Phase 1: Initial repolarization; sodium channels close, potassium channels open (K+ efflux).

    • Phase 2 (Plateau Phase): Balanced influx of calcium (Ca2+) via L-type channels and efflux of potassium leads to the plateau.

    • Calcium influx critical for further calcium-mediated calcium release, facilitating tension generation.

    • Phase 3: Final repolarization; opening of additional voltage-gated potassium channels returning to resting membrane potential (Phase 4).

Autorhythmic Cell Action Potential

  • Pacemaker Potential:

    • Spontaneous depolarization due to sodium funny channels (Na+) and T-type calcium channels (Ca2+).

    • Reaches threshold leading to rapid depolarization via L-type calcium channels.

    • Similar repolarization process as contractile cells.

Organization of Cardiac Cells and Conductivity

  • Autorhythmic cells are organized in specific areas:

    • Sinoatrial Node (SA Node): Primary pacemaker of the heart, initiates heart rhythm.

    • Atrioventricular Node (AV Node): Receives signals from SA node and transmits to ventricles.

    • Bundle of His and Purkinje Fibers: Conduct impulses throughout the ventricles, ensuring synchronized contraction.

Electrical Activity of the Heart and EKG

  • Electrocardiogram (EKG): Records electrical activity, including:

    • P Wave: Atrial depolarization initiated by SA node activity.

    • QRS Complex: Ventricular depolarization, larger deflection due to more muscular mass compared to atria.

    • T Wave: Ventricular repolarization, reflecting the return to baseline after contraction.

Summary of Electrical Signal Pathway

  • Signal Pathway: From SA Node → AV Node → Bundle of His → Bundle Branches → Purkinje Fibers → Contractile Cells.

  • Ensures orderly conduction and contraction of the heart muscle, vital for effective pumping mechanics.

  • Conclusion: This comprehensive understanding of cardiac muscle structure, function, and electrical activity is crucial for diagnosing and managing heart conditions effectively.