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Each muscle fibre is made up of smaller fibres called myofibrils. Myofibrils contain ‘sections’ called sarcomeres, which are joined end to end along the myofibrils. Sarcomeres are divided up by Z lines, as shown in Figure 1.
Each individual sarcomere is the functional unit of the myofibril, that is, the sarcomeres cause contraction. Each sarcomere contains two proteins: actin and myosin. In the contracted state (Figure 2) the Z lines move inwards, getting closer together. Learning the basics behind the sliding filament mechanism will help you understand how this occurs.
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