41.6:
Overview of Skeletal Muscle
Skeletal muscles coordinate with other structures, such as nerves, bones, and blood vessels, to allow vertebrates to perform voluntary movements.
Skeletal muscles are a highly structured arrangement of cells and connective tissue layers. Tendons are collagenous fibers that connect the muscle to the bone.
A sheath of connective tissue called the epimysium encloses the skeletal muscle and separates and protects the muscle from friction against other structures during movement.
Within the epimysium are numerous fascicles, or cell bundles, surrounded by another connective tissue layer — the perimysium.
Each fascicle contains multiple muscle cells individually enclosed in a specialized plasma membrane called the sarcolemma. The sarcolemma surrounds the sarcoplasm, the cytoplasm of skeletal muscle cells.
A single muscle cell contains many myofibrils composed of actin and myosin. A myofibril has repeating units called sarcomeres formed by an alternating arrangement of thin actin and thick myosin filaments. A sarcomere forms the functional contractile unit of the skeletal muscle.
41.6:
Overview of Skeletal Muscle
Skeletal muscles are composed of a bundle of muscle fibers and are attached to bones through tendons. Each skeletal muscle fiber is a single muscle cell. The sarcolemma, the plasma membrane of a skeletal muscle cell, consists of a lipid bilayer and glycocalyx that supports muscle fibers. The sarcolemma extends into the muscle cells to form tubular structures called transverse or T-tubules. Each side of the T-tubules consists of a membrane-bound structure called the sarcoplasmic reticulum, similar to the smooth endoplasmic reticulum. The storage and release of calcium ions in response to a neuronal signal.
Contractile elements called sarcomeres aid the contraction and relaxation of skeletal muscle fibers to control various body movements, protect internal organs, and maintain body posture. A sarcomere mainly consists of actin and myosin protein filaments. In a relaxed muscle cell, the active binding site of actin is masked by a protein called tropomyosin. This masking prevents actin from binding to myosin, preventing muscle contraction. When a nerve impulse stimulates a muscle fiber, the sarcoplasmic reticulum releases calcium into the sarcoplasm. The released calcium binds to a protein called troponin on the thin filaments of muscle fiber. This induces structural changes in troponin, allowing it to attach to tropomyosin. The binding of troponin to tropomyosin relieves the inhibition of actin and enables myosin to bind to actin, further causing muscle contraction.
Suggested Reading
- Cooper, G.M., Hausman R.M. The Cell A Molecular Approach. 4th ed.ASM Press. pp 487-492
- Karp, Gerald. Cell and Molecular Biology: Concepts and Experiments. 6th ed. John Wiley & Sons, 2010. pp 344-351