Bundles of individual muscle cells are known as fascicles. They are covered by a connective tissue sheath known as the perimysium. Fascicles bundle together to form large muscle groups.
Each muscle fascicle is comprised of several individual muscle cells known as myocytes. These individual muscle cells are also called myofibrils or muscle fibers. The myocytes are connected by connective tissue coverings to form fascicles. When the nervous system innervates the connective tissue coverings of the myocytes and the fascicles, a smooth contraction or relaxation occurs. This is due to the electrical excitability of the perimysium and the connective tissue sheaths of the myocytes.