The heart beats ~100,000 times a day, 35 million times a year, and billions over a lifetime.
Cardiac muscle must avoid fatigue to prevent cell death (e.g., brain cells) due to ceased circulation.
Focus on how the heart avoids exhaustion through three levels of 'security'.
Pacemaker Cells (Autorhythmic Cells)
Types of Cells: Autorhythmic (pacemaker) cells and myocardial contractile cells.
Function: Autorhythmic cells autonomously generate heartbeats, modulated by the autonomic nervous system.
Pacemaker Potential:
Autonomous activation leads to gradual increase in membrane potential (pacemaker potential).
Threshold at -40 mV triggers an action potential.
Action potential has a refractory period, limiting beat frequency.
Ionic Channels:
If Channel: Allows Na+ and K+ flux, more Na+ leads to depolarization.
Calcium Channels: Activated at threshold; influx of Ca2+ promotes action potential.
Potassium Channels: Promote repolarization.
Contractile Cells
Comparison with Skeletal Muscle:
Contractile cells show a longer refractory period (250 ms) than skeletal muscle fibers.
Prevents high-frequency contractions; refractory period is driven by calcium and low potassium permeability.
Function:
Tension follows the fixed refractory period; insensitive to high-frequency action potentials.
Refractory period guards against fatigue.
Anatomy and Length-Tension Relationship
Skeletal Muscle:
Length changes restricted by joint geometry.
Allows almost full length-tension range.
Cardiac Muscle:
Length changes driven by blood volume in chambers.
Surrounded by connective tissue, which restricts range, preventing excessive length-tension reductions.
Ensures positive slope in length-tension relationship, promoting increased force with muscle extension.
Frank-Starling Law
Experiment: Dog heart study shows relation between stroke volume and ventricular end-diastolic volume.
Observation:
Initial linear relationship; diminishes at higher volumes.
Increased pressure needed for larger stroke volumes achieved by greater muscle force.
Anatomical range prevents drop-off in force, supports increasing tension with extension.
Cardiac muscle can increase twitch tension unlike skeletal muscle.
Conclusion
Three Measures to Avoid Exhaustion:
Autorhythmic cells have action potentials with fixed durations, limiting beat frequency.
Contractile cells have refractory periods, preventing fatigue from high-frequency activation.
Anatomical range prevents muscle hyperextension, maintaining tension output.
The design of the heart's cellular components and anatomical setup work synergistically to prevent exhaustion and ensure continuous, efficient function across a lifetime.