The ‘Z’ line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction. It is clear from the above steps, that during shortening of the muscle, i.e., contraction, the ‘I’ bands get reduced, whereas the ‘A’ bands retain the length (Figure . ). The myosin, releasing the ADP and P goes back to its relaxed state.
A new ATP binds and the cross-bridge is broken (Figure . ). The ATP is again hydrolysed by the myosin head and the cycle of cross bridge formation Figure . Stages in cross bridge formation, rotation of head and breaking of cross bridge and breakage is repeated causing further sliding.
The process continues till the Ca ++ ions are pumped back to the sarcoplasmic cisternae resulting in the masking of actin filaments. This causes the return of ‘Z’ lines back to their original position, i.e., relaxation. The reaction time of the fibres can vary in different muscles. Repeated activation of the muscles can lead to the accumulation of lactic acid due to anaerobic breakdown of glycogen in them, causing fatigue.
Muscle contains a red coloured oxygen storing pigment called myoglobin. Myoglobin content is high in some of the muscles which gives a reddish appearance. Such muscles are called the Red fibres. These muscles also contain plenty of mitochondria which can utilise the large amount of oxygen stored in them for ATP production.
These muscles, therefore, can also be called aerobic muscles. On the other hand, some of the muscles possess very less quantity of myoglobin and therefore, appear pale or whitish. These are the White fibres. Number of mitochondria are also few in them, but the amount of sarcoplasmic reticulum is high.
They depend on anaerobic process for energy. Figure . Sliding-filament theory of muscle contraction (movement of the thin filaments and the relative size of the I band and H zones)