Myofibril and Sarcomere Function

Myofibril and Sarcomere Function is the description of how the intracellular contractile organelles of cardiac muscle cells, the myofibrils, and their repeating structural and functional units, the sarcomeres, generate the mechanical force responsible for cardiac contraction through the coordinated interaction of actin and myosin filaments.

Myofibrils as Contractile Organelles

Structural Composition

Myofibrils are long, cylindrical bundles of protein filaments that occupy the majority of the intracellular volume of a cardiac muscle cell, arranged in parallel along the long axis of the cell so that their combined shortening produces contraction of the entire cell.

Alignment and Striation

The precise, repeating alignment of myofibrils, with their sarcomeres in register across the width of the cell, produces the characteristic striated appearance of cardiac muscle when viewed microscopically, a pattern arising directly from the regular arrangement of thick and thin filaments within each sarcomere.

The Sarcomere as the Functional Unit

Boundaries and Zones

Each sarcomere is bounded by two Z-discs, to which thin actin filaments are anchored, and contains a central zone occupied by thick myosin filaments. The region where thick and thin filaments overlap, and the central region containing only thick filaments, together define the banding pattern visible along the myofibril.

Filament Composition

Thin filaments are composed of actin monomers arranged in a double helix, along with the regulatory proteins tropomyosin and troponin, while thick filaments are composed of myosin molecules whose globular heads project outward to form cross-bridges capable of interacting with the thin filaments.

The Sliding Filament Mechanism

Cross-Bridge Cycling

Contraction occurs as myosin heads bind to specific sites on the actin filament, undergo a conformational change that pulls the thin filament toward the center of the sarcomere, release, and then reattach further along the filament, a repeating cycle powered by the hydrolysis of adenosine triphosphate.

Filament Overlap Without Filament Shortening

During contraction, the thick and thin filaments themselves do not change length; instead, they slide past one another, increasing their zone of overlap and drawing the Z-discs closer together, thereby shortening the sarcomere as a whole while individual filament lengths remain constant.

Regulation of Cross-Bridge Formation

Troponin-Tropomyosin Regulatory Complex

In the resting state, tropomyosin blocks the myosin-binding sites on actin, preventing cross-bridge formation. Binding of calcium to troponin induces a conformational shift that displaces tropomyosin, exposing the binding sites and permitting cross-bridge cycling to proceed.

Calcium as the Triggering Signal

The concentration of intracellular calcium, elevated during excitation-contraction coupling, directly determines the proportion of troponin-tropomyosin complexes in the permissive configuration, linking the electrical activation of the cardiac cell to the mechanical response of its sarcomeres.

Length-Dependent Force Generation

Sarcomere Length and Filament Overlap

The force generated by a sarcomere depends on the degree of overlap between thick and thin filaments, which in turn depends on sarcomere length; within the physiological range of cardiac muscle, increased sarcomere length, produced by increased ventricular filling, increases the number of viable cross-bridge interactions and thus the force of contraction.

Cellular Basis of the Frank-Starling Mechanism

This length-dependent relationship at the level of the individual sarcomere provides the fundamental mechanistic explanation for the whole-heart Frank-Starling mechanism, linking the microscopic behavior of myofibrils directly to the macroscopic regulation of stroke volume.

Integration into Whole-Cell Contraction

The summed, synchronized shortening of thousands of sarcomeres arranged in series and parallel within each myofibril, and of the many myofibrils within each cardiac muscle cell, produces the coordinated cellular contraction that, when propagated across the electrically coupled myocardium, generates the macroscopic pumping action of the heart.