The human brain’s advanced cognitive abilities are typically attributed to our recently evolved neocortex. Comparisons of human and rodent brains show that the human cortex is thicker, contains more white matter, has larger neurons, and its abundant pyramidal cells have more synaptic connections per cell compared to rodents.
But scientists have yet to determine whether there are important differences at the biophysical level of the basic building blocks of the human neocortex, the pyramidal neurons. Do these cells possess unique biophysical properties that might impact on cortical computations?
A team of researchers built detailed 3D models of pyramidal cells from the human temporal neocortex to try to answer this question. These first-ever detailed models of human neurons were based on in vitro intracellular physiological and anatomical data from human cells.
Cortex Membrane Capacitance
The theoretical study, led by Prof. Idan Segev from the Hebrew University of Jerusalem, with colleagues at Vrije Universiteit Amsterdam and Instituto Cajal in Madrid, predicted that layer 2/3 pyramidal neurons from the human temporal cortex would have a specific membrane capacitance that is half of the commonly accepted “universal” value for biological membranes (~0.5 μF/cm2 vs. ~1 μF/cm2).
Since membrane capacitance affects how quickly a cell can respond to its synaptic inputs, this finding has important implications for the transmission of signals within and between cells. The theoretical prediction regarding the specific membrane capacitance was then validated experimentally by direct measurements of membrane capacitance in human pyramidal neurons.
Researcher Guy Eyal, a Ph.D. student at the Hebrew University’s Department of Neurobiology, said:
“This is the first direct evidence for the unique electrical properties of human neurons. Our finding shows that low membrane capacitance significantly improves the efficacy of signal processing and the speed of communication within and between cortical neurons in the human neocortex, as compared to rodents.”
To collect this data, fresh cortical tissue was obtained from brain operations at a neurosurgical department in Amsterdam, and additional data was obtained from light-microscope studies in pyramidal cells from post mortem studies at the Cajal Institute in Madrid.
The researchers suggest the distinctive biophysical membrane properties of human pyramidal neurons are an outcome of evolutionary pressure to compensate for the increase in size and distances in the human brain.