Saturday, November 19, 2016

86 Billion Neurons

Energy requirements and cooking our food:
Our 86 billion neurons need so much energy that if we shared a way of life with other primates we couldn’t possibly survive: there would be insufficient hours in the day to feed our hungry brain. It needs 500 calories a day to function, which is 25 percent of what our entire body requires. That sounds like a lot, but a single cupful of glucose can fuel the brain for an entire day, with just over a teaspoon being required per hour. Nevertheless, the brains of almost all other vertebrates are responsible for a mere 10 percent of their overall metabolic needs. We evolved and learned a clever trick in our evolutionary past in order to find the time to feed our neuron-packed brains: we began to cook our food. By so doing, more energy could be extracted from the same quantity of plant stuffs or meat than from eating them raw.
Brain soup:
Brain soup was the method Herculano-Houzel devised to deal with the problem of a brain’s heterogeneity. Her procedure was to dissolve a brain of whatever species, with its millions or billions of cell membranes, in detergent to create a homogeneous distribution of free-floating cell nuclei. She could then sample the suspension, use a blue dye to stain the nuclei, count them up, and confidently extrapolate to the number of cells in the entirety of the brain, or whatever part of the brain she had begun with.

Those cells would be of three types—neurons, glial cells, and endothelial cells. Glial cells are crucial to the synaptic transmission of information across neurons, while endothelial cells form the walls of the capillaries that take oxygen and nutrients to the brain via the blood. Fortunately, the neurons could be distinguished by tagging them with a red-colored neuron-specific antibody, one that attaches to the NeuN protein within the cell nuclei. By counting the number that turned from blue to red once the antibody was added to the suspension, she could establish the proportion of the total cell count that was neurons.
Human uniqueness:
Here are the numbers she found: the average human brain has 16 billion neurons in the cerebral cortex, 69 billion in the cerebellum, and slightly fewer than one billion in the rest of the brain. This fitted almost perfectly with the neuronal scaling rules derived for nonhuman primates: we have a perfectly normal primate brain, just the right number of neurons for the mass of our brain and also our body size.

That finding flew in the face of conventional wisdom, which argued that when correlations are drawn between body size and brain size for living primates (including the great apes), humans appear to have a brain size three times larger than expected. But Herculano-Houzel argues that it is the great apes, not humans, that are the exception. While the great apes also conform to the neuronal scaling rules—i.e., the average size of their neurons doesn’t increase exponentially as they gain more neurons—their brains are much smaller than should be expected for their body size.

The evolutionary story she tells by way of explanation is one of choosing between brain and brawn. Being restricted to eight hours of foraging a day, the ancestral great apes chose brawn (which, of course, means they underwent natural/sexual selection for a larger body size): the amount of energy that could be acquired was invested in building a bigger body rather than a bigger brain. At seventy-five kilograms a 30 billion–neuron brain was the maximum size that could be fueled. Ancestral Homo went a different way: it increased the energetic uptake from foraging by increased scavenging and hunting while maintaining a relatively small body size, enabling its brain to expand to an estimated 40 to 50 billion neurons for Homo habilis two million years ago. But that was the limit: there was no time left in the day and no other sources of food to exploit. Further expansion of the brain required securing more energy from the same type and quantity of foodstuffs. As from 1.5 million years ago that is just what our ancestors achieved by cooking their food.
Scaling rules:
But even though the human cerebral cortex constitutes 82 percent of the total brain mass, the largest when compared to all mammals, it was found to contain only 19 percent of the total number of neurons in the brain, the same percentage as in the guinea pig and capybara, and midway in the 15 to 25 percent range found in most mammals.

How can the human cerebral cortex have expanded so greatly in comparison to the rest of the brain while maintaining a proportion of neurons equivalent to that found in the cerebral cortex of other small-brained primates? Herculano-Houzel’s answer lies partly in the absolute number of neurons in the human cerebral cortex and partly in the fact that different scaling rules apply to the cerebral cortex and the cerebellum.

These rules are constant across all primates: when additional neurons are added to the brain, the cerebral cortex increases in mass at a much faster rate than does the cerebellum. This is because the cerebral cortex requires larger neurons than the cerebellum—neurons that have long-range connections of several centimeters to link different cortical areas; neurons in the cerebellum need to span no more than a few millimeters. As a result, the cerebral cortex becomes proportionally larger even though the ratio of cortical to cerebellar neurons remains the same. So with humans, the 16 billion neurons in the cerebral cortex result in its forming 82 percent of the total brain mass, despite the human brain’s remaining entirely typical for a primate with regard to the proportions of neurons in the cerebral cortex and in the cerebellum.

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