All human muscles are formed by a special tissue, the fibers of which are held together by connective tissue in bundles. They are all permeated with nerves and blood vessels. Muscle contractions are caused not only by their structure, but also by their interaction with the human skeleton.
Human muscles contract, primarily due to various irritations. This process is accompanied by thickening or shortening of muscle fibers, and, therefore, of the entire muscle as a whole. How can you cause this contraction? For example, you can send a short electrical discharge into the body, which causes an instantaneous muscle contraction. It will last no more than 0.1 seconds. Another way to cause muscle contraction is to be in a certain position for a long time. That is, while lying on the bed or sitting at the computer. In these cases, prolonged muscle contractions take place. All this is due to the position of the body, in which there is practically no movement. So, why do muscles contract? This process occurs in a living organism under the influence of excitation that enters the muscles from the nervous system through the centrifugal nerves. You can do two simple experiments to visualize how this happens: First, sit in a chair with one leg over the other. Ask to hit you on the knee with the edge of your hand. This action will irritate the knee tendon receptors. The excitation process that arose in them will be transmitted through the spinal cord along the nerves to the muscles of a person. They will contract for a while, which will lead to the "bouncing" of the lower leg. This process is also called the knee jerk. It is caused by excitement that has come from the central nervous system to skeletal muscle. The second simple example is muscle contraction. Call out to any person passing along the street. You will see how he turns his head, despite the fact that you may not know him at all. Moreover, it will turn exactly towards the sound. This is a reflex process that is caused by nervous excitement. It penetrates into the receptors of the hearing organs and goes through the brain to the muscles of a person. Thus, muscle contraction occurs.
