Final answer:
Skeletal muscles are made up of sarcomeres, myofibrils, muscle fibers, and fascicles, each enclosed in connective tissues like epimysium. Muscle size changes via hypertrophy and atrophy impact strength, with factors like aging and disuse influencing these processes.
Step-by-step explanation:
The units that make up skeletal muscle, in order of increasing size, are: sarcomeres, myofibrils, muscle fibers, and fascicles. Sarcomeres are the smallest contractile units found within myofibrils, which are rod-like units within muscle fibers. Muscle fibers, also known as myocytes, are the cells of the muscle tissue that contract to generate force. Finally, a group of muscle fibers bundled together by perimysium connective tissue make up a fascicle. Each skeletal muscle is enveloped by epimysium connective tissue which provides structure and integration with other tissues.
Muscle atrophy such as sarcopenia occurs when muscle fibers reduce in size and are replaced by connective and adipose tissue, leading to a loss in muscle strength. Conversely, muscle hypertrophy involves an increase in the size of muscle fibers, which can enhance muscle strength. The muscular system is a vital component of human anatomy, allowing for movement, posture, and daily physical activities.
Everyday muscle use, disease, disuse, or aging can cause muscles to change in size. Astronauts, for example, can experience rapid muscle atrophy in space due to the lack of gravity-induced resistance that stimulates the muscles on Earth.