Dendritic spines don’t go with the flow

Neurons receive incoming signals through synapses at hundreds of dendritic spines, the lollipop-shaped structures with thin necks and bubblelike heads that stud the surface of dendrites. Each spine serves as an antenna relaying the chemical and electrical signals at the synapse to the cell body. If the din is loud enough, the entire cell will rouse itself to fire an action potential.

Synapses hold the key to understanding how the brain perceives, records, and responds to incoming information. With the right stimulation, some of the synaptic signals grow stronger, like soloists in a chorus. And this regulation of synaptic strength allows the brain to change in response to experience.

Many studies have looked at the complex molecular changes that influence synaptic strength. But a study led by Bernardo Sabatini, Harvard Medical School assistant professor of neurobiology, suggests that part of the control may lie in the shape of the spines themselves. He and graduate student Brenda Bloodgood found that the necks of dendritic spines constrict or widen in response to different inputs, regulating the ability of molecules to flow from the spine into the cell body. This action, detailed in the Nov. 4, 2005 Science, could be a way that the spines control synaptic strength and give synapses some independence from the cell.

“One of the big questions in neuroscience is, how do neurons integrate all the synaptic inputs they get?” said Bloodgood. “Not all synapses on a neuron are equal.” The structure of dendritic spines keeps each synapse separate, marooned on its own peninsula at the cell surface. It is thought that this physical separation helps regulate the synapses, allowing each one to keep its own pool of molecular signals. But until now, it was difficult to study whether their isolation was a regulated property.