MUSCLE TISSUE Muscle tissue is a type of soft tissue in the body. It is by definition contractile tissue and is formed by the process of myogenesis in the embryo. As mentioned, the three types of muscle tissue include skeletal (striated) muscle, smooth (nonstriated) muscle, and cardiac (semistriated) muscle. Figure 22 shows what these different muscle types look like:
There is no conscious control over smooth muscle and cardiac muscle but there is activation of both muscle types by the central nervous system and the endocrine system. The contraction of skeletal muscle is voluntary with input from the central nervous system. Deep tendon reflexes (like the knee-jerk reaction) will happen involuntarily but depend on the central nervous system without actual thinking involved. Muscle cells are contractile and are called myocytes. They can be very short muscle cells or as long as ten centimeters in length. They can be connected by arrangements of myofilaments, which are regularly repeated to allow the muscle tissue to have definition. Skeletal muscle is anchored by tendons or aponeuroses to bone as is used to cause skeletal movement and to affect a certain posture. About 40 percent of the body by mass is made from skeletal muscle. It is known for striations. Smooth muscle is non-striated and is found within the walls of internal structures, such as the stomach, esophagus, bronchi, intestines, bladder, urethra, ureters, blood vessels, and the arrector pili in the skin (which makes goosebumps). Cardiac muscle is also striated and is found in the heart. Both cardiac and skeletal muscle contains sarcomeres, which are packed bundles into bundles of muscle. Sarcomeres are made from long, fibrous proteins that act as filaments that slide past one other when a muscle contracts or relaxes. Skeletal muscle has parallel bundles of cells,
39
