High-efficiency transporters that work like a shuttle system to constantly move ions into and out of neurons appear to slam into reverse following a stroke or other injury and start delivering instead too much water, scientists have found. It’s called spreading depolarization, a wave of death that can follow a stroke or traumatic brain injury, as neurons and their extensions, called dendrites, become bloated, dysfunctional and vulnerable, said Dr. Sergei Kirov, neuroscientist at the Medical College of Georgia at Georgia Regents University.
While swelling is clearly a result of trauma to the brain, just how water gets into neurons was largely a mystery. In a study published in The Journal of Neuroscience, Kirov and his colleagues report that a handful of these ion transporters – known to tote some combination of sodium, potassium and chloride – appear to be a missing link in how excess water gets inside. The differential distribution of sodium and potassium also is essential for neurons to generate electrical signals, called action potential, and communicate with other neurons or cells so humans and animals can think or move or otherwise function.
When action potential is generated, a neuron goes through a process called depolarization, which alters its electrical charge so it becomes positive inside. Sodium channels open and small amounts of sodium move inside and channels rapidly close. During the repolarization that follows, the opposite happens: potassium channels open and small amounts of potassium move out of the neuron and those channels close. Once again, the sodium-potassium pumps push the ions back in their correct location. It’s a continuous, efficient process in the healthy brain. But a traumatic brain injury, stroke, brain bleed or even a migraine can result in unrelenting, pathological spreading depolarization in which large amounts of sodium move inside and large amounts of potassium move out of neurons. Sodium-potassium pumps quickly get overwhelmed trying to straighten things out and neurons rapidly find themselves in trouble.