Maturation of fast and slow motor units during synapse elimination in the rabbit soleus muscle

In adult mammalian skeletal muscles, fast and slow muscle fibers are selectively innervated by single inputs from corresponding motor neuron types, giving rise to fast and slow motor units. At birth, however, muscle fibers are polyinnervated, and connections between motor neurons and muscle fibers are not as specific as those found in adults. Excess synapses are removed during the first few postnatal weeks. In addition to changes in the degree of polyinnervation, motor unit types undergo maturation in their contractile properties. In this study, we have investigated the maturation of motor unit types during postnatal synapse elimination in the rabbit soleus muscle. The ratio of twitch tension to tetanic tension in a motor unit is an indication of its contractile type. Our results indicate that during synapse elimination, the twitch/tetanus ratios for fast motor units increase while those for slow motor units decrease. The ratio of motor unit tension at polyinnervated ages to that at singly innervated ages has previously been used to estimate the degree of polyinnervation for fast versus slow muscle fibers. We found that twitch and tetanic tension yield conflicting estimates of polyinnervation. This discrepancy was resolved on the basis of intracellular recordings of endplate potentials. Using latencies to endplate potentials as an indicator of muscle fiber type, we found that fast and slow muscle fibers are polyinnervated to a similar extent during both early and intermediate stages of synapse elimination, suggesting that specific tension, and not polyinnervation, changes differently in fast versus slow muscle fibers. These changes are consistent with those we found in twitch/tetanus ratios. Furthermore, these intracellular recordings suggest a high degree of specificity at birth, which is further refined during synapse elimination.