Published Online: 15 SEP 2011
Copyright © 2001 John Wiley & Sons, Ltd. All rights reserved.
How to Cite
Squire, J. M. 2011. Muscle Contraction. eLS. .
- Published Online: 15 SEP 2011
Muscular contraction is one of the few biological processes that can be appreciated directly in our everyday lives. In the past few decades a detailed understanding of the underlying molecular processes that produce force and movement has been obtained. Striated muscles contain well organised repeating units called sarcomeres which each include overlapping arrays of filaments of the proteins myosin and actin. Myosin filaments have projections on their surfaces known as crossbridges. These are the myosin head parts of the myosin molecules which form the myosin filament. It is the myosin heads, which are enzymes (ATPases) which interact with actin filaments to produce force and movement. Actin filaments contain additional proteins troponin and tropomyosin which are involved in switching muscle activity on and off (muscle regulation).
Vertebrates have two types of striated muscle namely cardiac muscle, which drives our heart and skeletal muscles which move our skeleton.
Vertebrate skeletal muscles contain long thin fibres (multinucleate cells) about 20–100 μm in diameter and very long (mms to cms).
Muscle fibres are parallel arrays of long, thin myofibrils about 2–5 μm in diameter and sometimes as long as the fibres.
Cardiac muscle cells (myocytes) are much shorter than skeletal muscle fibres and link end-to-end to provide long lengths of muscle. They also contain myofibrils.
The basic repeating unit in all striated muscles is the sarcomere. In vertebrates this is about 2–2.5 μm long. A myofibril is a long string of sarcomeres.
The sarcomere comprises overlapping arrays of myosin and actin filaments.
Myosin filaments are made of myosin molecules which consist of a rod-shaped region with two heads on.
The myosin heads projecting from the myosin filament surface can hydrolyse ATP (adenosine triphosphate; they are ATPase enzymes), a process which is accelerated when heads interact with actin to produce force and movement.
In striated muscles the interaction of myosin heads with actin is controlled by the proteins troponin and tropomyosin on the actin filaments.
It is calcium ions released by the sarcoplasmic reticulum around myofibrils in response to nervous stimulus that interact with troponin, move tropomyosin and allow the actin–myosin interaction to proceed.
- crossbridge cycle;
- myosin ATPase