Each of our neurons compute inordinate amounts of information in compartmentalised sections, according to one of the first recordings of electrical activity in human brain cells.
Our understanding of neurons (brain cells) is almost exclusively from investigation into our small mammalian friends – the rodent. A study Published in the journal Cell, however, has revealed salient differences between the two species’ neurons that could contribute to our superior levels of intelligence.
Neurons, generally, communicate via electrical impulses that’re fired down its lengthy axon, causing the release of neurotransmitters. Studies have detected this electrical activity by inserting microscopic electrodes inside these cells – usually within mouse neurons that have been preserved in a dish. Mark Harnett’s study at the Massachusetts Institute of Technology in Cambridge set-out to see how our very own neurons compared to those of mice.
Using live neuronal tissue generously obtained from patients who were undergoing surgery to help alleviate their epileptic symptoms, Harnett’s team poked super-thin electrodes into the branched segments, known as dendrites, of the neurons’ cell bodies.
Each neuron has, on average, 50 dendrites that branch out and provide connection points, or synapses, to other adjacent neurons. The neurotransmitters that get released from these adjacent neurons are the initial spark of the eventual electrical signal that may, or may not, get propagated down the length of the neuron.
Compared with mice pyramidal neurons (massive cells that branch out widely), our human counterparts express larger dendrites with fewer ion channels: the cell’s slip roads of electrical activity. While this sounds counterintuitive, remember the old adage, “sometimes less is more.”
The electrical activity garnered within our dendrites, as this study suggests, is more dispersed along our elongated dendritic branches. This dendritic property causes a greater compartmentalisation of electrical activity, with a unique integration strategy that brings this activity together.
Take a mouse neuron; if a signal begins to gather activity and propagate down one dendrite, the sheer number of ion channels that conduct electricity will greatly help this signal along its journey to the main truck of the neuron. The human neuron, conversely, doesn’t share this property, as its less certain an initial electrical input will gather the necessary electrical momentum to proceed down the dendrite to influence the cell body. Because of the cells’ comparative elongation and spread of ion channels, the chance of a full electrical propagation relies on a greater integration of electrical information from other dendrites.
This complexity allows the input from thousands of synapses on each of the neuron’s dendrites to have an almost democratic say regarding whether a piece of information should allow the main branch to fire or not. This property provides our neurons with a richer source of information that can inform on the all-or-nothing final “decision” a neuron will make to either fire or hold-off.
Cell, DOI: 10.1016/j.cell.2018.08.045