The ultrastructure of cat fusimotor endings and their relationship to foci of sarcomere convergence in intrafusal fibres
Authors: Arbuthnott E. R. , Ballard K. J. , Boyd I. A. , Gladden M. H. , Sutherland F. I. 
Published: December 2014

1. Six muscle spindle poles, five from experiments in which foci of sarcomere convergence had been observed during stimulation of fusimotor axons, were serially sectioned for light and electron microscopy. Every somatic motor terminal was studied in ultrathin sections at several levels.

2. In all six poles static γ axons, or presumed static γ axons, supplying the static bag2 fibre and/or chain fibres had no terminations on the dynamic bag1 fibre. In five poles, the dynamic bag1 fibre was selectively innervated by dynamic γ or β axons save in one case where a dynamic γ axon also innervated one chain fibre.

3. Seventy‐seven motor endings were of four distinct ultrastructural types: ‘ma plates’ lay superficially on the surface of static bag2 or chain fibres; ‘mb plates’ were deeply indented into dynamic bag1 fibres; in ‘mc plates’, found on chain fibres only, the muscle surface was thrown into projecting fingers between which the axon terminals were embedded; one type ‘md plate’ was found, fully indented into a long chain fibre. A few plates of intermediate form (mab) were variants of ma and mb plates.

4. The muscle membrane beneath both ma and mb plates was smooth, or had a few wide, shallow folds; mc plates usually had wide, shallow subjunctional folds; numerous deep, narrow folds were characteristic of the md plate. The length of unmyelinated pre‐terminal axon or the number of sole plate nuclei were not useful diagnostic features.

5. Obvious foci of sarcomere convergence in the capsular sleeve region of dynamic bag1 and static bag2 fibres coincided with the location of motor plates. Additional contraction foci were observed in the extracapsular region of dynamic bag1 fibres where there was no motor innervation; contraction occurs principally in the outer half of these fibres. No foci of contraction or motor plates were observed in the extracapsular region of static bag2 fibres; contraction in these fibres is typically mid‐polar.

6. In some poles local contraction of chain fibres centred on the location of mc plates. In others, very localized contraction occurred distal to the sites of ma plates. Both ma and mc plates were never found on the same pole of a chain fibre.

7. Dynamic γ or β axons end in mb plates, probably equivalent to p2 plates. The concept of distinctly different p1 and p2 plates on dynamic bag1 fibres, supplied by dynamic β and γ axons, respectively, is not supported by ultrastructural evidence.

8. Some static γ axons end in multiple ma plates which correspond with ‘trail endings’, or in single large ma plates, on static bag2 or chain fibres. The mc plates are the terminations of other static γ, or occasionally dynamic γ, axons on chain fibres. Static β axons probably end in md plates on long chain fibres which may correspond with p1 plates.

9. It is proposed that there are two types of static γ motoneurone, one terminating in ma plates and the other in mc plates, possibly directed preferentially towards static bag2 fibres and chain fibres, respectively.