ASSESSING ULTRASTRUCTURE OF CRUSTACEAN AND INSECT NEUROMUSCULAR-JUNCTIONS

Motor nerve terminals of arthropods provide excellent models for study of synaptic transmission, and their ultrastructure can be investigate d in the same endings from which physiological recordings have been ob tained. An experimental procedure for marking a recording site for sub sequent ultrastructural analysis is described. The most commonly used procedure for ultrastructural analysis has been serial sectioning and three-dimensional reconstruction. This procedure has the advantage of providing information about the entire nerve terminal, including quant itative information on number, sizes, and relative positions of indivi dual synapses and presynaptic 'active zones'. However, several errors may be generated in the process of viewing the sections and making the reconstruction; these errors can in principle lead to overestimation of synapse and active zone size. The errors become relatively more ser ious for smaller structures. Procedures for alleviating some of the po ssible errors are outlined. It is desirable to have additional informa tion from other methods, such as freeze-fracture replication, to guide analysis of reconstructions from serial sections. Combined physiologi cal and ultrastructural analysis of arthropod terminals has shown that each terminal has many small synapses, differing in size and in numbe r of active zones, and that in some terminals, many of the observed sy napses have a very low probability of transmission when nerve impulses occur at low frequencies.