In ’09 Lu et al published their use of confocal microscopy to reconstruct the YFP-labeled axon branching of neurons in the interscutularis, a small muscle of the mouse. These connectomes are small, only 14.5 ± 1.5 axons per mouse, but since there are few recurrent connections from other areas, it is feasible to study all its connections.
One of their interesting comparisons is between the left and right counterparts of the same neuron in one animal:
As you can see there is a tremendous amount of variation between each of the axon traces in the left and right components of the same neuron in the same animal. This contrasts to invertebrates like nematodes whose connectomes are much more predictable.
They also analyzed the optimality of the axon connections, and found surprising amounts of sub-optimality, such as this example:
The sub-optimality is a clear prediction of the heterogeneity, as without a defined genetic map, stochastic effects will end up producing less efficient networks. The authors emphasize the difference between this mammalian model and the current models of invertebrate connectomes, which are more so although still not fully optimal.
What’s less clear are the benefits from the stochasticity and lack of genetic determination. Presumably this is why the nematode is able to accomplish so many behavior with so few neurons (~ 300), whereas the mouse has ~ 3 million in its cortex alone. Clearly there is a large metabolic cost to using so many more neurons. Perhaps what vertebrates gain from their huge increase in number of neurons is more behavioral variability, which among other advantages, makes their actions less predictable to competitors.
Lu J, Tapia JC, White OL, Lichtman JW (2009) The Interscutularis Muscle Connectome. PLoS Biol 7(2): e1000032. doi:10.1371/journal.pbio.1000032