muscle type: heart of a runner

Written By Anthony Jones

During running, the heart pumps oxygen-rich blood to working muscles, sustaining energy production and performance. As a specialized involuntary muscle, cardiac tissue contracts rhythmically and continuously without fatigue, thanks to its dense mitochondrial content and rich blood supply.

Running strengthens the heart by increasing stroke volume (amount of blood pumped per beat) and enhancing cardiac output. Over time, this leads to lower resting heart rate, improved circulation, and greater efficiency in oxygen delivery. The autonomic nervous system also adapts, improving the balance between sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) responses.

Properties:

1. Shape and Structure:

  • Branched and cylindrical in shape, allowing them to form a network.

  • Striated appearance due to organized sarcomeres (similar to skeletal muscle).

2. Mitochondria and Energy:

  • Numerous mitochondria occupy about 30–40% of the cell volume.

  • This high mitochondrial content supports aerobic respiration and continuous contraction without fatigue.

3. Excitability and Conductivity:

  • Some cardiomyocytes, especially in the sinoatrial (SA) node, can generate action potentials spontaneously (automaticity).

  • All cardiomyocytes conduct these signals to propagate contraction in a coordinated fashion.

4. Sarcoplasmic Reticulum and T-Tubules:

  • The sarcoplasmic reticulum stores and releases calcium, essential for contraction.

  • T-tubules help quickly transmit action potentials into the cell, triggering calcium release.

5. Refractory Period:

  • Cardiac muscle cells have a long absolute refractory period, which prevents tetanus (sustained contraction) and allows the heart to relax and refill with blood.

Anthony Jones
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