In details

Muscle contraction


The vast majority of animals have nerve and muscle cells. In all of them, the mechanism of operation of these cells is exactly the same.

The internal coordination of the organism and its relationship with the environment are functions performed by the nervous system. The sensory system is responsible for receiving messages from the environment. The answer, often represented by movement, is the role of muscle tissue.

Myofilaments: actin and myosin

The entire muscle cell contains contractile protein filaments of two types: actin and myosin. These myofilaments (or myofibrils) are differentiated from each other by molecular weight, higher in the myosin filament.

Under the electron microscope, actin appears as thin filaments, while the Myosin is represented by thick filaments. The interaction of actin with myosin is the major triggering event of muscle contraction.

The regular arrangement of these proteins along the fiber produces the pattern of alternating light and dark streaks typical of striated muscle.

The actin and myosin repeating units along the myofibril are called sarcomeres. The outermost bands of the sarcomeres, clear, are called band I and contain only actin filaments. The darker center strip is called band A. The ends of band A are formed by overlapping actin and myosin filaments, while their lighter median region, called band H, contains only myosin.

At Z lines constitute the point where actin filaments originate. The myosin filaments are interspersed with those of actin. Note that on both sides of the myosin filaments there is a space. This is the conformation when the muscle cell is relaxed. In contraction, the sarcomere shortens and the myosin molecules “touch” the Z lines. In this case, the typical striation changes momentarily. Returning to the state of relaxation.