Structure and distribution of fibre types in the external eye muscles of the rat.

Rat extraocular muscles are composed of two layers differing in muscle fibre diameter. In both layers multiply innervated fibres are found besides focally innervated ones. Significant differences in internal structure thus demand further subtyping. The global layer (predominantly larger fibres) contains 10" 'clear' fibres (multiple innervation, medium size) and a spectrum of focally innervated fibres, from small 'dark' fibres (30%, abundant mitochondria) over 'intermediate' fibres (30%) to large 'pale' fibres (30 %, few mitochondria). The orbital layer (exclusively small fibres) contains 80% focally innervated 'dark' fibres and 20% extremely small 'clear' fibres with multiple innervation. An ultrastructural characterization of the fibre types is given and possible functional implications are discussed.