Pittsburg, PA & New Haven, CT, USA. There is a fundamental mechanism used by neurons to communicate despite the brain's noisy environment, whether they are existing brain cells or altogether new adult growth neurons.

Tens of thousands of the billions of neurons in the brain might be trying to send signals to one another at any given time. Much like a person trying to be heard by a friend across a crowded room, individual neurons must select the best way to get their message heard above the noise.


Neurons communicate by sending out electrical impulses called action potentials (or spikes). These spikes code information much like a version of Morse code with only dots and no dashes. However, groups of neurons must solve a universal communications problem and choose to communicate information in one of two ways: by spiking simultaneously or by spiking separately. In essence, they can send short messages that are broadcast across a wide area or speak locally while delivering more complex content.



Existing Neuron Communication

Inhibitory circuits in the olfactory bulb use a novel, time-scale dependent strategy to mediate how neurons choose between encoding and propagating information.

The research was conducted at the Center for the Neural Basis of Cognition, a joint program between Carnegie Mellon University and the University of Pittsburgh.

The findings appear in the Proceedings of the National Academy of Sciences (PNAS). [C1]



New Neuron Communication

Scientists from the Yale School of Medicine had shown how newly created neurons in adults rely on signals from distant brain regions to regulate their maturation and survival.

The new neurons do this before the start of communication with the existing cells in their immediate neighborhood — a key finding that had important implications for using adult neural stem cells to replace brain cells lost by trauma or neurodegeneration.

Those earlier findings appeared in The Journal of Neuroscience. [C2]

All image adapted from originals courtesy of Sonya GiridharBoth approaches are important, but the scientists wanted to find out how the brain decided which method to use to process a sensory input. They looked at mitral cell neurons in the brain's olfactory bulb — the part of the brain that sorts out smells and a common model for studying global information processing.

What happens before new neuron growth?

Using slice electrophysiology and computer simulations, the researchers found that the brain had a clever strategy for ensuring that the neurons' message was being heard. Over the short time scale of a few milliseconds, the brain engaged its inhibitory circuitry to make the neurons fire in synchrony. This simultaneous, correlated firing creates a loud, but simple, signal. The effect was much like a crowd at a sporting event chanting, "Let's go team!"

Over short time intervals, individual neurons produced the same short message, increasing the effectiveness with which activity was transmitted to other brain areas. The researchers say that in both human and neuronal communication alike, this collective communication works well for simple messages, but not for longer or more complex messages that contain more intricate information.

The neurons studied used longer timescales (around one second) to convey these more complex concepts. Over longer time intervals, the inhibitory circuitry generated a form of competition between neurons, so that the more strongly activated neurons silenced the activity of weakly activated neurons, enhancing the differences in their firing rates and making their activity less correlated. Each neuron was able to communicate a different piece of information about the stimulus without being drowned out by the chatter of competing neurons.

It would be like being in a group where each person spoke in turn. The room would be much quieter than a sports arena and the immediate audience would be able to listen and learn much more complex information.

Researchers believe that the findings can be applied beyond the olfactory system to other neural systems, and perhaps even be used in other biological systems.

What happens after new neuron growth?

Certain important synaptic connections--the circuitry that allows the brain cells to talk to each other — do not appear until 21 days after the birth of the new cells, according to Charles Greer, professor of neurosurgery and neurobiology, and senior author of an earlier study on the effects on communication of new neuron growth. [C2]

In the meantime, other areas of the brain provide information to the new cells, preventing them from disturbing ongoing functions until the cells are mature.

It had been established in even earlier studies that several regions of the adult brain continue to generate new neurons, which are then integrated into existing brain circuitry. However the mechanisms that allowed this to happen were not known.

To answer this question, Greer and Mary Whitman, an M.D./Ph.D. candidate at Yale, studied how new neurons are integrated into the olfactory bulb, which helps discriminate between odors, among other functions.

They found that new neurons continue to mature for six to eight weeks after they are first generated and that the new neurons receive input from higher brain regions for up to 10 days before they can make any outputs. The other brain regions then continue to provide information to the new neurons as they integrate into existing networks.

The progress of neuron research

Regarding the earlier research [C2], the discovery of a previously unrecognized mechanism was significant, Charles Greer said at the time, because "if we want to use stem cells to replace neurons lost to injury or disease, we must ensure that they do not fire inappropriately, which could cause seizures or cognitive dysfunction."

The accumulation of these kinds research results over the years has proven significant in understanding how the brain constantly renews itself, guided by the developmental body plan to ensure consistency and renewal.

Remarking on the current research, [C1] Nathan Urban says "Across biology, from genetics to ecology, systems must simultaneously complete multiple functions. The solution we found in neuroscience can be applied to other systems to try to understand how they manage competing demands." Urban is Professor of Life Sciences and head of the Department of Biological Sciences at CMU.

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