The General Anatomy of a Skeletal Muscle Help

Myofilaments and the Sliding Filament Theory

Whether one is considering the contraction of muscles in humans, fish, amphibians, reptiles, birds, or practically any other vertebrate animals, the general mechanism of shortening is much the same. To understand its basic features, we must examine the interior of a sarcomere (Figure 14.7). Within the sarcomere are a number of myofilaments ( my -oh- FIL -ah-ments) or “muscle threads.” In reality, these myofilaments are thread-like collections of protein molecules. Attached to the Z-line at either end of the sarcomere are a series of thin actin ( AK -tin) myofilaments . Each of these thin actin myofilaments consists of two twisted strands of globe-shaped actin proteins.

Fig. 14.7 Myofilaments and the sliding filament theory of muscle contraction. (A) Resting (non-contracting position). (B) Contracting process: thin myofilaments slide inward; sarcomere shortens.

Study suggestion: Find a beaded pearl necklace and lay it upon a surface. Place both strands of the necklace side-by-side, then twist them around each other. The resulting double helix (two twisted strands) provides a rough model for a thin actin myofilament.

In the middle of each sarcomere is a series of thick myosin ( MY -oh-sin) myofilaments . These are stacked vertically above-and-below one another, with narrow gaps between them. Each myosin myofilament consists of dozens of individual myosin protein molecules. Each myosin protein somewhat resembles a golf club with two heads. The double-heads are tiltable, as if they were poised upon a chemical hinge. The tiltable double-head of each myosin molecule is technically called a myosin cross-bridge . [ Study suggestion: Visualize two golfers, each carrying their own bag of golf clubs. Each club has a double-head at one end, which is attached by a tiltable hinge. The two golfers stand back-to-back, in the center of a sarcomere, and then each gives his golf bag a heave. If the golfers keep hold of their bags, their clubs will come flying out in both directions, some with their double-heads pointing upward, and some with their double-heads pointing down. The resulting highly orderly arrangement provides a rough model for the thick myosin myofilament.]

The myosin cross-bridge is the chief contact point between the thin actin and thick myosin myofilaments. It is also vitally important because of its close functional relationship with the high-energy ATP molecule. You may remember (Chapter 4) that the ATP molecule is split by a special kind of enzyme, called ATPase. In the case of muscle, the enzyme is myosin ATPase .

The Sliding Filament Theory

According to the sliding filament theory of muscle contraction , muscles shorten due to the inward sliding of the thin actin myofilaments over the tilted cross-bridges of the thick myosin myofilaments. “How is this accomplished?” you might reasonably question. The answer involves the action of myosin ATPase.

When the skeletal muscle fiber is excited by a nerve ending, myosin ATPase enzyme splits ATP molecules in the region of the myosin cross-bridges. The resulting free energy tilts the myosin cross-bridges inward. As the cross-bridges tilt, the overhanging thin actin myofilaments slide over their tips. The sliding occurs at both ends of the sarcomere. Hence, each sarcomere shortens. Since each myofibril organelle consists of a series of sarcomeres hooked end-to-end, the whole myofibril shortens. And as all of their myofibril organelles shorten, the entire skeletal muscle fiber (cell) also shortens.

It is this shortening of many muscle fibers that yanks upon a tendon, creating a pulling force upon a bone that results in body movement.

Fig. 14.7 Myofilaments and the sliding filament theory of muscle contraction. (A) Resting (non-contracting position). (B) Contracting process: thin myofilaments slide inward; sarcomere shortens.

After the muscle contracts, the cross-bridges tilt back into their vertical positions, causing the thin actin myofilaments to slide outward, again. The sarcomere re-lengthens, and the muscle fiber relaxes. This same highly orderly sequence of muscle contraction (shortening) followed by relaxation (lengthening) repeats itself over and over again. And each time, the trigger for the contraction–relaxation sequence to begin is a sufficiently strong level of muscle fiber excitation by nearby nerve endings.

Practice problems for these concepts can be found at:  The Neuromuscular (Nerve-Muscle) Connection Test