Used to maintain posture since they can sustain long contractions. Fast fibers have short duration twitches and are used in fast muscles for rapid, powerful contractions. Some are able to sustain long periods of repeated contractions without fatiguing. Other Types of Muscles Vertebrate cardiac muscle is found only in the heart. Muscle cells are branched, and the junction between cells contain intercalated discs that electrically couple all heart muscle cells, allowing coordinated action.
Cells can also generate their own action potentials.
Rhythmic depolarizations due to pacemaker channels in the plasma membrane trigger action potentials which last up to 20 times longer than those for skeletal muscle and have long refractory periods. Generates less tension but can contract over a greater range of lengths. Contractions are relatively slow but there is greater range of control. Found mainly in the walls of blood vessels and digestive tract organs. Invertebrates have muscles similar to the skeletal and smooth muscles of vertebrates, but with some interesting adaptations.