Videos of the Simple Machines in Action (click here)
In order to demonstrate and elaborate on the “describe how muscle contraction is induced” learning objective, I created a simple machine to illustrate the process. I expanded on the material by going into much greater depth on the individual steps that occur in the process of muscle contraction. I decided to isolate my project solely to skeletal muscle. Cardiac muscle and smooth muscle also contract; actually, all muscle cells contract. In fact, it’s the only thing they do. Muscle cells are highly specialized for contraction alone. (Cooper, 2000). The reason I chose to focus on skeletal muscle is due to it being the only muscle type responsible for voluntary movements. (Cooper, 2000). I chose to make a simple machine to depict muscle contraction because much like action potentials and muscle contraction, simple machines are all or nothing processes. If your action piece does not reach its targeted destination, the next phase will not occur. Each step is contingent on the last, and it makes it, or it doesn’t.
Muscle contraction in skeletal muscle tissue is started when the nervous system generates action potentials. (Larson, n.d.). Action potentials are impulses that travel through motor neuron cells. The action potential then arrives at the neuromuscular junction, the motor neuron releases acetylcholine (a type of neurotransmitter) which binds to the receptors on the most external layer of the muscle fiber. (Krans, 2022). The action potential is represented by the orange ball in the first simple machine, the motor neuron cells are represented as the large tube, the neuromuscular junction is marked, the acetylcholine is represented by the two marbles, and the outer muscle fiber is labeled.
Acetylcholine binding to the outer receptors of the muscle fiber membrane causes the internal proteins of the muscle fiber to open the membrane channels (represented by wood slots in the second simple machine) and allow sodium ions (represented by marbles) into the muscle fibers’ cytoplasm (represented by funnel). (Krans, 2022). Most of the cytoplasm contains myofibrils which are two types of cylindrical bundles of filaments. (Cooper, 2000). The first type, thick filaments are myosin and the second, thin filaments are actin. Sarcomeres are chains of contractile units organizing each of the myofibril. (Cooper, 2000). The sarcomeres are split into various distinct regions. Z disc, for example, defines the end of each sarcomere, A bands (containing thick myosin filaments) and I bands (containing only thin actin filaments) are located within the sarcomere and respond to the presence or absence of myosin filaments. (Cooper, 2000). The myosin and actin filaments overlap in some peripheral areas of the region of A band, while the H zone (located in the middle region) contains only myosin. (Cooper, 2000).
The insurgence of sodium causes the release of calcium ions that had been stored in the muscle fiber. The concentration of the ions will impact the contraction force generated by the contractile elements. (Hijikata et al., 2021). The calcium ions (represented by marbles in the second simple machine) subsequently diffuse into the muscle fiber (represented by the glass vase). The plus ends of actin filaments are attached to the Z disc, and myosin filaments are anchored to the middle of the sarcomere. (Cooper, 2000).
It is the release of calcium that allows our myosin (represented by marble 1 in the third simple machine) and actin (represented by marble 2) to interact and cross-bridges forming (represented by the large and medium sized tubes) while powered by ATP. Myosin chains of proteins within the muscle cell are able to communicate with each other and lengthen and shorten as the muscle contracts and eventually relaxes. (Krans, 2022). The motor activity of the myosin moves along the actin filament in the direction of the plus end which slides the actin filaments from each side of the sarcomere toward the M line which subsequently shortens the sarcomere which results in muscle contraction. (Cooper, 2000). This reorganization of length in protein chains will cease when the stimulation of the impulse is exhausted. In order to relax the muscles, the chemical processes in the muscle fiber reverses, allowing for muscle relaxation.