Human Anatomy
Muscle
It is originated from Latin musculus "little mouse" and is contractile tissue of the body and is developed from the mesodermal layer of embryonic germ cells. Its function is to produce force and cause motion within internal organs. Much of muscle contraction occurs without conscious thought and is necessary for survival, like the contraction of the heart, or peristalsis (stomach, intestinal movements). Voluntary muscle contraction is used to move the body, and can be finely controlled, like movements of the finger, or gross movements like the quadriceps muscle of the thigh.
Muscle is mainly composed of muscle cells. Within the cells there are myofibrils; myofibrils contain sarcomeres, which are composed of actin and myosin. Individual muscle fibres are surrounded by endomysium. Muscle fibres are bound together by perimysium into bundles called fascicles; the bundles are then grouped together to form muscle. Muscles are distributed throughout the body and provide sensory feedback information to the central nervous system.
Skeletal muscle is muscle attached to skeletal tissue, distinct from heart or smooth muscle. It is arranged in discrete muscles, an example of which is the biceps brachii. It is connected by tendons to processes of the skeleton. In contrast, smooth muscle occurs at various scales in almost every organ, from the skin (in which it controls erection of body hair) to the blood vessels and digestive tract (in which it controls the caliber of the lumen and peristalsis).
Cardiac muscle is the muscle tissue of the heart, and is similar to skeletal muscle in both composition and action, being comprised of myofibrils of sarcomeres. Cardiac muscle is anatomically different in that the muscle fibers are typically branched like a tree branch, and connect to other cardiac muscle fibers through intercalcated discs.
There are approximately 650 skeletal muscles in the human body. The number of muscle fibres are not increased through exercise; instead the muscle cells simply get bigger. Muscle fibres have a limited capacity for growth through hypertrophy and some believe they split through hyperplasia if subject to increased demand.
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