The demonstration of different types of muscle fibers in human extraocular muscle by electron microscopy and cholinesterase staining

Surgical specimens of normal human extraocular muscle repealed in electron microscopic cross sections two distinct types of extrafusal fibers: (1) a fibrillar type (Fibrillenstniktur) and (2) an essentially afibrillar type (Felderstruktur). Fascicles of autopsy muscle, stained with a modified Koelle technique to demonstrate cholinesterase activity, showed by light microscopy two kinds of nerve endings: (1) large, heavily staining, compact discs (typical motor end. plates or "en plaque" endings), which usually occurred singly within the distance teased, and. (2) smaller, lighter staining droplets in clusters or chains ("en grappe" endings), tuhich occurred multiply on a single fiber. The two nerve terminal types were never seen together on the same muscle fiber. Differential staining, with selected substrates and an inhibitor (DFP), showed, the presence of both acetyl and butyryl cholinesterases in each ending. Electron microscopy of muscle stained for cholinesterase correlated the fibrillar ultrastructure with the fibers possessing "en plaque" endings and the afibrillar ultrastructure with the fibers possessing "en grappe" endings. These morphologic features strongly suggest that human extrinsic eye musculature is organized into two separate contractile systems similar, if not identical, to the fast and slow striated muscle systems conclusively established in the frog. A brief discussion of the oculomotor implications is included.