Molecular mechanism of muscle contraction
In the contracted state, these actin filaments have been pulled inside the myosin filaments, so that their ends overlap to the maximum extent.
Muscle contraction occurs by a sliding fibre mechanism. The figure shows the basic mechanism of muscle contraction. It shows the relaxed state of a contractile unit (above) and the contraction state (below).
In the dilated state, the ends of the actin filaments pulled from two consecutive Z-discs barely overlap. In contrast, in the contracted state, these actin filaments have been pulled inside the myosin filaments, so that their ends overlap to the maximum extent. In addition, the Z-discs were pulled by actin filaments all the way to the terminals of myosin filaments. Thus, muscle contraction occurs by a sliding fibre mechanism.
Figure. The relaxation and contraction states of a myofibril are shown (at the top) the sliding of the actin filaments (pink) into the space between the myosin filaments (red) and (bottom) the pulling of the Z membranes into each other.
But what causes the actin filaments to slip inside the myosin filaments? This activity is caused by the force generated by the interaction of the cross-bridges from the myosin filaments with the actin filaments. Under resting conditions, these forces are inactive, but when an action potential travels along with the muscle fibre, this causes the sarcoplasmic reticulum to rapidly release large amounts of calcium ions from around the myofibrils. The calcium ions in turn activate the forces between the myosin and actin filaments, and contraction begins. However, energy is required for the contraction to proceed. This energy comes from high-energy binding in the ATP molecule, which is broken down to adenosine diphosphate (ADP) to release energy. In the following sections, we will describe the molecular processes of contractility.