Chapter 4: Muscle
Muscle is one of the four primary tissue types. There are three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle. The three muscle types have some properties in common. They all exhibit excitability as their plasma membranes change their electrical states (depolarization) and send an electrical signal in a wave-like pattern called an action potential along the entire length of the skeletal muscle cell membrane. Skeletal muscle depends on the nervous system to work properly. This is referred to as voluntary motion. In contrast, the nervous system influences the activity of cardiac and smooth muscle to a lesser degree. On the other hand, both cardiac muscle and smooth muscle can respond to other stimuli, such as catecholamines (epinephrine and norepinephrine) from the blood, hormones and local stimuli.
Myoepithelium and myofibroblasts are specialized types of epithelium and fibroblasts, respectively, that have contractile ability. Myoepithelial cells are often located in and around glands where extracellular product produced by the epithelial cells needs to be moved by contractile action towards the duct system. Myofibroblasts are often seen in healing wounds and assist in maturation and contraction of the granulation tissue (immature fibrous connective tissue) as it matures and contracts.