Scientists were surprised to discover that primate neurons in the visual cortex have fewer synapses than mice.

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A study analyzing individual synapses in macaques and mice shows primate neurons have two to five times fewer synapses than mice in the visual cortex.

A UChicago and Argonne National Laboratory study analyzing over 15,000 individual synapses in macaques and mice found that primate neurons have two-to-five times fewer synapses in the visual cortex compared to mice — and the difference may be due to the metabolic cost of maintaining synapses.

Primates are generally considered smarter than mice. But in a surprising finding, neuroscience researchers at the and Argonne National Laboratory have discovered that mice actually have more synapses connecting the neurons in their brains.

In a study comparing the brains of macaques and mice at the synaptic level, the researchers found that the primates had far fewer synapses per neuron compared to the rodents, in both excitatory and inhibitory neurons in layer 2/3 of the primary visual cortex. Using artificial recurrent neural network modeling, the team was further able to determine that the metabolic cost of building and maintaining synapses likely drives larger neural networks to be sparser, as seen in primates versus mouse neurons. The results were published on September 14, 2021, in Cell Reports.

The research team, made up of scientists from the laboratories of David Freedman, Ph.D., at UChicago and Narayanan ​Bobby” Kasthuri, M.D., at Argonne, leveraged recent advances in electron microscopy, as well as existing publicly available data sets, to compare the connectivity in both species. They chose to examine both excitatory and inhibitory synapses, as most previous research had focused on only excitatory synapses. Focusing on layer 2/3 neurons in the adult primary visual cortex made it easier to compare across species, as these neurons have distinct morphologies that are similar in both primates and mice.

After reconstructing the microscopy images and measuring the shapes of 107 macaque neurons and 81 mouse neurons, the researchers identified nearly 6,000 synapses in the macaque samples and over 9,700 synapses in the mouse samples. Upon comparing the datasets, they found that primate neurons receive two-to-five times fewer excitatory and inhibitory synaptic connections than similar mouse neurons.

The reason why this is surprising is that it there’s this quiet sort of assumption among neuroscientists and, I think, people in general that having more neuronal connections means that you’re smarter,” said Gregg Wildenberg, Ph.D., a staff scientist in the Kasthuri… Brinkwire News Summary.

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