Neuroscientists have discovered a new signal process in the brain that they suggest may be key to what makes us human.
The human brain is a complex system of cells called neurons that exchange information using electrical and chemical signals. In a new study published in Science, researchers found that certain cells in the human brain transmit signals in a way not seen in corresponding rodent cells.
Demonstrable differences in human compared to other mammal brains helps us pin down why human brains may be unique, which can lead to a better understanding, and even better modeling, of our brains.
The researchers looked at slices of brains from patients who had either epilepsy or tumors. They focused on the dendrites – the branch-like extensions of brain cells that connect to other brain cells, allowing information to be exchanged – in the second and third layers of the brain’s cortex (outermost layer).
Most of what we know about dendrites is from rodent studies, so the opportunity to study human samples is vital. Patients with epilepsy were chosen due to epilepsy surgeries providing enough cortex tissue to analyze, and the tumor samples were to ensure that the results weren’t only found in those with epilepsy.
Thanks to the human brain’s expansion as we evolved, it has an unusually thick cortex (around 3 millimeters), which is disproportionately thick in the second and third layers, leading to large and elaborate dendrite trees.
The synapse is a structure that allows electric nerve impulses to be sent between two neurons. Neurons communicate via electrical events called “action potentials” – a burst of electrical activity when a neuron sends information away from the cell. Hundreds of synaptic inputs to a neuron determine whether an action potential results. The active electrical properties of these dendrites determine the numerous transformations from synaptic input to action potential, which means they are key to a neuron’s computational power.
The team used a patch clamp to construct an electrical circuit for the cells and a fluorescing imaging technique to investigate the properties of the cells. They discovered previously unknown classes of action potentials in the dendrites of these neurons, which means their activity is much more complex than previously realized.
Among the revelations, the researchers noted that one of the new kinds of action potentials traveled using just calcium ions, instead of both calcium and sodium ions, something not seen before in mammal cortex cells.
They studied the behavior of the action potentials by creating a computer-simulated model and discovered another surprising aspect – they could perform a "computational" function that researchers had previously thought required an entire network of neurons, not just one. This, they say, could have huge implications on artificial neural networks in that it could simplify calculations.
The research does have its limitations. As Gizmodo points out, the researchers didn't model a complete neuron, and the work was carried out in human cells, not actual humans. This could also mean other mammals have the same signal, we've just not observed it in lab conditions yet. However, on the continual quest to find out how and why humans are unique, this is an interesting step.