Already Past Due Thanks to "WRITEPRO", so who else will do it? Critique/Response (short) PART II ONLY

Contraction & Relaxation of Skeletal Muscle Fibers

            All muscles in your body contract at some point. When skeletal muscle fibers contract, the thick and thin fibers become smaller and shorten when protein filaments interact with sarcomeres (McKinley, 2013). As muscles fibers contract, eventually they will have to relax. This is called skeletal muscle relaxation. This involves returning all muscle fibers to their resting state (McKinley, 2013). Contraction and relaxation of skeletal muscle fibers include excitation of a skeletal muscle, excitation-contraction coupling, and crossbridge cycling (McKinley, 2013).

            The first step of skeletal muscle contraction and relaxation is excitation. This is also called the neuromuscular junction. When muscle excitation occurs, the release of neurotransmitter acetylcholine, or ACh, from synaptic vesicles bind (McKinley, 2013). This happens because a nerve impulse travels down an axon and releases acetylcholine (McKinley, 2013). The first thing that happens during this step is calcium enters the synaptic knob which binds to proteins in the synaptic vesicle membrane (McKinley, 2013). Next, acetylcholine is released from the synaptic knob, which “triggers the synaptic vesicles to merge with the synaptic knob plasma membrane” (McKinley, 2013). The final thing that happens during excitation is the binding of ACh to ACh. Acetylcholine diffuses across the synaptic cleft and binds with ACh receptors (McKinley, 2013).

            The second step of skeletal muscle contraction and relaxation is excitation-contraction coupling. This has to do with the sarcolemma, T-tubules, and sarcoplasmic reticulum (McKinley, 2013). The word “coupling” means the myofilaments, which are in the sarcomeres, are sliding (McKinley, 2013). There are three events that occur during excitation-contraction coupling. The first is development of an end-plate potential. This is when sodium, or Na+, diffuses into the skeletal muscle fibers and K+, or potassium, diffuses out the get a net gain of positive charge, which reverses the polarity and causes EPP, or end-plate potential (McKinley, 2013). Next is the initiation and propagation of action potential. This is when “the EPP triggers an action potential that is propagated along the sarcolemma and T-tubules of the skeletal muscle fiber” (McKinley, 2013). Action potential goes through depolarization and repolarization which is a period of time called the refractory period (McKinley, 2013). The last event of excitation-contraction coupling is the release of calcium from the sarcoplasmic reticulum into the sarcoplasm (McKinley, 2013).

            The final step of skeletal muscle contraction and relaxation is crossbridge cycling. This is when muscle contract and then move to a relaxed state (McKinley, 2013). There are four steps repeated in crossbridge cycling. The first step is crossbridge formation. This is when myosin heads attach to myosin binding sites of actin (McKinley, 2013). Next is power stroke, which is the pulling of thin filament by moving the myosin head (McKinley, 2013). Then is the “release of myosin head from actin” (McKinley, 2013). The final step is resetting the myosin head. ATPase splits, ATP and ADP and Pi and this provides energy to reset (McKinley, 2013). After muscles contract, they eventually return back to normal, this is called relaxation. ACh is then cut off and the ACh receptor closes (Mckinley, 2013).

            When ATP is not available to the skeletal muscle fibers, a stiffening of the body occurs (McKinley, 2013). This is called Rigor Mortis. All of the skeletal muscles lock into one position and will not release. Rigor Mortis usually last for 15-24 hours (McKinley, 2013). It only goes away after that time period because lysosomal enzymes are released in the muscle fibers (McKinley, 2013). This causes a breakdown of the myofibrils and allows movement once again (McKinley, 2013). Rigor mortis relates to this topic because contraction and relaxation would not occur in the order that it is supposed to.

            All of these steps make up the process of skeletal muscle contraction and relaxation. The muscles would not function properly without these steps in order. Here is an overview of what was stated. First excitation of a skeletal muscle fiber occurs, then excitation-contraction coupling occurs, then crossbridge cycling occurs, and lastly, relaxation occurs (McKinley, 2013). The muscles would not work the way they are supposed to without all these different events.