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.