Efferent Innervation of Mammalian Muscle-spindles

Abstract

MAMMALIAN intrafusal muscle fibres are innervated by the small γ-efferent fibres of Leksell¹, which have nerve-trunk diameters of 2–8µ. Fibre-size histograms of the intramuscular diameters of both intra- and extra-fusal efferents in fresh and gold chloride preparations show that, while both share the same peak at 3.5µ, about 30 per cent of the intrafusal efferents lie outside the extrafusal range at 4–6µ (ref. 2). These larger fibres have been reported in various descriptions of spindle innervation^3–6; in cat rectus femoris, they take part in the innervation of about one-third of the spindles present (Adal and Barker, unpublished). They could be members of either the 2–8&µ γ-group, or the 8–18µ α-group, which have not undergone such extensive branching as the majority of their fellows. The work of Granit, Pompeiano, and Waltman⁷ on the α-activation of cat spindles directs attention to the possibility that some members of the α-group may branch so as to innervate both intra- and extra-fusal muscle fibres. Such a claim has been made by Weiss and Dutil⁸, Cilimbaris⁴, Wilkinson⁹, Denny-Brown¹⁰, and, more recently, Häggqvist¹¹. Wilkinson (echidna spindles) and Häggqvist (monkey spindles) have each published a photograph purporting to show such innervation, but they are unconvincing and open to alternative interpretation. We have examined fifty gold chloride preparations of echidna (Tachyglossus aculeatus) spindles without finding instances of collaterals from α-fibres innervating intrafusal muscle fibres, and examination of more than two thousand gold chloride cat spindles, accumulated during work on various aspects of this receptor, has also failed to yield any instance of such innervation. Examination of bat, monkey, opossum, rabbit, and rat spindles has also proved negative in this respect. When apparent examples of such branching have been encountered, closer examination and further tracing has always shown the presence of two discrete fibres; in such instances, the two fibres are usually superimposed one above the other during their course before diverging, so as to give the impression of a single fibre branching into two. Similar instances occur with afferent fibres, and have led to claims^8,12–14 of a single fibre branching so as to terminate in a spindle primary ending and a nearby tendon-organ. If branches of α-efferent fibres do innervate the mammalian spindle, we are of the opinion that the collaterals must originate some distance away from the receptor, either within the muscle nerve or within intramuscular nerve trunks. However, experimental evidence for the innervation of the spindle by a-efferents is conflicting¹⁵, and it seems, on balance, more probable that in mammalian muscle there are discrete intrafusal (γ) and extrafusal (α) efferent systems.