Muscle and nervous tissues are considered excitable tissues. They are capable of transmitting electrical signals across their cell membranes by altering the membrane potential, the charge difference between the inside and outside of the cell. This electrical impulse is called an action potential and occurs when the inside of the cell becomes more positive and then negative by moving ions across the cell membrane. In muscle tissue, the electrical impulse sets off a series of events that leads to force generation within muscle cells. In nervous tissue, the electrical impulse can travel long distances and is used as a form of communication.
Muscle tissue is an excitable tissue that generates force leading to voluntary and involuntary movements of the body. Muscle tissue is divided into three types based on location and function: skeletal, cardiac, and smooth. The three types have cellular differences that are important for carrying out their distinct functions. Skeletal muscle is attached to bones and skin. The contraction of skeletal muscle allows the body to stand upright, walk, run, and perform other voluntary movements. Skeletal muscle fibers are multinucleated and rod-shaped with striations. These cells have multiple nuclei because cells fuse together during skeletal muscle development. When viewed microscopically, stripes called striations are visible. Striations result from the alignment of sarcomeres, the structural units of muscle fibers that facilitate contraction.
Cardiac muscle is a striated muscle type located in the walls of the heart. Contraction of cardiac muscle is under involuntary control. Cardiac cells (cardiomyocytes) are uninucleate and branched. An intercalated disc connects cardiac muscle cells and supports cardiac muscle contraction by allowing the electrical impulse to travel more rapidly.Smooth muscle is located in the walls of internal organs and blood vessels. It is responsible for involuntary movements including those of the digestive, urinary, and reproductive systems. Smooth muscle cells are narrow and spindle-shaped with a single central nucleus and no striations. Smooth muscles contain the same contractile proteins as skeletal and smooth muscle fibers, but they are not aligned to appear striated.