Tuesday, November 24, 2009
Firing at Whisker Barrels
Wednesday, September 19, 2007
 Pyramidal neurons in the hippocampus, a brain area that is involved in learning and memory. |
 Neurons in an area of the hippocampus called the dendate gyrus expressing Green Fluorescent Protein. The green band on the right is the axon of other neuron far away that's making synaptic contacts with the neurons shown in the image. |
 One millimeter of the cortex of a mouse brain with neurons stained with a fluorescent indicator. The three red objects are patch- pipettes, small hollow glass pipes with extremely fine tips that are used to examine single neurons. |
| Images provided by Helmut Koester. |
There’s a gap in our knowledge of how the brain works, says Koester, an assistant professor of neurobiology and member of the Center for Learning and Memory. He says we know a lot about how individual brain cells change as we learn and how sections of the brain change as we learn, too.
But what happens in between–at the level of networks of neurons–we know almost nothing about.
It’s like trying to figure out how a computer program works by looking at a single binary digit as it switches back-and-forth from “One” to “Zero,” rather than studying lines of code.
“You only get meaning within the context of what several thousand neurons are doing,” Koester says. “I’m trying to close the gap from the cellular world to the systems world.”
The particular network he studies is in the barrel cortex, which handles sensory input from the mouse’s whiskers. The cortex is formed of barrel-shaped structures, all lined up in a row, like a wine cellar. Mice typically have 40 to 60 whiskers, and, accordingly, 40 to 60 barrels. Each whisker corresponds to a barrel.
The barrel cortex is part of the mouse’s neocortex, where learning and memory takes place in all mammals, including humans. Humans do not have a barrel cortex, but similar structures play a role in how humans process vision, for example.
Koester studies one barrel at a time. A barrel comprises about 2,000 neurons, making it the smallest independent neuronal network known to science, just small and simple enough for its activity to be recorded.
To study how the network changes in response to stimulation from electrodes, Koester uses a souped-up version of fluorescent imaging, a combination of glowing dye with a short-pulse laser beam. Electrodes simulate physical stimuli, and the network responds as if it were receiving input from a whisker.
Because tiny neurons fire so quickly within a relatively enormous network, limitations in imaging technology made it impossible to study neuronal networks at all until about 10 years ago.
Even now, only 50 to 150 of the neurons can be pictured at a time. The stimulus must be repeated several times to get a complete map of the network’s activity.
Then the questions become: “How are the neurons talking to each? How do they determine who’s firing? Who’s not? How much?” says Koester.
“What we’re trying to get at is the basic mechanism of how the neocortex works to signal information and to store information, how it changes with experience,” Koester says.
