 This video will cover the following objective from muscle physiology, describe the role of ATP in muscle fibers, describe three ways ATP is generated during muscle activity. These are direct phosphorylation, anaerobic respiration, and aerobic respiration. Myosin is a major enzyme in muscle fibers that uses ATP. Remember from the power stroke cycle of the sliding filament theory, that in the third step cross-bridged attachment, ATP attaches to the myosin head, stimulating cross-bridged attachment. Then myosin performs hydrolysis of ATP in order to fuel cocking of the myosin head in the fourth step of the power stroke cycle in the sliding filament theory. What causes rigor mortis is depletion of ATP Without ATP available to bind to the myosin heads, the third step cross-bridged attachment is not possible. So without cross-bridged attachment, myosin heads remain bound to actin, causing the rigid muscles characteristic of rigor mortis. Another major enzyme found in muscle fibers that uses a large amount of ATP is the sarcoplasmic reticulum calcium pump, also known as the sarcapump, standing for sarcoplasmic slash endoplasmic reticulum, calcium ATPase. So ATPase tells us that this enzyme performs hydrolysis of ATP and this is a calcium pump. So the sarcopump performs active transport in order to move calcium ions from the cytosol into the sarcoplasmic reticulum. So the sarcopump is important for sequestering calcium inside of the sarcoplasmic reticulum and this is important in order to lower the calcium levels of the cytosol enabling muscles to relax. The sodium potassium pump is another enzyme that uses a large amount of ATP in muscle fibers. The sodium potassium pump performs hydrolysis of ATP and uses the energy released in order to force sodium out of the cytosol and potassium into the cytosol, creating the concentration gradients necessary for the action potential. Direct phosphorylation provides a rapid source of ATP in order to fuel muscle contraction. In the mechanism of direct phosphorylation, the enzyme creatine kinase transfers a high energy phosphate from the molecule creatine phosphate onto a molecule of adenosine diphosphate in order to form ATP. This mechanism provides enough energy to fuel contraction for a short amount of time around 10 to 15 seconds. When the muscle is at rest, creatine phosphate can be regenerated as creatine can be phosphorylated from ATP. Then when the muscle needs to use ATP rapidly during contraction, creatine phosphate can be used in order to rapidly regenerate ATP. Aerobic cellular respiration provides another pathway for the generation of ATP. Glucose enters glycolysis and as glucose is broken down to produce pyruvate or pyruvic acid as a product, two ATP molecules are generated. Then the pyruvate molecule can either enter aerobic respiration to generate more ATP if oxygen is available, or if there's not sufficient oxygen to fuel aerobic respiration, the pyruvate will be converted to lactic acid that can be transported out into the blood. If muscle fibers do have sufficient oxygen availability, pyruvic acid from glycolysis can then enter the mitochondria and mitochondria will perform aerobic cellular respiration using oxygen to break down the pyruvic acid, producing the waste products of carbon dioxide and water and generating a large amount of ATP that can be used to fuel muscle activity.