The Contractile Function of Muscle
In previous posts we saw that, although muscles contract to do work, this traditional concept cannot explain how muscles function to support the skeleton as a whole. To support the skeletal framework, muscles do not simply pull on bone but lengthen between their bony attachments and, in this context, maintain tone in order to create postural support. Muscle tissue thus possesses the dual property of being able to contract and to lengthen.
But what does it mean for a muscle to contract, and what does it mean for a muscle to lengthen? Muscle fibers are long, thread-like cells bundled together to form muscles, which are attached to bones to produce movement. In highly specialized organs, like the eye, muscle fibers are a fraction of an inch long, and only a very few are sufficient for the job at hand; in other parts of the body, such as the thigh, these fibers can be up to two feet long, and many thousands are bundled together to form powerful, bulky muscles that make it possible to walk, jump, and run on two feet.
Just as a muscle is made up of many individual muscle cells, each muscle cell is made up of tiny, thread-like myofibrils, which are the contractile units within each muscle cell that make it possible for the muscle to perform work. The illustration to the right shows the bundling of the myofibrils into individual cells, which in turn are bundled into larger bundles, which in their turn are bundled into the units we call muscles.
Within each fibril are two types of molecular chains, the thin actin and the thick myosin chains, which are stacked together in such a way that the two types of filaments interdigitate, giving the muscle the banded or striated pattern after which it is named.
This illustration shows the cylindrical arrangement of an individual myofibril, composed of chains of molecules.
It is the movement of the thin filaments in relation to the thick ones that produces contraction of the muscle, which is set off by chemical activity within the myofibrils. The myosin filaments have regularly protruding globular heads which are capable of binding at particular regions on the actin filaments. In a relaxed muscle, the myosin heads do not come into contact with these sites on the actin molecules because they are blocked by troponin molecules. In this state, the myofilaments are able to slide alongside each other, allowing the muscle to be passively lengthened. When the muscle fiber receives a signal from its motor nerve, however, this releases calcium ions into the myofibril space, which bind with the troponin molecules, changing their shape and exposing the binding sites along the actin molecules. The globular heads of the myosin molecules now bond at regular intervals along the actin molecules, forming cross-bridges between the actin and myosin strands.
One of two things can now happen. If the myosin heads remain bonded along the actin strands, the cross-bridges will now create stiffness or tone in the muscle because the interdigitating strands are now linked together and cannot slide alongside each other. The muscle will then resist being lengthened--exactly what we see when a muscle is not entirely relaxed but maintains tone, or very gentle contraction which maintains postural support. In this case, the muscle does not actively contract but maintains stiffness or tone because the myosin and actin strands are connected.
The second possibility, of course, is that the muscle can actively contract or shorten. In this case, the myosin heads will only momentarily form cross-bridges because they will continue to bond at adjacent sites along the actin chain, rotating and drawing the actin chain along the myosin chain.
This happens because, when the myosin head attaches to the actin filament, it pivots or rotates, which moves the actin filament in relation to the myosin filament. After each rotation, the myosin head detaches, straightens itself out, re-attaches at a new point on the actin chain and, through this continuous “ratcheting” action, draws the actin chain along the myosin chain. This telescoping of the actin and myosin strands, which can be seen on an electron microscope as a narrowing of the striations in the muscle, causes the muscle fiber as a whole to shorten, or contract.
All of this is what we call normal muscle activity. Muscle contraction is produced at a molecular level by the interaction between the interdigitating strands of myosin and actin molecules that make up the myofibrils, and is caused when the interdigitating strands of myosin and actin molecules begin to slide over one another and thus shorten the muscle.
In the next post, we’ll look at how muscles lengthen.