Energy Sources for Contraction

Energy Sources for Contraction

Muscle fibers have just enough energy (ATP) for short-term contraction. ATP must be regenerated when fibers are active, using existing ATP molecules in the cells. ATP is regenerated from ADP and phos-phate. Creatine phosphate accomplishes this with high-energy phosphate bonds. It is between four and six times more abundant in muscle fibers than ATP; however, it does not directly supply energy. It stores excess energy from the mitochondria in the phosphate bonds. Therefore, muscle cells store more creatine phosphate than ATP. Excitation-contraction coupling requires both ATP and calcium ions.

When ATP breaks down, energy from creatine phosphate is transferred to ADP molecules to con-vert them back into ATP. Creatine phosphate stores are exhausted rapidly when muscles are active; there-fore, the muscles use cellular respiration of glucose as energy to synthesize ATP. Energy stored in creatine phosphate is eventually used to recharge ADP and convert it back to ATP. The enzyme that causes this conversion is creatine kinase. Muscle damage causes creatine kinase to leak across plasma membranes into the bloodstream. High blood levels of creatine kinase usually mean that serious muscle damage has occurred.

Nearly all (95%) of the ATP demands of a resting cell are provided by aerobic metabolism. This involves the absorption of oxygen, ADP, phosphate ions, and organic substrates such as pyruvate from the sur-rounding cytoplasm by the mitochondria. These mol-ecules then enter the citric acid cycle. Resting skeletal muscle fibers rely almost totally on aerobic metabo-lism of fatty acids absorbed from the circulation to generate ATP.

Oxygen Use and Debt

Oxygen is required for the breakdown of glucose in the mitochondria. Red blood cells carry oxygen, bound to hemoglobin molecules. Hemoglobin is the pigment that makes blood appear red. The pigment myoglobin is synthesized in the muscles to give skeletal muscles their reddish-brown color. Myoglobin can also com-bine with oxygen and temporarily store it to reduce muscular requirements for continuous blood supply during contraction.

When skeletal muscles are used for a minute or more, anaerobic respiration is required for energy. In one type of anaerobic respiration, glucose is broken down via glycolysis to yield pyruvic acid, which reacts by producing lactic acid. Lactic acid can accumulate in muscles but diffuses in the bloodstream, reaching the liver, where it is synthesized into glucose. This acid is a three-carbon molecule, which dissociates into one hydrogen ion and one negatively charged lactate ion. The movement of lactate to the liver and glucose back to muscle cells is called the Cori cycle.

When exercising strenuously, oxygen is used mostly to synthesize ATP. As lactic acid increases, an oxygen debt develops, also referred to as excess postexercise oxygen consumption. Oxygen debt is equivalent to the amount of oxygen that liver cells require to convert the lactic acid into glucose and the amount needed by muscle cells to restore ATP and creatine phosphate levels.

Endurance is defined as the amount of time during which a particular muscular activity can be performed without fatigue. Anaerobic endurance is the amount of time contraction of muscles can continue to be supported by glycolysis and the reserves of ATP and creatine phosphate. Aerobic endurance is the amount of time muscle contraction can continue while being supported by the activities of the mitochondria. Anaerobic endurance promotes hypertrophy, whereas aerobic endurance does not.

It may take several hours for the body to convert lactic acid back into glucose. Muscles may experience a change in their metabolic activity as exercise levels change. Increased exercise raises the muscles’ capacity for glycolysis. Aerobic exercise increases the muscles’ capacity for aerobic respiration. This is summarized in TABLE 9-3. For optimal health, you should alternate between aerobic and anaerobic exercise.

Muscle Fatigue

Prolonged exercise may cause a muscle to become unable to contract. This condition is called muscle fatigue, and it may also occur because of interruption of muscular blood supply or occasionally a lack of ACh in the motor neuron axons. Lactic acid accumulation is the usual cause of muscular fatigue. As lactic acid lowers pH levels, muscle fibers cannot respond to stim-ulation. When a muscle becomes fatigued and cramps, it experiences a sustained, involuntary contraction. Although not fully understood, muscle cramps appear to be caused by changes in the extracellular fluid sur-rounding muscle fibers and motor neurons.

In a muscle’s recovery period, conditions return to the normal levels present before the exertion occurred. After moderate activity, muscle fibers may need several hours to completely recover. Sustained, higher level activity may require as much as one week for recovery.

Production of Heat

Most of the energy released in cellular respiration becomes heat. Muscle tissue generates a lot of heat because muscles form much of the total body mass. Body temperature is partially maintained by the blood transporting heat generated by the muscle to other body tissues. Heat energy produced from muscle con-traction is released through the integumentary system.