Regeneration by supernumerary axons with synaptic terminals in spinal motoneurons of cats

Axons in the central nervous system (CNS) of mammals do not normally regrow if they are cut, which severely limits restoration of function after injury. We have studied the reactions of adult cat spinal α-motoneurons after chronic transection of their axons inthe periphery by labelling single cells with horseradish peroxidase. Twelve weeks after the operation, about a third of the axotomized cells had developed a 'supernumerary' axon originating from the cell-body region. These supernumerary axons had variable trajectoriesand termination fields in the ipsilateral spinal cord but generally anomalous projections. Ultrastructural examination shows that they give rise to boutons that form morphologically normal synaptic contacts with neuronal profiles, although they contain dense-coredvesicles not normally seen^1,2 in central terminals of α-motor axons. We conclude that axotomized neurons in the mammalian CNS may be able to form new synaptic contacts by means of supernumerary axons in the absence of local damage.