Recent electron microscopic studies have demonstrated the presence of two distinct types of muscle fiber, Fibrillenstruktur and Felderstruktur,1 in the extraocular muscles of guinea pig,2 cat,3 and man.4 In the Fibrillenstruktur fibers, the myofibrils have a punctate appearance, and the sarcoplasmic reticulum surrounds clearly the individual fibrils. The Felderstruktur fibers, on the other hand, have larger myofibrils which are not regularly separated from each other and appear to be fused together. Physiologically, the Fibrillenstruktur fibers appear to be associated with relatively rapid twitch responses, whereas the Felderstruktur fibers respond to stimulation with prolonged contracture.3There are, in addition, two kinds of nerve endings in the extraocular muscles which can be histochemically differentiated by cholinesterase staining; these are the "en plaque" and "en grappe" endings.2-7 The "en plaque" endings stain intensely and show the same configuration as the subneural apparatus of motor end-plates in
CHENG K, BREININ GM. Fine Structure of Nerve Endings in Extraocular Muscle. Arch Ophthalmol. 1965;74(6):822–834. doi:10.1001/archopht.1965.00970040824019
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