What determines motor neuron number? Slow scaling of facial motor neuron numbers with body mass in marsupials and primates
How does the number of motor neurons in the brain correlate with the muscle mass to be controlled in the body? Numbers of motor neurons are known to be adjusted during development by cell death, but the change in the percentage of surviving motor neurons in response to experimental changes in target muscle mass is relatively small. Here we address the quantitative matching between final numbers of motor neurons in the facial nucleus and body mass (which we use as a proxy for the muscle mass). In 22 marsupial species, we found that the number of facial motor neurons is strongly correlated with body mass, and scales across species as a power function of body mass with a very small exponent of 0.184, which is close to the exponent found in primates from previously published data. With such an exponent, doubling the body mass is accompanied by a modest increase of only 14% in numbers of facial motor neurons, while halving body mass results in a decrease of only 12%. These numbers are remarkably similar to the 15–20% increase or 8% decrease in the number of spinal cord motor neurons that results from experimental or natural doubling or reducing by half the target muscle field of birds and amphibians. The scaling rule presented here might thus account for the quantitative matching of motor neurons to their target muscle mass in evolution. With this small scaling exponent, our data also raise the possibility that larger animals will have larger motor units.