Editing Neuropil (section)

===Efficiency in the brain===
Neurons are necessary for all connections made in the brain, and thus can be thought of as the "wires" of the brain. As in computing, an entity is most efficient when its wires are optimized; therefore, a brain which has undergone millions of years of natural selection would be expected to have optimized neural circuitry. To have an optimized neural system it must balance four variables—it must "minimize conduction delays in axons, passive cable attenuation in dendrites, and the length of 'wire' used to construct circuits" as well as "maximize the density of synapses",<ref name="Dmitri">{{cite journal|first1=Dmitri B.|last1=Chklovskii|first2=Thomas|last2=Schikorski|first3=Charles F.|last3=Stevens|title=Wiring Optimization in Cortical Circuits|journal=Neuron|date=25 April 2002|volume=34|issue=3|pages=341–347<!--|access-date=20 April 2012-->|doi=10.1016/s0896-6273(02)00679-7|pmid=11988166|s2cid=1501543|doi-access=free}}</ref> essentially optimizing the neuropil. Researchers at Cold Spring Harbor Laboratory formulated the optimal balance of the four variables and calculated the optimal ratio of axon plus dendrite volume (i.e. the "wire" volume or neuropil volume) to total volume of grey matter. The formula predicted an optimal brain with 3/5 (60%) of its volume occupied by neuropil. Experimental evidence taken from three mouse brains agrees with this result. The "fraction of wire is 0.59 ± 0.036 for layer IV of visual cortex, 0.62 ± 0.055 for layer Ib of piriform cortex, and 0.54 ± 0.035 for the stratum radiatum of hippocampal field CA1. The overall average is 0.585 ± 0.043; these values are not statistically different from the optimal 3/5."<ref name="Dmitri" />