A muscle is a "contractile organ" of the body, according to Princeton.edu. Muscles make up between 40 and 50 percent of our total body weight and work to help us move, maintain posture and produce heat. Muscles respond to stimuli, contract, extend and have elasticity to return to their original shape and length after contracting or extending. Each type of muscle has different characteristics and structures.
Skeletal
Skeletal muscle is attached to the bones and moves the skeleton. It is a striated muscle; when it is viewed under a microscope there are lines or striations seen. Skeletal muscles are voluntary muscles, which means they can be moved on command. These muscles connect to the bones with tendons made up of connective tissue. Within skeletal muscle are bundles of fascicles that surround connective tissue and are made up of many muscle fibers. Thousands of these tiny, round fibers go from the origin of the muscle to the bone to the insertion, where the muscle connects to another bone.
Within the skeletal muscle type there are type I and type II fibers. Type I fibers have a lot of mitochondria, depend on cellular respiration to make energy and are resistant to fatigue. They are known as slow-twitch fibers and are largely found in muscles that require endurance, like the leg muscles and those used for keeping us upright. Type II fibers have few mitochondria, depend on phosphate and glycolysis for energy production and fatigue easily. They are fast-twitch fibers and are seen largely in muscles used for rapid movement, like moving the eyes.
Cardiac
Cardiac muscles are those that make up the wall of the heart. They contract about 70 times and pump about five liters of blood each minute. Like skeletal muscles, cardiac muscles are striated. Cardiac muscles are made up of single cells, each with a nucleus, and the cells are branched. These branches then attach and interlock with adjacent fibers using adherens junctions. This enables the heart to contract forcefully without damaging the fibers. The heart beats when the action, or contraction, passes from one fiber to another through the junctions. Fibrillation occurs when something gets in the way of the fibers communicating and passing through junctions. Cardiac muscles have more mitochondria than skeletal muscles, which means that there is more dependence on cellular respiration for energy.
Smooth
Smooth muscles do not have the lines or striations like cardiac or skeletal muscles. They are made up of single, spindle-shaped cells and have thick and thin filaments that move against each other to make the cell contract. Motor neurons can reach the muscle and make it active or relax depending on the type of message sent. Smooth muscles are also influenced by other substances released in the body near it, like an allergen that causes a sneeze or cough, or by hormones in the blood, as when the signal to start childbirth causes contractions. Smooth muscles contract more slowly than cardiac or skeletal muscles.
