Muscular activity: Energy requirement and Cori cycle

Muscular activity requires energy, which is provided by the breakdown of glycogen in the skeletal muscles. The breakdown of glycogen, a process known as glycogenolysis, releases glucose in the form of glucose-6-phosphate (G-6-P). G-6-P is readily fed into glycolysis, a process that provides ATP to the muscle cells as an energy source. During muscular activity, the store of ATP needs to be constantly replenished. When the supply of oxygen is sufficient, this energy comes from feeding pyruvate, one product of glycolysis, into the Krebs cycle.

When oxygen supply is insufficient, typically during intense muscular activity, energy must be released through anaerobic respiration. Anaerobic respiration converts pyruvate to lactate by lactate dehydrogenase. Most important, fermentation regenerates NAD+, maintaining the NAD+ concentration so that additional glycolysis reactions can occur. The fermentation step oxidizes the NADH produced by glycolysis back to NAD+, transferring two electrons from NADH to reduce pyruvate into lactate. Refer to the main articles on glycolysis and fermentation for the details

Instead of accumulating inside the muscle cells, lactate produced by anaerobic fermentation is taken up by the liver. This initiates the other half of the Cori cycle. In the liver, gluconeogenesis occurs. From an intuitive perspective, gluconeogenesis reverses both glycolysis and fermentation by converting lactate first into pyruvate, and finally back to glucose. The glucose is then supplied to the muscles through the bloodstream; it is ready to be fed into further glycolysis reactions. If muscle activity has stopped, the glucose is used to replenish the supplies of glycogen through glycogenesis.