Module 7: Figure skeletal muscle structure
Structure of skeletal muscle fibres.
A. The multinucleate muscle fibres are held together in a connective tissue matrix. The nuclei (N) are located close to the cell surface. Each fibre is innervated by a motor neuron that terminates at the neuromuscular junction (NMJ). B. Each muscle fibre has characteristic striations that reflect the precise organization of the contractile proteins actin and myosin along the length of the myofibril. C. The basic contractile unit is the sarcomere. At either end of each sarcomere are the Z discs to which the actin filaments (yellow) are attached. D. Actin has a helical structure made up of spherical subunits [globular actin (G-actin)] joined together into two strands that wind around each other to form the F-actin helix with a diameter of 8 nm. The other major contractile protein is myosin, which is organized into filaments that lie in the middle of the sarcomere between the actin fibres. Each filament contains about 200 myosin molecules. Each myosin molecule has a head region and a long tail. The long tails of two myosins wrap around each other to form a dimer, which in turn aggregates to form the myosin filament, with the head regions lined precisely opposite the actin filaments. When viewed in transverse sections, the actin filaments form a hexagonal pattern around each myosin filament.