Structure of the Extraocular Muscles
Extraocular muscles differ histologically from most other skeletal muscles in that they are made up of 2 different types of muscle cells. Each muscle cell is composed of groups of myofibrils called sarcomeres. Fibrillenstruktur (or fast-twitch) muscle fibrils generate fast eye movements and are composed of well-defined myofibrils with well-developed sarcomeres. Felderstruktur muscle fibrils generate slow or tonic eye movements and are composed of poorly defined myofibrils with poorly developed sarcomeres.
Cholinergic motor neurons supply both types of muscle fibers. The innervation to fibrillenstruktur fibrils is thick and heavily myelinated, with a single (en plaque) neuromuscular junction, whereas the innervation to felderstruktur fibrils is thin, with multiple grapelike clusters of neuromuscular junctions.