Sliding Filament Theory
The sliding filament theory explains the organization and structure of the sarcomere (the smallest function unit of a muscle cell) and the contractile proteins. It helps us understand muscle cell contraction via a power stroke. The power stroke (discussed in excitation-contraction coupling ) is the process of the myosin head attaching to actin and pulling the actin filament toward the center of the cell (sliding the "actin" filament across the "myosin" filament).
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Contractile Protein Arrangement: In the diagram, note the thick myosin filaments (horizontal and red) and the thin actin filaments (horizontal in blue). The bumps on the myosin proteins represent the myosin heads. This is the portion of myosin that binds to actin. Note that there are multiple myosin heads ready to bind to each actin protein. The multiple heads on the myosin protein are due to the myosin chains being comprised of many myosin proteins "braided" together, and the myosin protein "heads" staggered and sticking out, ready for a power stroke. The power stroke (discussed in excitation-contraction coupling ) is the process of the myosin head attaching to actin and pulling the actin filament toward the center of the cell (sliding the "actin" filament across the "myosin" filament).
Rower Analogy: The best analogy for describing the sliding filament theory is likely the "rowers of a boat." The rowers' legs in the boat are like the majority of the myosin protein, with the oars hanging out of the boat like the myosin heads. In this analogy, we equate the action of the multiple myosin heads repeatedly attaching to actin, performing a power stroke, detaching, and reattaching further along the actin to repeat the process, as similar to the oars (myosin) stroking the water (actin), coming out of the water, resetting their position, and performing another stroke. As the water (actin) is pulled behind the oars (myosin heads) toward the center of the sarcomere, the Z-disks are pulled along with them.