As more motor neurons are recruited by the brain, tension in the muscle progressively increases. Duration of the muscle contraction depends on how long the concentration of calcium remains elevated. Slow fibers have longer lasting twitches because they have less sarcoplasmic reticulum. The calcium remains in the cytoplasm longer. They have many mitochondria, rich blood supply, and myoglobin.
Fast fibers have short duration twitches and found in fast muscles for rapid, powerful contractions.
It is striated. Muscles cells are branched, and the junctions between the cells contain intercalated discs that electrically connects all heart muscle cells, allowing coordinated action. Cells can also generate their own action potential. Contractions are slow but there is a greater range of control. Calcium must enter the cell through the plasma membrane during an action potential.