The cross-bridges pull on the actin filaments, causing them to slide between the myosin filaments. As the actin filaments slide, they pull the ends of the muscle toward its middle, making the muscle fibers shorten. ATP provides the energy to slide actin filaments. It is produced when oxygen in muscle fibers combines with chemicals from food. How muscles are stimulated.
Muscle cells are excitable because the membrane of each cell is electrically charged.
Thus, a muscle cell is said to have electric potential. Potassium ions easily move through the membrane and accumulate in the cell. Sodium ions do not enter the cell as easily. In addition, the membrane has a special mechanism that pumps potassium into the cell and pumps sodium out. Consequently, the cell normally contains much potassium but little sodium.