Scaling of Brain Metabolism with a Fixed Energy Budget per Neuron: Implications for Neuronal Activity, Plasticity and Evolution
PLoS ONE 6(3): e17514. doi:10.1371/journal.pone.0017514
The human brain takes up 20% of our bodies total metabolism rating third after the skeletal/musculature system and the liver. Does metabolism put a constraint on brain size? Do larger neurons take up more energy?
Just what is meant by larger brains? Human skull size is actually getting smaller and this seems to be a selection driven trend. The author looks into this question with a bioinformational study of current literature. Neuron size is the total volume of a neuron, including arborization. The current belief is that larger brains have larger neurons but this turns out not to be the case. The true measure of brain size is the number of neurons and brain size as number of neurons scales linearly with total glucose use. Thus, the size of individual neurons doesn’t matter, every neuron regardless of size uses the a “fixed energy budget.”
Since synaptic activity is the main consumer of metabolic energy in the neuron, the number of synapses is constrained, larger, more arborized neurons would not necessarily get more synapses. Neurons using larger energy budgets would suggest that other neurons would get less. Larger neuron do have less arborization and synapses, this means that firing rates are fairly constants with different neuron types. It is suggested that human brain growth was helped along by advances in food consumption like cooking food and later by agriculture. Also, selection for efficiency in cell metabolism should occur, which does in humans.
Synapse plasticity, changes in the number of synapses at any given moment and the use of sparse coding, very few high frequency firings at any given moment could be one consequence of this constraint. This and recent confirmations of neuronal growth/electronic optimization are putting the form and function of neurons and the special needs of human intelligence into perspective.