A Parametric Study of the Neuromuscular Junction During Ontogenesis and Under Different External Conditions

Abstract

1.1. In a morphometric study on axonal and ChE stained cutaneus pectoris muscles of frog, synaptic and muscle fiber parameters were determined. 1.2. Synaptic length, synaptic complexity (=number of intraterminal branches and number of contributing axons), features of sprouting (sprouts and jumping fibers) and the abandoned gutter length as well as muscle fiber diameter were measured. 1.3. Values for parameters obtained from a single muscle scatter considerably. Coefficients of variation range from 40% (muscle fiber diameter) to 56% (number of intraterminal branches). This diversity is clearly counterbalanced by correlations between muscle fiber and synaptic parameters as well as between different synaptic parameters. Larger muscle fibers bear larger junctions and large junctions have higher numbers of intraterminal branches, contributing axons and have longer abandoned gutters. 1.4. In order to investigate growth and age related changes, muscles from 5 small frogs (body size 4.5 to 6.0 cm) were compared with muscles from 5 large frogs (body size 6.1 to 7.0 cm). In the average, there is an increase in muscle fiber diameter, synaptic length, synaptic complexity and abandoned gutter length in larger frogs. 1.5. To exclude growth-related differences and reveal changes with aging, muscle fibers of identical size were compared. Especially for large muscle fibers, synaptic lengths and abandoned gutter lengths are larger in the older (=larger) animals. 1.6. It appears, therefore, that with aging abandoned gutters on a muscle fiber accumulate. This also indicates that there is formation of new synaptic contacts rather than reoccupation of previously abandoned gutters. Alternatively, or in addition, remodelling of synaptic contacts (Wernig, Pecot-Dechavassine & Stoever, 1980a,b) could be more elaborate in large fibers of older animals. 1.7. In animals kept in the laboratory under certain conditions for 16 weeks there is an increase — as compared to freshly caught frogs — in the number of intraterminal branches and in the features of sprouting while abandoned gutters seem to be reduced. 1.8. This supports previous observations that remodelling, i.e., the extent of nerve sprouting and nerve retraction, is influenced by external factors.