Researchers have discovered a surprising difference between the neurons of humans and other mammals, which may make our brains work more efficiently than theirs.

The cells in the nervous system are the building blocks of the brain. These cells are also called neurons and they communicate with each other via electrical pulses. These are produced by so-called ‘ion channels’: proteins that ensure the transport of ions across a cell membrane and, for example, regulate the flow of ions such as potassium and sodium. In a new study researchers have now discovered something interesting. Because human neurons appear to have far fewer such ion channels than expected.

More about neurons
As mentioned, neurons are the fundamental cells of the nervous system. Neurons are actually the body’s information and signal processors and are responsible for receiving and transmitting nerve impulses. Together they form long fibers. They consist of a cell body containing the nucleus and one or more neurites and dendrites. A specific characteristic of nerve cells is that they are excitable; they can receive and transmit signals without loss of signal strength. The brain contains circuits of nerve cells that control many bodily functions and are also responsible for our thinking abilities. It is estimated that a human has about 100 billion neurons.

In their study, the researchers analyzed the neurons of ten different mammals: eyelash shrews (fun fact: this is one of the smallest mammals known), gerbils, mice, rats, guinea pigs, ferrets, rabbits, marmosets and macaques. They also examined human tissue removed from patients with epilepsy during brain surgery.

Density

The analysis shows that as the size of neurons increases, the density of the channels in the neurons also increases. However, human neurons appear to be a striking exception to the rule. Here the researchers found far fewer ion channels than they had expected. “Previous studies had shown that the human brain is structured in the same way as the brains of other mammals,” said study researcher Lou Beaulieu-Laroche. “We were therefore very surprised to find strong evidence that human neurons are special.”

Mammals

An example. In the eyelash shrew’s cerebellum — which is full of tiny neurons — a given volume of tissue houses many more neurons than the same tissue volume of the rabbit brain, which has much larger neurons. However, rabbit neurons have a higher density of ion channels. And so, down the line, the total density of ion channels in a given volume of tissue is exactly the same in both species. “This building plan is consistent for nine different mammal species,” said study researcher Mark Harnett. “It seems that the cortex keeps the number of ion channels per unit volume the same. This means that for a given cortex volume, the energetic costs are the same – at least in terms of ion channels.”

Neuroscientists analyzed the neurons of several mammalian species, including (from left to right) ferret, guinea pig, rabbit, marmoset, macaque and human. Image: Beaulieu-Laroche et. already.

The human brain, however, is a striking departure from this construction plan. Instead of an increased density of ion channels, the researchers found a dramatic decrease in the expected density of ion channels for a given volume of brain tissue.

Energy saving

Why do human neurons have fewer ion channels? “If the brain can conserve energy by reducing the density of ion channels, it can spend that energy on other neuronal processes,” Harnett said. In other words, this lower density means less energy has to be spent pumping ions, allowing the brain to use that energy for something else, such as creating more complicated synaptic connections between neurons. It means that this adaptation may make our brain work more efficiently than that of other mammals, enabling us to perform more complex, cognitive tasks.

The researchers hope to find out in follow-up studies where exactly that extra energy could go. They also want to look for specific gene mutations that help neurons of the human cortex to achieve a higher efficiency. Finally, they would also like to investigate other primates more closely related to humans. Because the question is whether a comparable decrease in the density of the ion channels in their brains can also be found.