Muscle Physiology*This chapter discusses how the muscles work to move our body and perform autonomic activities, such as cardiac rhythms. *Content Summary:Stucture of Skeletal Muscles: Skeletal muscles are what control your day to day movement and are known as voluntary muscles. These types of muscles are attached to bones by tendons, which are made up of connective tissue forming a sheath called the epimysium. Skeletal muscles are made up of fasicles which are again covered by a sheath of connective tissues, called perimysium. Fasciles are made up of many muscle cells or also called fibers. These cells are more elongated than regular cells and have multiple nuclei. They run the stretch of the entire muscle. Muscle cells are made of many myofibrils which are compsed of myofilaments. Myofilaments are classified as either thick of thin. Muscle cells are then surronded by a plasma membrane known as the sarcolemma.

*Muscle Fatigue: Muscle fatigue is the decline in ability of a muscle to generate force. When muscles are at maximum contraction, short-term fatigue can by caused by an accumulation of extracellular potassium, interfering with the fiber's ability to produce action potentials. An example of short term muscle fatigue is bench-pressing weights. The biceps may become very fatigued for a short amount of time, but the lifter can usually continue with the weight training after a couple of minutes. It has also been said that an accumulation of lactic acid may cause muscle fatigue, saying that it has a "pickling effect" on the muscles from the decrease in pH. *Sliding Filament Theory: The sliding filament theory explains how the muscles produce force. Thick and thin filaments within the sarcomere slide pas one another, shortening the length. In order to do so, the myosin heads will interact with the actin filaments using ATP and bend to pull past the actin. I found these six steps explaining cross bridge cycling:*1. Influx of Calcium, exposing binding sites on Actin.2. The binding of myosin to Actin.3. Power stroke of the cross bridge that causes the sliding of thin filaments. 4. The binding of ATP to the cross bridge, which causes the bridge to disconnect from the actin.5. The hydrolysis of ATP, leading to the reorganization of the cross bridge.6. The transport of Calcium ions back to the sarcoplasmic reticulum. *(*Here is a video talking about the Sliding Filament Theory. I had a hard time understanding this chapter so watching a virtual video helped!

*Application: As a nursing student, I had the amazing opportunity to do my clinicals on the Transitional Care Unit (TCU). On this unit, we had alot of hip fractures and people recovering from surgeries. I worked closely with physical therapy so I got to see first hand how these people were doing and why they needed the rehabilitation. It's important to understand how the muscle works in order to make sure that it is working correctly and prevent any complications. I have always been taught to watch and prevent muscular atrophy with the elderly and partially immobile.

Critical Thinking Questions:1. Describe the sliding filament theory of muscle contraction and why it is called the sliding filament theory. Describe the action of the cross bridge that causes the power stroke.A: I briefly described the sliding filament theory above before I read the questions. Now, let's dig deeper! Here is the process: Action potentials are sent from motor neurons making their way to the sacroplasmic reiculum. The calcium release channels open and the calium diffuses inward to the sacroplasm. Ca2 binds to the tropinin attaching to the tropomyosin. This changes the shape and allows the actin binding site on the myosin to be exposed. Myosin heads then bind to actin forming the cross bridge. When phosphate is released, the myosin changes shape causing a power stroke. This power stroke then pulls the thin filaments over the thick filaments toward the center of the sacromere. As this happens, the sacromere shortens leading to the shortening of muscle fibers. This theory is named so because thick and then filaments don't shorten, they stay the same length. 2. What is the role of Calcium and ATP in muscle contraction and relaxation?A: Calcium binds with tropin to allow the binding sites of actin to be exposed, eventually causing the muscle contraction. For muscles to relax, calcium must be pumped back into the SR from the sacroplasm. To do this, ATP is required as active transport powering the pump. ATP is needed for muscle contraction because it is broken down into ADP and phosphate allowing the myosin head to change shape causing the binding with actin.