Figure 4.

In the cardiac muscle, the basic unit is called a ‘sarcomere’. It is mainly composed of two protein chains: actin and myosin (see lower-middle panel). Protrusions called bridges, originating from the myosin filament and previously activated by an ATP molecule, will attach to the actin chain. These bridges will then behave like springs exerting a force that will make actin and myosin slide on each other, producing cardiac contraction. This action is repeated cyclically during a heart beat by hundreds and thousands of attachments and detachments (note: a single bridge attaches and detaches several times during a beat and consumes one ATP per attachment). This is called the ‘cross bridges cycle’. The contractile protein mechanisms, represented by a 2PC: two ‘ports’ connect to two different domains and scales, chemical and mechanical. The energy storage/conversion will be achieved with a coherent set of equations.