Muscle contraction

Contraction of striated muscle

Under a muscle contraction is defined as the active shortening of a muscle.

In a broader sense tension of the muscle, the effect of shortening, but the muscle against resistance in a given length will keep ( isometric contraction) and those that offer resistance to a force acting on the muscle force, during which he is lengthened ( eccentric contraction ), the muscle contractions attributed.

Basics

Thus, it is generally a biological process, are generated in the mechanical forces in muscle tissue. In case of the skeletal muscle, these forces are transmitted through the bone tendon.

The forces are generated by converting chemical energy into mechanical energy by means of the actin -myosin complex in individual muscle cells, which in turn derives its energy from the chemical hydrolysis of ATP.

Thus, to ensure a contraction of the muscle tissue as a whole part, and the transmission of the force thus generated, it is necessary to synchronize and coordinate the contraction of the muscle (fiber ) cells and a transfer of the force generated by each of muscle fiber in the chord in question.

A muscle is a part of the fabric in most multicellular animals, which is usually a bone or other muscles rests ( displacement caused by layers of loose connective tissue, fascia and lodges ) and through his connections with tendon is able to move as a whole, limbs, internal body areas ( abdominal pressure during coughing, micturition, defecation, parturition, etc.; breathing motion ) to move and ultimately enables the individual to exert forces on its environment ( eg hitting a nail into the wall) and move around themselves.

Muscle contraction in the context of the musculoskeletal system of the extremities

To permit movement of parts of the body against resistance, such as lifting a leg, which has a considerable weight, or even slowing down of the running or jumping, the muscle on the tendon system power must be exerted on the points of the bone to. For this purpose, a continuous power circuit is required, which has to include all the parts of the tendons and of the muscle.

This power circuit must according to the principle " The chain is only as strong as its weakest link" all the elements include both the coarse and the fine structure of muscle formation. This therefore includes various levels: the muscle than whole tissue section, the muscle fiber, the myofibrils and in longitudinal outline the sarcomere as the smallest portion of the myofibril. At the transitions muscle / muscle fiber ( endomysium ), fibril / fibril endomysium / tendon, perimysium / tendon, etc. up to the transitions of the sarcomeres within the fibril structures need to control the forces and partially redirect ( shear forces) can. The muscle can exert a force of up to 40 N per cm ² muscle cross -section and are passively up to 100 N / cm ² resilient.

It is striking that of the tendon forces supporting structures, in particular the endomysium contributes to the transmission, by being directly connected to the radiating tendon ends. Since the endomysium about Myotendinöse compounds takes on the ends of actin filaments (see below) forces exerted directly and the traction is guaranteed here.

In addition to the power transmission in the direction of contraction, the prevention or redirection of shear forces is an important task that meets the muscle through mechanisms of the mechanical connection, but also the control of the contraction of the muscle fiber cells. Here come the transverse structures that ensure the cohesion of the fibrils into fibers and the fibers turn to muscle strands, of great importance. From a mechanical point here are the Costamere and desmin filaments mentioned, with respect to the control and thus reduce shear forces to synchronize the work of the sarcomeres and thus the fibrils by the rapid transmission of the action potential generated by the end plates in the longitudinal and transverse directions about the so-called triads. This is a the fibrils to comprehensive, transversal structure of two terminal cisterns of the sarcoplasmic reticulum and the transverse tubules, the morphologically represents an invagination of the plasma membrane and transfers the action potential both in length and in depth. There he provides for the opening of specific Ca channels, resulting in the activation of the contractile mechanism.

Description of the contractile mechanism

After Gleitfilament - or Filamentgleittheorie by Andrew F. Huxley and Hugh E. Huxley sliding filament proteins during contraction without altering the proper length with each other and thus shorten the length of the muscle. When the filament proteins are actin, the outer, thin filament, and myosin, the inner, thick filament which pushes past on the thin filament, thereby allowing the contraction. This movement is made possible by changes in the chemical configuration and thus the shape of the myosin molecules: Myosin has small projections ( "heads" ), can also change its angle to the rest of the molecule ( " stem "). The heads can in turn bind to the actin filaments and move them to so-called " rowing movements." The contraction is initiated by a nerve impulse. In addition, is required for the solution of the actin myosin from energy in the form of ATP. Is this no longer available, the molecules can no longer be detached from one another and it comes to rigor mortis.

In detail, the contraction caused by the so-called cross- bridge cycle ( Griffin letting go cycle) is declared between the actin - myosin filaments and. The name derives from the function of the myosin heads as a cross- bridges between the actin and myosin filaments ago.

A cross-bridged cycle lasts from 10 to 100 ms and shifts the filaments around 10-20 nm, corresponding to only about one percent of its length. To allow for greater variation in length, the cycle must therefore be passed through several times. By about 50 gripping letting go cycles, the sarcomere can shorten by about 50% of its rest position in significantly less than a second.

If free Ca2 concentration below 10-7 mol / l, the Tropomyosinfäden loop back to the actin filament, so that no new bonds can form with the myosin heads - the muscle relaxes, then one speaks of muscle relaxation. For this it is necessary to transport the calcium ions by active pumping from the muscle tissue. The involvement of calcium ions in muscle contraction was first demonstrated by Setsuro Ebashi.

Some of the details of Gleitfilamenttheorie are not yet fully understood. For example, the exact geometrical configuration of the myosin heads the subject of current research.

Contraction types

Depending on the power (voltage ) and change in length of the muscle can be multiple types of contraction differ:

From these elementary types of contraction to contraction complex shapes can be put together. They are used in everyday life most often. These are, for example,

In terms of the resulting change in length of the muscle and the speed with which this is done to contractions can be characterized as follows, for example: