Muscle Contraction

A rotator cuff injury, an injury to the group of four muscles in the shoulder, is the irritation or overuse of these four muscles or tendons. These tears are typically a results of a significant injury or progressive deterioration due to wear and tear of the tissues, typically due to a sport’s related injury or aging. In my previous paper, I mentioned some of the physiological mechanisms under normal circumstances. For example, calcium binding to troponin and exposing binding sites which results in a cross-bridge cycle. In addition to that, I mentioned principles such as the length-tension relationship and force summation. On the contrary, when a tear or injury to the muscle tissue occurs some of the pervious aspects mentioned are altered.
The normal physiology of the shoulder joint follows the skeletal muscle contraction process. This process begins with an action potential traveling down the axon, which will release acetylcholine. The acetylcholine will cause the impulse to spread across the sarcolemma, and enter the T tubules and sarcoplasmic reticulum. Calcium enters the muscle cell and binds with troponin, exposing the binding sites which allows the actin and myosin to bind together. The next step in the process after this binding is the sliding of actin and myosin using ATP as energy that activates the myosin cross bridges causing contraction. These steps will continue to take place as long there is a stimulus to activate the cycle, calcium is present in the muscles, and ATP is available. The deviation from the normal physiology that occurs from a rotator cuff injury is the tearing of one or all of the shoulder muscles, causing the muscles of the shoulder joint to no longer be able to generate as much force as expected.
The two main processes involved in muscle contraction are cross-bridge cycling and the length-tension relationship, which normally function to maximize contraction in the muscle. Beginning with the activation of a motor unit, the ions flow from the motor unit towards the muscle fibers that it innervates by the motor neuron in the same way as a normal muscle would. However since the tissue is damaged, the force that is generated by these muscle fibers will not be as great, leaving the fibers less effective. Now, there is a situation in which the myosin head is damaged and there are less actin binding to less myosin effecting the force production. The length tension relationship is dependent on the number of cross-bridges that are formed, which decreases when they are not generating at the usual amount. In relation to the length-tension relationship, when a muscle is contracting at an angle in which the muscle should have greater force, it will not because of the lack of actin-myosin overlap. So when the myosin head pivots, there are now less pivoting myosin heads. A major cause of this deviation is due to a lack of calcium flooding into the muscle cell. If there is not enough calcium for the troponin to bind to in order to expose the
myosin-actin binding sites, the attaching and detaching of myosin and actin groups is affected causing the filaments in the sarcomere to be unable to bind together. In an attempt to return back to the normal physiology, there are a few treatment options for a rotator cuff tear. In some cases rest, ice, and physical therapy may be all that is necessary to return to a normal state but another treatment option may be steroid injections to reduce pain. However in some tears, a surgery to repair the torn tendons and return cross-bridge cycling back to a normal state could be required. Physical therapy is also successful in repairing the skeletal muscles by stimulating protein synthesis and allowing the tendons to repair themselves. In relation to physiological processes, a successful therapy for traumatic or over-use injuries like rotator cuff tears is prolotherapy.
Prolotherapy is an irritant that is used to stimulate regeneration. According to the American Association of Orthopaedic Medicine, it “stimulates the body’s natural healing processes to strengthen joints weakened by traumatic or over-use injuries”, repairing the damaged tendons by producing new fibrous tissues. This is done by injecting an irritant solution directly on the torn ligaments on the shoulder. Prolotherapy works by stimulating satellite cells in an attempt to proliferate the injured muscles. These satellite cells are activated by the disruption in muscles cells from an injury, and are stimulated by growth factor hormones to increase muscle fiber size by regulating the cell activity. Growth factor hormones including hepatocyte growth factor, fibroblast growth factor, and insulin-like growth factor (I and II) work in various ways to repair and regrow the muscles. The satellite cells will fuse together which will increase hypertrophy, they go through this process to form new fibers and repair the damaged
fibers. In conclusion, I prefaced normal physiology then discussed the deviation away from normal physiology and the mechanisms including cross-bridge cycling and the length-tension relationship. Following this, I talked about the therapies and specifically prolotherapy, an irritant therapy that is used to stimulate satellite cell proliferation, which restores the normal physiology of the muscle where we see adequate cross bridging and force summation when the motor unit activates the muscles by the use of a nerve impulse and those innervated muscle fibers can cross-bridge and generate force again.