On the classification of motor endings in mammalian muscle spindles

Abstract

In mammalian muscle spindles, motor neuromuscular junctions (i.e. the composite structures of axonal terminals and the muscle fibres below them) are much more varied in form than those on the surrounding extrafusal muscle fibres. Figure 1 shows transverse sections of six intrafusal neuromuscular junctions, each extensively sampled by electron microscopy. In each case the surface of the muscle fibre is both indented by the motoneuronal terminals and secondarily folded by various amounts. Sole-plate nuclei are usually present. The neuromuscular junctions on the bag2 and the three chain fibres were all associated with the same axon, yet their form varies considerably. Thus, secondary folding appears to be most developed on the bag2 fibre and least on the two chain fibres (4 & 5), whose motor endings were located closest to the spindle’s primary sensory innervation. In addition, the post-junctional membrane of chain 1 is thrown into finger-like processes, but similar processes did not occur in the neuromuscular junctions on the other two chain fibres. In contrast, the two neuromuscular junctions on the bag1 fibres, though superficially similar, were associated with different types of axon, one purely fusimotor (γ) the other mixed skeletofusimotor (β).KeywordsNeuromuscular JunctionMuscle SpindleMotor EndingIntrafusal FibreChain FibreThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.