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Please complete the following writing assignment using these guidelines:
- 500-700 words in length
- contains introduction with thesis statement or hypothesis statement, supporting paragraphs, and a conclusion paragraph
- plagiarized work will receive a zero and will be reported to college
- quotes can only make up 10% of the writing
- you must cite statements, facts, and quotes with the author’s last name and the year of the publication
- Example: Horses should eat smaller more frequent meals (Mulligan, 2013).
- OR: In 2013, Mulligan found that horses should eat smaller more frequent meals.
- OR: Mulligan stated “Horses should eat smaller, more frequent meals,” (Mulligan, 2013).
- references must be cited using MLA or APA format (find the Citation Builder link under Course Information tab in Blackboard)
- each assignment will be worth 100 points (all 3 writing assignments make up 5% of your total grade in the class)
- points will be awarded using the rubric that you can view in My Grades
2.
Part I: Explain the key steps involved in contraction and relaxation of a skeletal muscle fiber.
What is rigor mortis and how does it relate to this topic?
Due – 11:59PM on November 2nd (Saturday)
Part II: Respond to at least 2 classmates’ posts to
discuss/ theorize other conditions that might affect or influence the topic that was discussed (i.e. NMJ, muscle contraction, etc). Please choose from the list of diseases, disorders, toxins and drugs located in the Course Information folder.
Due – 11:59PM on November 9th (Saturday)
I can give you examples of responses if needed.
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.
Muscular contraction stmarts out with the release of ACh by the motor neuron. The signal moves down the axon and causes the calcium channels to open up, moving the calcium down the gradient and into the channels outside. The calcium then binds with the proteins in the channels and triggers the bonding of the synaptic vescicles to the synaptic knob. The ACh then begins to move throughout the bonding site down to the receptors by the motor end plate.
The next step in the process is exitation-contraction coupling. Once the ACh has binded and caused the ACh receptor channels to open, sends sodium and potassium out into the gap. This causes the end-plate potential that separates the positive and negative charges in the motor end plate. The next step from the potential is the action potential. This deals with depolarization, changing the polarity inside the sarcomere, and then repolarization, the act that switches the polarities. After both of the polarity changes have occured and the motor end plate has recieved its final change in polarity, the muscle then goes into the refractory period. The refractory period is the break between polarity changes where the muscle is basically inactive.
Next, the action potential has continued traveling and is now beginning to open calcium channels again and is preparing the muscle fibers for crossbridge cycling. Crossbridge cycling is basically when the componets of the sarcomere contract and move closer, far apart, and finally back to normal. The inner process is the myosin head moving and connecting, making ATP, and then letting go, moving to the next area and releasing ADP and phosphorus.
The final step in the process is muscle relaxation. The muscle at this point returns to its normal position(McKinley 2013). The channels and gates begin to close and the final remanents of calcium are dispursed stopping the binding process of the myosin head to the sarcomere.
Rigor mortis is a post-mortum condition that occurs within a few hours after death(Mulligan 2013). This is the condition where the ATP receptors lose control to contract again and so the left over calcium continues to seep out into the body creating a problem that causes the contractions within the body to lock together and are not able to let go and contract again, resulting in stiffness within the body. (Mulligan 2013).
EXAMPLE of RESPONSE:
“I think you did a great job of explaining muscle contraction and relaxation and connecting it to rigor mortis. I like the way you broke things down into events and steps. The sequential steps really helped me understand what I was reading. I think your report was well executed with an introduction and conclusion. In adding to your comments on ACh within the process of contraction, I want to highlight Parkinson’s disease. Parkinson’s is a disease stimulated by the degeneration of nerve cells that control movement (Web MD, 2013). The cells of the brain that control movement require a balance of dopamine and ACh, which are both involved in the transmission of nerve signals and therefore the instigation of the muscle contraction process. With the disease, cells that produce dopamine begin to degenerate, throwing off the balance causing muscle rigidity, tremors, and alterations in gait and speech (Web MD, 2013). Although medicine can be given to help with the symptoms of Parkinson’s disease, there is no cure (Web MD, 2013).
Web MD. (2013, April 03). Parkinson’s disease health center. Retrieved from
http://www.webmd.com/parkinsons-disease/
”
You did a wonderful job explaining all the steps of muscle contraction! It helped me to clearly understand all the steps. I really liked how you describe ATP production, which was also really helpful. One thing that you might be able to change would be dividing up the steps in a different way. Although you put in all the steps, it was kind of confusing when trying to keep up which ones went first. One disorder that you might could add is muscular dystrophy. Muscular Dystrophy is a genetic disorder that destroys how the muscle functions (Answers Corporation). The muscle is destroyed by this disorder and is replaced with fat and connective tissue (Answers Corporation). It causes muscle weakness and affects to the nervous system as well (Answers Corporation). Over all you did an excellent job. Keep up the good work!
These are responses classmates posted. Just to give you general idea. They are not long responses.
