Opinion – Suzana Herculano-Houzel: The problem of having too many neurons for too little energy

For two weeks now I’ve been talking about my most recent discovery: the double restriction of the brain’s energy economy by the constant flow of blood through the internal carotid artery, and by the local distribution, limited by the density of capillaries in the brain.

If the internal carotid is the access road through which all the cars pass in constant flow through the city of the brain, the capillaries are the vehicles, exclusive distributors of all the food and water to each of the houses on the edge of all the streets.

Our paradigm of a limited brain energy economy, developed with my Yale University collaborator Douglas Rothman, explains several open questions in neuroscience, from why it is not possible to pay attention to two things at once to why even small “strokes ” in the brain are so problematic.

Now, why do some parts of the brain seem to be so much more susceptible to these small strokes than others? Why, for example, are short-term memory loss and loss of balance and coordination so common in cerebral ischemia, when certain streets or avenues in the brain are temporarily blocked?

Because the number of houses served by the streets of the brain varies enormously, according to the second study I have just published in the same issue of the journal Frontiers in Integrative Neuroscience, and where there are more houses vying for the same resources brought by a limited number of cars, the risk neighborhood to suffer the consequences of a lockdown is especially great.

The study, conducted by Brazilian postdoctoral fellow Lissa Ventura-Antunes in my laboratory, demonstrated that, in the rat brain, small local variations in capillary density correspond to proportional variations in blood flow and energy consumption in the resting brain. In other words, the density of streets actually predicts the rate of resource allocation to the brain’s neighborhoods.

This density of streets —the capillaries— is not proportional to the density of houses —the neurons— in each neighborhood. It’s an arrangement that makes very little teleological sense: whoever planned such a city should be fired. The reason is that in dense neighborhoods, with more houses necessarily smaller between streets of the same density, each house receives fewer resources. Therefore, in neighborhoods with more houses per street, each house is more vulnerable to possible shortages, and the risk of neighborhood collapse is greater.

Which neighborhoods have we found to have more houses per street, or neurons per capillary? The hippocampus and cerebellum cortex, precisely two preferred targets of cerebral ischemia.

I would love to be able to complain to management and demand bigger streets in my brain, proportionate to the number of houses in each neighborhood, to end this particular vulnerability of the hippocampus and cerebellum. Since I can’t, I’ll do what I can: religiously attend my pilates classes to keep the precious streets of my brain in good shape.