Human neurons are different from those of other mammals


A neuron generates an electrical impulse with the help of ion channels – special proteins that sit in the cell membrane and pass certain ions through themselves. Under the influence of some kind of stimulus – mechanical pressure, a chemical, or an impulse coming from another neuron – ion channels open or close, the concentration of ions on both sides of the membrane changes, and the electrical properties of the membrane itself, accordingly, also change.

Neurons differ from each other in size, length and branching of the processes, with the help of which they receive and transmit impulses. A large neuron will have more ion channels than a small one if these channels are counted individually. But if we take a large neuron in an elephant and a small neuron in a mouse, then, despite the difference in the absolute number of channels, they should work in the same way.

For most animals this is true, but everything changes when it comes to humans. A few years ago, employees Massachusetts Institute of Technology found that rat and human neurons differ in electrophysiological properties, and these differences are due to some peculiarities in dendrites – branched neuronal processes that act as antennas, collecting signals from many other nerve cells. Further studies showed that the density of ion channels on the membrane of human neurons is lower than on the membrane of rat neurons.

In a new article in Nature the neurons of already ten species of animals are compared: the pygmy white-toothed shrew (the smallest mammal on earth), gerbils, mice, rats, guinea pigs, ferrets, rabbits, marmosets, macaques and humans (human samples were taken from patients with epilepsy who had to undergo brain surgery ). The researchers compared the density of two types of ion channels that allow potassium and sodium ions to pass through the membranes of pyramidal neurons in the fifth layer of the cerebral cortex. The thickness of the cortex and the size of neurons increase from shrew to primates, and at the same time, the density of ion channels in each individual neuron increased. Moreover, if we consider not a separate neuron, but some volume of the brain, then it turns out that the density of ion channels in the volume of the brain in the shrew and macaque is the same. White-toothed neurons are small in comparison with macaque neurons, therefore, in a given “cube” of brain tissue, there will be, roughly speaking, more white-toothed neurons, and fewer macaque neurons. However, due to the difference in the density of ion channels in the large and small neurons, the density of the same channels in the “cube” of the brain will be the same. And if you look at the cerebral cortex without disassembling it into individual neurons, it turns out that the electrical properties will be the same for a mouse, a rabbit, or a monkey – because they all will have the same density of ion channels per unit the volume of the bark.

A person falls out of this rule: his density of ion channels in the “cube” of the cortex turned out to be less than one might have expected – because the density of ion channels on the neurons of our cortex is also lower than it would be expected in comparison with poppies and in accordance with our size. Ionic channels require energy, and the more there are, the more energy is spent on synthesizing them and making them work. If the brain has abandoned some of the ion channels, it has the opportunity to direct excess energy to something else – for example, to the formation of more complex synapses between neurons. In addition, those ion channels that the human brain already has, could learn to use energy more efficiently to excite an electrical potential.

One way or another, the human brain – at least as far as we can judge from the cells of one of the layers of the cortex – differs from the brain of other mammals at the level of individual neurons, and this difference should affect some features of the functioning of the brain as a whole. In the future, the authors of the work want to study the brains of great apes, which are the closest to humans in evolutionary terms, in the same way – in this way it will be possible to understand exactly when the density of ion channels in the brain decreased and what genetic changes contributed to this.


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