Evidence for Transdifferentiation of Alpha Motoneuron Terminals during Reinnervation of Muscle Spindles

Abstract

It is quite clear that during synaptogenesis, the transformation of growing axons into specialized nerve terminals is largely regulated by their synaptic targets. This is especially true for vertebrate neuromuscular junctions where evidence points to local control of peripheral axon terminal differentiation by the target’s basal lamina. Such dynamic influence by the target apparently exists in mature adult tissue as well. For example, the results earlier experiments on cat muscle indicate that sometimes single regenerating alpha motoneurons can reinnervate two different types of intrafusal muscle fibers simultaneously (slow twitch and fast twitch) with focal plates and ‘en grappe’ terminals occurring on different branches of the same motoneuron, simultaneously. This led to the converse series of experiments (using rats) in which motoneurons were left intact while their targets were made to degenerate and subsequently allowed to regenerate around the axons. By transplanting a very different target (slow tonic extraocular eye muscle) into the normal regenerating leg muscle target, the intact neurons and their terminals have the option to innervate this very different target as well and my observations indicate that they do. The results also suggest that some existing plate terminals are modified to multiple ‘en grappe’ terminals on the transplanted target, thus providing evidence for the continuing dynamic influence of targets on adult neuromuscular junctions. However, the stability of the new junctions over time is unknown.