In their study, Sabatini and Svoboda could detect if single calcium channels opened or, by chance, remained closed following stimulation. Measuring the probability of channel opening, "like tossing a coin, where heads is open and tails is closed," says Svoboda, enabled him and Sabatini to determine the number of calcium channels per spine. The scientists discovered that depending on their size, spines contain from one to twenty, and typically three, calcium channels.
"Visually examining calcium fluctuations in a single dendritic spine in the brain as we have done is akin to examining the wrinkles on a raisin sitting on the 50 yard line of a football stadium from the Goodyear blimp," says Sabatini.
But nothing is that simple in the brain. Which type of channel is causing these changes in calcium? There are at least six known varieties of calcium channels that could be present in spines, each having different properties. Using chemical probes, Sabatini and Svoboda were able to demonstrate that one specific type of channel (the R type) is solely responsible for the influx of calcium that they observed. "We are looking at the behavior of single calcium channels in their natural environment, the brain." said Sabatini.
"The local influx of calcium we have observed in spines is a fundamental measure of the information carried in one particular brain neuron and how it is processed locally. The information encoded in the messages passed between neurons is simple," says Sabatini. "It's not unlike computer programming code where a single command can be either one for a positive response or zero for a negative response." When an action potential causes a calcium channel to open, that's a one, when it fails to open that's a zero.