What makes human brain cells unique?

The Spanish neuroscientist Santiago Ramón y Cajal revolutionized the study of the brain when he observed neurons for the first time. His research, now over 100 years old, has revealed intricate nerve cell details in many animals, including humans – root dendrites attached to bulbous body bodies, from which to expand long thin axons.

Cajal's examinations also revealed that dendrites (through which nerve cells receive signals from other neurons) were much longer in humans than in rodents and other animals, even in other non-human primates. A new study, published this week in Cell, shows that, in humans, these antennal projections also have distinct electrical properties that can help explain how the brain processes incoming information.

Scientists have been studying the dendrites thoroughly since Cajal's first observations. Nevertheless, "the only thing we really knew about human dendrites was their anatomy," says the neuroscientist at the Massachusetts Institute of Technology. "The human dendrites could do something different because of their length, but there was no published work, to my knowledge, on their actual electrical properties."

Harnett and his colleagues therefore sought to determine whether the length of the dendrites affected the electrical signals transmitted through them. With the help of a neurologist, Sydney Cash from Massachusetts General Hospital, they were able to obtain brain tissue that had been removed from epileptic patients undergoing routine surgery to help relieve convulsions – a procedure in which Doctors regularly remove a portion of the temporal cortex to get to the hippocampus, a structure located inside the brain, causing seizures. Once the research group obtained the resected tissue, it was then quickly transported to the laboratory where they cut and analyzed the samples. Since human tissue can only be kept alive for a few days, experiments usually continue for 48 consecutive hours. "We were going to work in teams and go home to sleep, then come back and continue recording," says Harnett.

In total, Harnett's team examined brain slices of nine patients and 30 rats. To study the electrical properties of neurons contained in these samples, researchers used patch-clamp recording, which involves attaching small glass needles to nerve cells in order to measure their activity. These probes revealed that although human and rodent dendrites have common basic characteristics, such as the ability to produce action potential, there were some essential differences between the two species. When the researchers injected an electric current into the dendrites of the neurons, they found that the somas (cell bodies) of human cells were much less active than those of rodents. This immediately suggests that [signaling is] much more compartmentalized [in human dendrites]Said Harnett. "This means that any local treatment in the dendrites can take place regardless of what happens in the soma."

Harnett compares these dendritic compartments to the cups of a lock: as the locks get complicated, you get more cups and you need a more sophisticated key to unlock it. . In the same way, human dendrites may require very specific signals to strongly influence the soma. In the end, the properties of human dendrites could give neurons more computing power than rodents. Since the signals are more easily transmitted from one end of the cell to the other in the rat, this suggests that the processing of electrical signals in the dendrites of these animals is less compartmentalized, notes Harnett.

Michael Hausser, a neuroscientist at University College London who did not participate in this work, says that this new study supports decades of research on animals – mostly rodents – that have shown that dendrites can separate the signals in this way. According to these observations, scientists expect dendrites from human neurons to be more compartmentalized than many other animals because they are much longer. And subsequent work with computer models has suggested having more independent processing compartments within dendrites, which could provide higher computational power in a single cell.

However, current calculations by dendrites – and the behaviors related to the activity of these neural branches – are still unclear. But scientists have ideas: one possibility, Hausser says, is that the electrical activity in the dendrites can detect the simultaneous appearance of separate signals, such as incoming information about the smell and shape of a rose. In addition to identifying different inputs into the neuron, dendrites could also be involved in linking this information and storing it.

Of course, these ideas still need to be tested experimentally. But Harnett's study "is a first step in a new era of exploration of our own dendrites," says Hausser. "And it's extremely important to understand how the human brain works."

Javier DeFelipe, a neuroscientist at the Cajal Institute in Madrid who did not participate in the work, says that this study highlights the importance of studying human tissue. Many neuroscience studies are based on rodent work, but the brains of animals differ from humans in many ways. This work shows that besides size differences, there are also differences in the functioning of the human organ. "Our brain is not a bigger mouse brain," explains DeFelipe.