Muscle Contractions

We have three types of muscles in our bodies, skeletal, smooth and cardiac. Each type of muscle performs different functions in our bodies. Some of the characteristics of our muscles are that they respond to stimuli, they are able to shorten in length, they stretch when they are pulled and they tend to return to their original shape after contracting or extending them. Muscle contractions occur when our nervous system sends a signal via the neuromuscular junctions the signal arrives at the end bulb, the vesicles release acetylcholine and diffuse across the neuromuscular cleft. The transmitter molecules fill receptor sites in the membrane of the muscle and increase the membrane permeability to sodium. The sodium then diffuses into the membrane and the membrane potential becomes less negative. If the threshold potential is reached, an action potential occurs and a nerve impulse moves along the muscle cell membrane and the muscle will contract. While skeletal muscles will only contract by neurons, smooth and cardiac muscles will contract without nervous stimulation. The nervous system can however influence smooth and cardiac muscle contractions.
These muscles are attached to our bones and skin by tendons and move our skeleton. Skeletal muscles consist of many bundles called fascicles. The fascicles are surrounded by the perimysium, or connective tissue. The fascicles are made of muscle fibers or cells. The muscle cells are made of myofibrils. The myofibrils are made of long molecules of protein called myofilaments. There are thick myofilament and thin myofilaments. Skeletal muscles shape and size depends on the arrangement of these fibers. For a skeletal muscle to contract it must receive an impulse from a