How are myofibrils arranged?
This is essentially the crucial piece of information in understanding muscle contraction – how, and why, myofibrils are arranged in a particular way. Being the functional component of muscle contraction, it is inevitable that their structure is tailored to the task they have to perform. Myofibrils are made up of units known as sarcomeres. These are formed by the arrangment of the proteins myosin and actin. As hopefully shown in the image on the left, the myofibrils contained in a muscle fibre are made up of filaments of these two proteins; so short, thin strands of myosin and actin are lined up in the myofibril. The way that these are lined up is very important for understanding how muscles contract. Along each myofibril are a series of Z-discs or Z-lines. These mark the start of a new sarcomere; so a sarcomere is made up of everything between two Z-discs. Half way between these two Z-discs is an M-line. Myosin (thick) filaments are attached to the M-line, while actin (thin) filaments
