Skeletal muscle metabolism

Introduction

Adenosine triphosphate (ATP) powers muscle contraction and relaxation. The ATP stored in the cytosol of muscle cells is enough to fuel only a few muscle twitches. To contract for longer, muscle cells need to replenish their intracellular ATP. They do so using three metabolic pathways:

• Creatine phosphate reaction: creatine kinase stores phosphate bonds in creatine phosphate at rest, and regenerates ATP from creatine phosphate when needed. The muscle fibers’ finite creatine reserves limit the use of this reaction to power up to 10 seconds of muscle contraction.
  
• Glycolysis: converts blood glucose to pyruvate yielding two ATP. If oxygen is available, pyruvate enters oxidative catabolism; otherwise, it is converted to lactate. Glycolysis powers up to a minute of muscle contraction before fatigue occurs.
  
• Oxidative catabolism: metabolizes biomolecules like pyruvate to regenerate 28-30 ATP. This process requires oxygen and takes place in mitochondria, predominantly in type I fibers. Oxidative catabolism can power muscle contraction for hours.

These pathways let muscle fibers regenerate ATP quickly at the start of contraction and make them energy-efficient in the long term.

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Metabolic pathways

Skeletal muscles replenish ATP using three metabolic pathways.

Glycolysis and oxidative catabolism

The pyruvate produced by glycolysis enters the mitochondria to undergo oxidative catabolism when oxygen is present. Otherwise, most of the pyruvate is converted to lactate.

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Summary