Rat extraocular muscle. 1. Three dimensional cytoarchitecture, component fibre populations and innervation

Rat superior oblique extraocular muscle was examined in serial sections by light and electron microscopy. By such analysis, it was possible to discriminate single versus multiply innervated fibres, characteristics of the internal structure of fibres, and topographical distribution of the respective fibre populations within the muscle. The superior oblique muscle was observed to consist of two distinguishable muscle portions; one portion originated proximally and the other muscle portion originated distally after passing through the trochlea tendon. Seven morphologically distinct fibre populations were discernible. Four populations, three singly innervated fibre populations and one multiply innervated fibre population, were localised in the global region; these differed in terms of diameter, mitochondrial richness, development of the sarcoplasmic reticulum and myofibrillar size. The orbital surface layer contained two singly innervated and one multiply innervated populations; all of these exhibited systematic morphological variability. The multiply innervated fibres displayed multiple, relatively superficial, neuromuscular junctions along their length, as well as end plate-like junctions which were apparently confined to the region of the singly innervated fibre innervation band, thus suggesting polyneuronal innervation. In the latter region, the multiply innervated fibres displayed morphological features characteristic of "twitch' fibres; away from this region these cells appeared to have morphological features typical of "tonic' fibres. In contrast, the global multiply innervated fibres were constant diameter and form along their length, and displayed only relatively superficial neuromuscular junctions.