Recent work has revealed some of the many crucial functions of astrocytes, brain cells that were once thought to play primarily supportive roles for neurons. Astrocytes do handle debris around neurons and help supply them with nutrients. But they have other important roles too, and scientists have now suggested that astrocytes could play a major role in storage in the brain. This work can help explain how the human brain holds so much information, which is far more than would be expected on the basis of the volume of neurons it holds. This work has been reported in the Proceedings of the National Academy of Sciences (PNAS).
Astrocytes have many extensions called processes, and each can link with a synapse, the junction of neurons where signals are sent. This connection creates a three-part or tripartite synapse. Research has indicated that when astrocytes' links to neurons are disrupted in the hippocampus, there are memory problems.
Neurons signal to one another with neurotransmitters, and electrical impulses called action potentials that are created by ion gradients. Astrocytes rely on calcium signaling to send signals to neurons; astrocytes appear to sense neuronal activity and modify calcium levels in response. This may lead to the release of molecules that are similar to neurotransmitters, known as gliotransmitters, into the synapse.
The brain's storage capacity has been modeled with networks, and synapses with more than one link are needed to explain the vast storage ability of the human brain. But synapses are typically thought of as places where only two neurons meet. So the missing storage space may lie in astrocytes.
In this study, the researchers developed a model that could show how the brain's storage capacity can be fully explained with astrocytes. The researchers suggested that astrocytes could encode memories with changes in calcium flow patterns; these changes can be signaled to neurons with gliotransmitter release at a synapse.
"By careful coordination of these two things: the spatial temporal pattern of calcium in the cell and then the signaling back to the neurons, you can get exactly the dynamics you need for this massively increased memory capacity," explained first study author Leo Kozachkov Ph.D.
Since astrocytes have many processes and can link up with many different synapses, they may dramatically increase the storage capacity of the brain while also keeping it very efficient.
However, more research will be necessary to determine whether this model is accurate. If it is, the findings explain not only more about the roles of astrocytes and gliotransmitters, but also more about how memory works in the brain.
"We hope that one of the consequences of this work could be that experimentalists would consider this idea seriously and perform some experiments testing this hypothesis," noted senior study author Dmitry Krotov, a research staff member at the MIT-IBM Watson AI Lab.
Sources: Massachusetts Institute of Technology (MIT), Proceedings of the National Academy of Sciences (PNAS)