Vertebrate animals (those with backbones) have three different types of muscles. The first two include the heart muscle, which is also referred to as cardiac muscle, and smooth muscle. Smooth muscle is the muscle that is in the walls of hollow organs, except the heart. The contraction of smooth muscle is usually involuntary. Smooth muscle allows for contraction and helps regulate blood flow and move food through the gastrointestinal tract. Smooth muscle is also used in urination. The third type is skeletal muscle, which is under voluntary control and is attached to the skeleton.
Skeletal Muscle Fiber
Skeletal muscle fibers are cylindrical. There are thousands of these fibers in a muscle. Connective tissue binds the muscle fibers together. Within the connective tissue, there are blood vessels and nerves.
Fiber Contents
Muscle fiber is derived from the fusion of cells and therefore has many nuclei. A muscle cell is called a myofiber and can reach a few centimeters in length. Myofibers are densely packed with energy. They are the smallest unit of contraction within a muscle and require biological systems for metabolism and contraction signaling. Within the myofiber are myofibrils, which contain the proteins that perform the actual production of force within the muscle. The myofibrils give the muscle fiber an appearance of parallel lines, which is why skeletal muscle fiber is also called striated voluntary muscles.
Skeletal Muscle Activation
Skeletal muscles are under voluntary control. The nervous system is responsible for skeletal muscle contraction and is connected to skeletal muscle at junctions that are types of synapses. Although the autonomic branch of the nervous system is connected to heart and smooth muscle, the effect is moderating strength of contraction or rate of contraction or both.
Heart Muscle
Heart muscle also has myofibrils and is also called striated muscle, but they are striated involuntary muscles. The cells in heart muscle have nuclei that are located in the center of the cells. The myofibrils of heart muscle, unlike skeletal muscle, branch out. The myofibril branches interlock with neighboring myofibrils, creating strong junctures. This allows for forceful heart contractions while keeping the interlocking branches secure.
Smooth Muscle
Smooth muscle is made up of single cells that have spindle shapes. The name of smooth muscle comes from the fact that there are no striations visible in them. The cells in smooth muscle have thin and thick filaments that slide against each other, producing contractions of the cell.
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