HOW A MOTOR NEURON WINS THE COMPETITION
In the adult mammals each motor neuron innervates one or more muscle fibre-cells (motor units), although during prenatal life things are quite different. The immature neuromuscular junction receives inputs from several motor neurons, but these are gradually whittled away during early post-natal life, so that each muscle fibre-cell receives the axon by a single neuron. Two new studies investigated what determines which input will escape elimination and if the competition is mediated by local factors at the neuro-muscular junction level, or by some global property of the motor neuron (Kasturi N. & Lichtman J.W. The role of neuronal identity in synaptic competition. Nature 424, 426-430, 2003; Buffelli M. et al. genetic evidence that relative synaptic efficacy biases the outcome of synaptic competition. Nature 424, 430-434, 2003).
Kasthuri and Lichtman demonstrated that as a motor neuron won the first competition could also beat the competitor at all the other neuromuscular junctions that were co-innervated by these two neurons. It would be interesting to study a competition among three neurons.
Which is the decisive factor in the competition? Buffelli and his colleagues showed that synaptic efficacy might be such a factor.
Since Kasthuri and Lichtman showed that the ranking of a motor neuron in the competitive hierarchy was inversely proportional to the size of its axonal tree, the findings of the two works might imply that each motor neuron has a finite supply of neurotransmitter, which is spread more thinly as the size of the motor neuron increases.
Further studies are needed to establish if natural variations in neurotransmission are sufficient to drive the competition or whether factors distributed in motor neurons are required.