Synaptic differentiation in a regenerating crab-limb muscle.

Properties of synapses on regenerating nerve terminals of the single excitatory axon to the stretcher muscle were studied in the regenerating second walking leg of the shore crab, Grapsus. In the adult condition these synapses vary in physiological properties, ranging from high release, poorly facilitating types to low release, highly facilitating types. Synapses on regenerating stretcher-muscle fibers show a distinct temporal pattern of differentiation. In early limb buds, a characteristic fluctuating, excitatory postsynaptic potential, punctuated by failures of transmission, is seen, indicating a developmentally "naive" synapse with low quantal content. In these early stages proportionately more synapses are of the poorly facilitating type; the highly facilitating synapses appear increasingly in later stages. Thus, the type of synapse that will form seems likely to be related to the time of innervation. Synapses of early developmental stages found by electron microscopy are significantly smaller than those seen in adult muscles; thus, the synaptic contact area must increase during development. We postulate that contacts formed by the primary branches of the axon early in development differentiate into relatively large, poorly facilitating synapses, while contacts formed by secondary branches slightly later in development differentiate into smaller, highly facilitating synapses.