How nerve cells form connections

Scientists at the University of California, San Diego have produced images of brain cells forming temporary and permanent connections in response to various stimuli.

This illustrates for the first time the structural changes between neurons in the brain that, many scientists have long believed, take place when we store short-term and long-term memories.

"The long-term memories stored in our brain last our entire lives, so everybody had assumed that there must be lasting structural changes between neurons in the brain," says Michael Colicos, a postdoctoral fellow at UCSD. "Although there has been a lot of suggestive evidence to indicate that this is the case, it has never before been directly observed."

"While most people assumed that some sort of rearrangement of nerve cell connections took place in the brain, this was extremely difficult to demonstrate experimentally," says Yukiko Goda, a professor of biology at UCSD.

To resolve this problem, the UCSD researchers focused their attention on individual nerve cells, specifically neurons from the hippocampus - the portion of the human brain crucial to forming particular types of memory - and filmed them as their synapses made new connections to other nerve cells in response to electrical impulses.

The ability of the scientists to do this without impairing the normal physiological functions of the cells depended on two techniques implemented in Goda's lab to study synaptic connections.

One was a method of visualizing the rods and filaments of actin-the girders that make up the cytoskeleton, the internal skeleton of the cell. Using molecular biology techniques, fluorescent versions of actin were constructed and visualized as the neurons grew and changed shape to establish new connections.

The second development, was a method of stimulating nerve cells in a manner that mimicked their stimulation in the brain. This involved using the photoconductive properties of silicon in a way that allowed the researchers to deliver a short, high frequency burst of electricity to a specific area of a neuron on a silicon chip by simply shining light on that area. Light excitation in that area of the silicon created a narrow pathway through which Colicos and his colleagues could apply a tiny voltage below the chip to target the neuron.