THE vertebrate nervous system is characterised by the existence of specific patterns of interconnections established during embryonic development. When nerve pathways are disrupted, axonal regeneration re-establishes the original patterns in some cases1-3 but not in others4-6. Little is understood about how the specific patterns develop and why they are, or are not, reproduced after injury. Where it has been studied, it seems that target cells are initially innervated by a greater number of axons than in the fully developed tissue and that the 'excessive' synaptic inputs disappear with maturation7-9. Regeneration of motor axons after damage to muscle innervation recapitulates this developmental process10, although the original pattern of specific connections may not be restored. Comparable information about reinnervation in nerve-nerve circuits is not known. For example, are more synaptic connections than normal also established during regeneration after damage to presynaptic axons? And if so, is the specificity of the connections thereby altered? We report here our studies on the parasympathetic cardiac ganglion of adult frogs which show that neurones do receive more synaptic inputs during regeneration, and that this excess is associated with an apparent failure to restore the original innervation pattern.
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