The structure and innervation of sheep extraocular muscles

Abstract

The structure and innervation of the extrafusal muscle fibres and the muscle spindles in sheep superior rectus, levator palpebrae superioris, and peroneus brevis muscles are compared. Superior rectus is organized into three layers: A central core of mainly large-diameter fibres contains three plate-innervated types of 'twitch' fibre, with 7% large-diameter grape-innervated fibres; an orbital rim of small-diameter fibres contains mainly plate-innervated ‘twitch’ fibres, together with a third of small diameter grape-innervated fibres; and a thin peripheral patch at either end of the muscle is composed mainly of intermediate- diameter grape-innervated fibres. Levator palpebrae and peroneus brevis are not layered and they are composed of three plate-innervated types of 'twitch' fibre, the 'slowest' of which are not represented in superior rectus. The functional significance of the histochemical and ultrastructural characteristics of the fibre types is discussed. In superior rectus, about 180 spindles are peripherally distributed in the orbital rim and peripheral patch layers, throughout the length of the muscle. The ultrastructure, histochemical profile, and innervation of the nuclear-bag and nuclear-chain fibres are the same as the extrafusal peripheral grape-innervated (G) and plate- innervated (G) fibre types, respectively. Motor innervation is collateral and segregated with grape innervation distributed to the bag fibres, and plate innervation to the chains. Levator palpebrae has about 60 evenly-distributed spindles. The bag fibres correspond histochemically and ultrastructurally to the bag fibres of superior rectus, and receive a purely fusimotor grape innervation. The chain fibres correspond to those of superior rectus, and receive a similar plate innervation. Sheep extraocular spindles receive both a primary and secondary sensory innervation. In peroneus brevis the spindles are composed of typical bag, intermediate bag and nuclear-chain fibres, and these receive an innervation typical of mammalian hindlimb muscle spindles.