Muscle Contraction
The ability of muscles to contract makes movement possible. The association of muscle contraction with movement has been known for centuries. In the second century A.D., the physiologist Galen understood that muscles work by pulling rather than by pushing. However, the mechanism remained unresolved until the 1950s.
Muscle contraction does operate by a pulling force. The basic mechanism is widespread in nature, being found in humans and other animals, in smooth, skeletal and heart muscles as well as in movement events at the level of the single cell. Although there are differences in the mechanism of contraction of smooth and skeletal muscle there are also a number of similarities. Basically, two sets of filaments slide over one another, allowing the muscle to shorten. When the filaments slide back in the other direction the muscles attains its original length.
Skeletal muscle is composed of fibers that are in turn each made up of myofibrils. Each myofibril is composed of myofilaments. The filaments are called thick and thin filaments, and were named after their appearance under microscopic examination. Thick filaments are composed of a protein called myosin. Myosin is a fibrous protein. That is, the proteins tend to be arranged in a relatively linear fashion. Thin filaments are composed of two interwoven strands of a protein called actin. Each actin strand is arranged as a helix, somewhat like the arrangement of a spiral staircase. The two helical actin strands wind around each other to form the thin filament.
The myosin and actin filaments are organized in a muscle as hundreds of layers, with the thick myosin filaments sandwiched between the thin actin filaments. Viewed in cross section the filaments resemble a honeycomb. The length of each filament is only a few millionths of a meter. This short stretch is known as a sarcomere (also called an A band). At either end of the sarcomere is a wall structure called the Z disc. The Z disc provides an anchor for the myosin and actin filaments. In the middle of each sarcomere is a structure called the M line. The M line is composed of another protein and acts as an anchor for the thick filaments.
In this so-called sliding filament model, because the myosin and actin filaments can slide past one another, they themselves do not shorten in the process of muscle contraction. The pulling together of the Z discs is what shortens the muscle.
The overlapping sliding of filaments occurs for only a short distance between adjacent Z discs. But the millions of such events occurring along the many sarcomeres are sufficient to cause contraction. The energy of these many contraction is transmitted through a system of connective tissue wrapping around individual muscle fiber and blankets of connective tissue that wrap around the entire muscle. This allows the contraction energy to be translated into anatomical motion.
The energy for the process is provided by adenine triphosphate, which is commonly designated as ATP. The ATP is able to bind to power the binding of myosin to actin. Numerous such events act to ratchet the actin filament past a myosin filament. The dissolution of ATP to adenosine diphosphate (ADP) releases the tension holding the filaments together, allowing relaxation of the filaments to occur. Also involved in this process in skeletal muscles are the so-called accessory proteins trophomyosin and troponin.
Smooth muscle lacks troponin, but does have a protein called caldesmon that acts in a similar fashion. Calcium is also necessary to cause a change in shape of the accessory proteins that exposes a site on the actin filament to which the myosin filament can bind, in the ATP-dependent process. Smooth muscle contraction also involves different molecules and unique myosin subunits, particularly the light chain or p-light chain.
This is the complete article, containing 617 words
(approx. 2 pages at 300 words per page).
