Energy supply per neuron is constrained by capillary density in the mouse brain

Neuronal densities vary enormously across sites within a brain. Does the density of the capillary bed accompany the presumably larger energy requirement of sites with more neurons, or with larger neurons, or is energy supply constrained by a mostly homogeneous capillary bed? Here we find evidence for the latter across various sites in the mouse brain, such that the ratio of capillary cells per neuron, and thus presumably blood and energy supply per neuron, decreases uniformly with increasing neuronal density across sites. These findings suggest that it is not that larger neurons demand more energy, but simply that larger (and thus fewer) neurons have more energy available per cell than smaller (and thus more numerous) neurons due to competition for limited resources supplied by capillaries.