How efficient is the human brain?

Neurons in the brain communicate with each other via electrical pulses. These pulses, generated as ions such as potassium and sodium, enter the cell through an ion channel. The channels act as an “on-off” switch by changing shape to alter the flow of the ions, either letting them in and generating an electrical signal, or closing to prevent the signal getting through.

Now, a team of researchers from Massachusetts Institute of Technology have found that the number of ion channels in the human brain are at a lower density than in other mammals’ brains. They have hypothesised that this has helped human brains become more efficient.

The researchers analysed neurons from 10 mammals – the most extensive electrophysical study of its kind to date – and found that bigger neurons lead to more ion channels in a relatively constant ratio of size-to-channels.

What this means is that bigger neurons can still handle all the electrical pulses they are bombarded with because they have the ion channel capacity to process the ions. No matter the brain size, the energetic cost to run each mammal’s brain was about the same.

The human brain is the exception

That is, in every mammal except humans, who had a much lower density of ion channels than expected.

“Previous comparative studies established that the human brain is built like other mammalian brains, so we were surprised to find strong evidence that human neurons are special,” says former MIT graduate student Lou Beaulieu-Laroche.

Graphical abstract. Created on imgflip by Cosmos.

One benefit of a lower density of ion channels may be that less energy is used on pumping ions into neurons, which could then be diverted to other processes, like creating more complicated synaptic connections.

“If the brain can save energy by reducing the density of ion channels, it can spend that energy on other neuronal or circuit processes,” says Mark Harnett, an associate professor of brain and cognitive sciences, a member of MIT’s McGovern Institute for Brain Research, and the senior author of the study.

“We think that humans have evolved out of this building plan that was previously restricting the size of cortex, and they figured out a way to become more energetically efficient, so you spend less [energy] per volume compared to other species.”

In this case, the human brain could run on the same amount of power as other mammal brains but perform more complex procedures with the excess energy diverted from ion channels.

Harnett hopes to study where that extra energy is being used and whether there are any specific genes responsible for ion channel density exclusive to humans.