Mammalian skeletal muscles contain several types of muscle fibres, each characterised by its contraction speed and molecular properties. Individual motor axons innervate a few dozen muscle fibres, usually all of the same type. How this striking `motor unit homogeneity' is established is incompletely understood but, on , Joshua Sanes and colleagues reveal that, in mice, signals from the muscle fibres influence the molecular properties of motoneurons that innervate them. The lack of markers for motoneuron types has impeded the study of motor unit homogeneity. Here, however, the researchers show that the motoneurons that innervate slow muscle fibres selectively express the synaptic vesicle protein SV2A and carry it to their nerve terminals. Notably, overexpression of the transcriptional co-regulator PGC1α in muscle fibres, which converts them to a slow phenotype, increases the number of SV2A-positive motoneurons. The researchers propose, therefore, that retrograde signals from muscles integrate with previously described anterograde influences of the nerve on the muscle fibre to match the properties of these synaptic partners to each other.
