Current understanding of how muscles contract is based on the sliding filament model. This model applies to skeletal, smooth and cardiac muscle.
Very simply, during muscle contraction, a change in muscle length is caused by the thin filaments being pulled along the thick filaments.
So although the length of the overlap of the thick and thin filaments (the sarcomere) changes, the lengths of the filaments themselves remain the same. The length of the sarcomere is a result of how far these filaments overlap.
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The key to the sliding filament model is the structure of the myofilaments (within the myofibrils, within the muscle fibres, within the muscle). The two kinds of myofilaments, thick and thin, differ from each other in diameter and in the protein they are made from:
- Thick myofilaments are thick in diameter and are made of several hundred molecules of a fibrous protein called myosin
- Thin myofilaments are thin in diameter and are made of two linear polymers of a globular protein called actin wound in a spiral
Muscle tissue can be described in terms of units called sarcomeres, which are the portion of overlapping thick and thin filaments. This means that the length of a sarcomere is determined by the positions of the thick and thin filaments relative to each other.
What starts the contraction?
In skeletal muscle, this contraction is stimulated by electrical impulses carried by nerves. In cardiac and smooth muscles, the contractions are stimulated by internal pacemaker cells, which regularly contract without any conscious control.
See an animation and explanation of the sliding filament model.
In-depth information on muscle contraction.