Skeletal muscles are voluntary muscle tissue that comprise a significant portion of the human body’s musculature. They are responsible for walking, running, lifting, and maintaining posture. Skeletal muscle tissue is organized hierarchically, with multiple levels of organization, from the whole muscle down to individual muscle fibers.
1. Whole Muscle:
– At the highest level of organization, a skeletal muscle is considered a whole muscle. Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerves, and connective tissue.
– The connective tissue surrounding the entire muscle is known as the epimysium.
2. Muscle Fascicles:
– The muscle fibers are grouped into bundles called fascicles within each skeletal muscle.
– A layer of connective tissue surrounds the bundles called the perimysium.
3. Muscle Fiber:
– A muscle fiber, also known as a muscle cell or myofiber, is the individual contractile unit of skeletal muscle.
– Each muscle fiber is a long, cylindrical, multinucleated cell with a high content of myofibrils.
– Myofibrils are contractile structures within muscle fibers composed of repeating units called sarcomeres.
4. Sarcomere:
– The sarcomere is the functional unit of muscle contraction and is responsible for the striated appearance of skeletal muscle.
– Sarcomeres are composed of thick and thin myofilaments. Thick filaments are primarily made of myosin, while thin filaments are primarily made of actin.
– The interaction between myosin and actin during contraction is the basis of muscle contraction.
5. Myofibrils:
– The Myofibrils are thread-like structures that run the muscle fiber’s length and contain numerous end-to-end sarcomeres.
– Myofibrils are composed of repeating sarcomeres, giving them a striated appearance.
6. Myofilaments:
– Myofilaments are the protein filaments within a sarcomere that generate muscle contraction.
– Thick filaments (myosin) and thin filaments (actin) are organized within the sarcomere to allow them to slide past each other during muscle contraction.
7. Muscle Proteins:
– In addition to myosin and actin, skeletal muscles contain various proteins involved in muscle contraction, including troponin and tropomyosin, which regulate the interaction between myosin and actin.
– Other structural proteins like titin provide elasticity and maintain the alignment of sarcomeres.
8. Muscle Fiber Membrane:
– The plasma membrane of a muscle fiber is called the sarcolemma.
– The sarcolemma contains numerous invaginations called T-tubules that help transmit action potentials deep into the muscle fiber.
9. Sarcoplasm:
– The cytoplasm of a muscle fiber is called sarcoplasm, and it contains the usual cellular organelles such as mitochondria, nuclei, and the endoplasmic reticulum.
– The endoplasmic reticulum in muscle fibers is called the sarcoplasmic reticulum (SR), which stores and releases calcium ions necessary for muscle contraction.
10. Motor Units:
– A motor unit consists of a single motor neuron and all its innervated muscle fibers.
– Motor units vary in size, with small motor units controlling fine motor movements and larger motor units used for more powerful movements.
11. Neuromuscular Junction:
– The neuromuscular junction is the point of contact between a motor neuron and a muscle fiber.
– When an action potential reaches the neuromuscular junction, it triggers the release of acetylcholine, which initiates muscle contraction.
The organization of skeletal muscle from the whole muscle down to the individual sarcomeres allows for precise control of muscle contraction and movement. This hierarchical structure enables the body to produce a wide range of movements with varying degrees of force and precision, depending on the functional requirements of different muscles and activities.