Human neurons are electrically compartmentalized, study finds

Oct. 19 (UPI) -- Neurons inside the human brain are significantly larger than those in rodent brains. According to new research, the enhanced size allows for electrical compartmentalization.

Compartmentalized electrical signaling can help explain the advanced cognitive capabilities of the human brain.

"We've known for over 100 years that these human neurons had different shapes and were much longer than those found in rodent cortex," Mark Harnett, an assistant professor or cognitive sciences at the Massachusetts Institute of Technology, told UPI in an email. "This was first described by Santiago Ramon y Cajal, who is considered the grandfather of neuroscience. What we didn't know was if these changes in structure are associated with changes in the functional properties of the neurons."

Because a human neuron is longer, the distance between dendrites and the center of a neuron is greater. Thus, the electrical signals received and propagated by dendrites have farther to travel, altering the input-output function.

Harnett and his colleagues also determined more distant dendrites feature fewer ion channels, which dictate signal processing.

"This helps to preserve strong electrical compartmentalization in the distal dendrites of human neurons, which we think is important for computational power, by creating an increased number of semi-independent processing units within each neuron," Harnett said. "Rodent neurons already have some of this capability, but the human neurons seem to have an extreme version of it."

But while the unique size and structure of the human neuron suggested an enhanced level of computational sophistication, scientists wanted to measure the differences in electric signaling.

"The structure can only give you clues about compartmentalization," Harnett said. "You have to actually record the electrical signals propagating from one area of the neuron to another to make rigorous claims about compartmentalization."

Researchers used tiny glass electrodes to measure the electric signals traveling through the dendrites and the neuron's cell body, or soma -- a technique called patch-clamping.

Compartmentalization allows parts of a neuron to be active while other parts remain silent. Neurons with limited compartmentalization are forced to employ all of its dendrites at once, or none at all.

The electrical signals recorded by Harnett and his colleagues -- and described this week in the journal Cell -- demonstrated compartmentalization.

"A pulse is big in one part of the dendrite, but then decrements substantially as it propagates. Once its small enough, that's compartmentalized," Harnett said. "The different areas of the neuron have a limited impact on one another, until a specific set of conditions are fulfilled and then they change how they communicate -- for example, a dendritic spike in the voltage -- and the compartmentalization breaks down."

The differentiated abilities of longer neurons and distantly spaced dendrites is one reason why the human brain is so powerful.

"It allows for more computational subunits, sort of like a small neural network, but within a single cell," Harnett said.