Spanish neuroscientist Santiago Ramón y Cajal revolutionized the study of the brain when he observed neurons for the first time in history. His investigations, now more than 100 years old, revealed intricate details of nerve cells in many different animals, including humans, root-like dendrites attached to bulbous cell bodies, from which long, thin axons extend.
Ramón y Cajal’s studies also revealed that dendrites (by which nerve cells receive signals from other neurons) were much longer in humans than in rodents and other animals, even other non-human primates.
Now, a new study published in the journal Cell shows that in humans these antennae projections also have different electrical properties than other animals that can help explain how the brain processes information.
Although scientists have been meticulously studying dendrites for many decades, “the only thing we really knew about human dendrites was their anatomy,” says Massachusetts Institute of Technology neuroscientist Mark Harnett, leader of the work. “There was a lot of potential for human dendrites to do something different because of their length, but there was no published work that I know of on their actual electrical properties.”
Harnett and his team set out to investigate whether the length of the dendrites affected the electrical signals transmitted through them. They obtained brain tissue taken from epilepsy patients undergoing routine surgery to help relieve seizures.
Once the experts had the resected tissue (removing moisture from the brain tissue), they quickly transported it to the lab, where they cut and analyzed the samples. Because human tissue could only stay alive for a few days, the experiments continued for 48 hours straight.
In all, they examined brain slices from nine patients and thirty rats . To study the electrical properties of neurons within these samples, the researchers used patch registration, which involves attaching tiny glass needles to nerve cells to measure their activity. These probes revealed that although human and rodent dendrites shared basic characteristics, there were some key differences between the two species.
When the researchers injected an electrical current into the dendrites of neurons, they found much less activity in human cells than in rodent cells. “That immediately suggests that the signaling is much more compartmentalized in human dendrites, ” says Harnett. “That means that any local processing that occurs in the dendrites can occur regardless of what is happening in the soma (the cell body of the neuron).”
Harnett likens these dendritic compartments to locks: As locks get more complicated, you need a more sophisticated key to unlock them. Similarly, human dendrites may require very specific signals to greatly influence soma. Ultimately, the properties of human dendrites could endow neurons with more computing power than those of rodents. Because signals are more easily transmitted from one end of the cell to the other in rats, this suggests that electrical signal processing in the dendrites of these animals is less compartmentalized, Harnett notes.
This work supports decades of research in animals, mostly rodents, that have shown that dendrites can secrete signals in this way.
Still, the actual calculations that dendrites perform, and the behaviors linked to activity in these neural branches, are still unclear. However, this work shows that, in addition to differences in size, there are also differences in the way the human organ works.
“It’s not just that humans are smart because we have more neurons and a larger cortex . From top to bottom, neurons behave differently,” says Harnett.
“In human neurons, there is more electrical compartmentalization, and that allows these units to be a little more independent, potentially leading to increased computational capabilities of individual neurons. If this architecture can explain the differences in how our species information processing remains to be seen, but it is a hypothesis worth exploring, “concludes Harnett.
Referencia: Enhanced Dendritic Compartmentalization in Human Cortical Neurons. Lou Beaulieu-Laroche, Enrique H.S. Toloza, Marie-Sophie van der Goes, Matthew P. Frosch, Sydney S. Cash, Mark T. Harnett. CELL 2018. DOI: https://doi.org/10.1016/j.cell.2018.08.045
