“It’s not just that humans are smart because we have more neurons and a larger cortex. From the bottom up, neurons behave differently,” says neuroscientist Mark Harnett with MIT’s McGovern Institute for Brain Research. “In human neurons, there is more electrical compartmentalization, and that allows these units to be a little bit more independent, potentially leading to increased computational capabilities of single neurons.”
A new study, published this week in Cell, shows that in people dendrites (via which nerve cells receive signals from other neurons) have distinct electrical properties that may help explain how the brain processes arriving information.
Javier DeFelipe, a neuroscientist at the Cajal Institute in Madrid who was not involved in the work, says this study shows that in addition to size differences, there are also differences in the way the human organ functions. “Our brain is not a bigger mouse brain.”
Scientists have been studying dendrites since Spanish neuroscientist Santiago Ramón y Cajal revolutionized the study of the brain when he revealed more than 100 years ago intricate details of nerve cells in many different animals, including humans—rootlike dendrites attached to bulbous cell bodies, from which extend long, slender axons.
“The only thing we really knew about human dendrites was their anatomy,” Massachusetts Institute of Technology says Harnett, Fred and Carole Middleton Career Development Assistant Professor of Brain and Cognitive Sciences.. “There was a lot of potential for human dendrites to be doing something different because of their length, but there was no published work, as far as I know, on their actual electrical properties.”
Dendrites in the cortex of the human brain are much longer than those in rats and most other species, because the human cortex has evolved to be much thicker than that of other species. In humans, the cortex makes up about 75 percent of the total brain volume, compared to about 30 percent in the rat brain.
Although the human cortex is two to three times thicker than that of rats, it maintains the same overall organization, consisting of six distinctive layers of neurons. Neurons from layer 5 have dendrites long enough to reach all the way to layer 1, meaning that human dendrites have had to elongate as the human brain has evolved, and electrical signals have to travel that much farther.
Using hard-to-obtain samples of human brain tissue, MIT neuroscientists have discovered that human dendrites have different electrical properties from those of other species. Their studies reveal that electrical signals weaken more as they flow along human dendrites, resulting in a higher degree of electrical compartmentalization, meaning that small sections of dendrites can behave independently from the rest of the neuron.
These differences may contribute to the enhanced computing power of the human brain, the researchers say.
Harnett and his colleagues set out to investigate whether the length of dendrites affected electrical signals transmitted through them. With the help of a neurologist, Sydney Cash of Massachusetts General Hospital, they were able to obtain brain tissue that had been removed from epilepsy patients undergoing routine surgery to help allay seizures—a procedure in which physicians routinely remove part of the temporal cortex to get to the hippocampus, a structure deep inside the brain where seizures typically originate.
Once the research team obtained the resected tissue, reports Diana Kwon in Scientific American, they then hurriedly transported it back to the lab, where they sliced and tested the samples. Because the human tissue could only be kept alive for a few days, experiments usually continued for 48 hours straight. “We would work in shifts and go home and sleep then come back and keep recording,” Harnett says.
Michael Hausser, a neuroscientist at University College London who was not involved in this work, says scientists expected there would be a greater degree of compartmentalization in the dendrites of human neurons than many other animals,’ because they are much longer. And subsequent work with computational models suggested having more independent processing compartments within dendrites could provide greater computational power within a single cell.
Still, the actual computations dendrites carry out reports Berlin-based Kwon, and the behaviors linked to the activity in these neuronal branches—are unclear. One possibility, Hausser says, is the electrical activity within the dendrites could detect the simultaneous occurrence of separate signals—say, incoming information about the smell and shape of a rose. In addition to identifying different inputs to the neuron, dendrites could also be involved in binding this information together and storing it.
Of course, these ideas have yet to be tested experimentally. But Harnett’s study “represents a first step in a new era of exploring our own dendrites,” Hausser says. “And that’s incredibly important for understanding how human brains work.”