WHAT MAKES THE BRAIN TICK

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 cell8217;s branching arms.

Brain cells communicate with their neighbours 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 forward8212;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 neighbours more sensitive to chemical signals8212;standing ready, in effect, to digest any spillover of information. Imagine every neighbour 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 theorised that this effect, called clustered plasticity, might help account for the tremendous capacity of the brain, but they had not seen it in action.

8220;The traditional view was that each synapse functioned independently, and the strength of individual connections modulated memory storage,8221; said Harvey, a graduate student at the Cold Spring Harbor Laboratory on Long Island in New York. 8220;What we8217;ve shown is that neighbouring synapses may function together, which leads to the idea that information is stored in a clustered manner, with related things concentrated in the same neighbourhood.8221;

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 neighbourhood of cells, swamping any of the subtle effects that happen naturally.

In this study, 8220;glutamate is delivered to individual spines located relatively deep within the brain tissue, under conditions that mimic8221; processes in the body, wrote Bernardo L. Sabatini, a neurobiologist at Harvard, in an editorial titled 8220;Neighborly Synapses8221; accompanying the study.

After firing the synapse, the researchers found that receptors on neighbouring cells remained extra sensitive to stimulation for 10 minutes. That makes sense, Harvey said, given the brief impressions that people need to process when walking into an unfamiliar room, say, or navigating a party.

8220;An hour or more would be too long,8221; he said. 8220;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.8221;Benedict Carey NYT