How does the brain process all of the information being taken in by our senses every second of every day?
A region deep in the brain called the hippocampus tracks, sorts and stores the onslaught of information pouring through the senses every waking minute. A large question in neuroscience is one a kindergartner would ask: How?
How does a dollop of tissue containing a small fraction of the brain’s neurons possibly absorb and hold so much, even temporarily?
A study published last week in the journal Nature provides the first step toward an answer, as well as a showcase for some of the most advanced methods available to study the brain.
Researchers at the Howard Hughes Medical Institute in Maryland stimulated not a single cell but a single dendritic spine, one of the hairlike growths that sprout from a cell’s branching arms.
Brain cells communicate with their neighbors by sending a chemical burst from the tips of these spines, across a space called the synapse to the tip of a spine on the next cell. If the chemical bath is strong enough, the receiving spine bulges forward — strengthening the connection between the spines. This is thought to be the fundamental process underlying learning.
But the researchers, Christopher D. Harvey and Karel Svoboda, found something unusual when they stimulated a single spine. Not only did the spine bulge, but it also somehow made its neighbors more sensitive to chemical signals — standing ready, in effect, to digest any spillover of information. Imagine every neighbor on the block calling up to offer a corner of his basement for storage, just in case.
The combined effect of these helpers multiplies the capacity of any single brain cell, the authors concluded. Neuroscientists had theorized that this effect, called clustered plasticity, might help account for the tremendous capacity of the brain, but they had not seen it in action.
“The traditional view was that each synapse functioned independently, and the strength of individual connections modulated memory storage,” said Mr. Harvey, a graduate student at the Cold Spring Harbor Laboratory on Long Island. “What we’ve shown is that neighboring synapses may function together, which leads to the idea that information is stored in a clustered manner, with related things concentrated in the same neighborhood.”
The ability to watch a synapse in action is itself a scientific accomplishment. The average human brain has about 100 billion neurons, and about 1,000 times that many synapses. To zero in on a single one, the researchers used mice that were genetically engineered so their brains produced a fluorescent protein that glowed only in specific cells of the hippocampus. Peering through a high-powered microscope at a slice of this tissue, the researchers could zero in on a single synapse.
Using a laser, they triggered a burst of glutamate, a brain chemical, into the synapse. The entire slice of brain tissue was soaked in a form of inert glutamate, and the laser activated the chemical in precisely the area the scientists were focusing on.
Previous efforts to observe this cell-to-cell communication in action used electrical stimulation, which sends a brush fire of activity through a neighborhood of cells, swamping any of the subtle effects that happen naturally.
In this study, “glutamate is delivered to individual spines located relatively deep within the brain tissue, under conditions that mimic” processes in the body, wrote Dr. Bernardo L. Sabatini, a neurobiologist at Harvard, in an editorial titled “Neighborly Synapses” accompanying the study.
After firing the synapse, the researchers found that receptors on neighboring cells remained extra sensitive to stimulation for 10 minutes. That makes sense, Mr. Harvey said, given the brief impressions that people need to process when walking into an unfamiliar room, say, or navigating a party.
“An hour or more would be too long,” he said. “There would be all sorts of information pouring in, unrelated stuff. And a few seconds would be too short, not enough time to take in something you were paying attention to. Ten minutes is just about right.”