Diversity and homogeneity within endplates associated with physiologically identified static gamma-axons in cat tenuissimus muscle.

Abstract

The terminations of static gamma-axons on chain fibres may be associated with a muscle surface contour thrown into complex folds at one extreme, ranging through lesser degrees of folding to being apparently uninfluenced by the presence of the terminals. The folding is not necessarily confined to the postsynaptic membrane. As a quantitative indicator of the degree of folding seen in cross-sections, the perimeters of the two halves of the intrafusal fibre with, and without the ending were compared. Both complexity of endplate structure and an undifferentiated appearance could co-exist within individual endings (of six static gamma-axons on thirteen chain fibres), associated with axons that supplied only chain fibres in the spindle isolated. (It should be noted, however, that the type of intrafusal fibres innervated by a particular axon was definitively identified only for the spindle isolated.) Endings of three other static gamma-axons on six chain fibres had a more homogeneous and less complex endplate structure; these supplied bag2 fibres in addition to the chain fibres and their endplates on the bag2 fibres were less folded than those of three axons which supplied bag2 fibres only. A sensory inhibitory influence on folding was not apparent because complexly folded endplates on chain fibres lay close (less than 500 microns) to sensory endings. All the endings studied were functional. In our experimental conditions all nine static gamma-axons which innervated chain fibres, alone, or together with a bag2 fibre, drove the Ia afferent at some stimulus frequencies; none of the three static gamma-axons innervating bag2 fibres alone caused driving at any frequency. These findings are discussed in relation to the concept of a dynamic remodelling of ending structure during life, to the relationship between the motor axon and the intrafusal fibre it innervates, and to the possibility that subgroups of static gamma-motoneurones might exist which could release different amounts of the trophic substances responsible for moulding the endplate structure.