In vivo studies of motor nerve re‐growth following skeletal muscle damage by lengthening contractions

Abstract

Very little is known about the pattern of changes in motor neurons that occur following skeletal muscle injury and during muscle regeneration during ageing in rodents or man. Our hypothesis is that physiologically relevant episodes of damage to muscle lead to a transient loss of innervation of the damaged muscle fibres with subsequent re‐growth of the terminal axons, interaction with the regenerating muscle fibres and formation of new neuromuscular junctions. Studies were undertaken in yellow fluorescent protein (YFP)‐H mice that only express YFP in neuronal cells. These mice provide a novel approach to assess the extent of motor neuron degeneration and regeneration. One extensor digitorum longus (EDL) muscle of adult YFP‐H mice was subjected to a protocol of 450 lengthening contractions involving a stretch to 120% of fibre length. The EDL muscle from the contralateral leg was used as control. This protocol produces an equivalent level of damage to 60–70% of the muscle fibres in muscles from adult and old mice. Maximal force generation by the EDL muscle in vivo was measured to examine the extent of functional denervation at 3, 14 and 28 days following damage. Mice were then killed, the EDL muscle was removed and the extent of loss and regrowth of peripheral motor neurons was examined by visualisation of labelled axons by fluorescence microscopy in the EDL. Supported by Research into Ageing, UK.