Neurons cooperate to get their voices heard

Researchers at Carnegie Mellon University and the University of Pittsburgh have investigated how neurons are able to relay their messages without them getting lost among the other signals sent out by the billions of other neurons in the brain.

Neurons interpret messages from the sensory organs: eyes, ears, nose, tongue and skin, and respond to them by releasing an electrical signal, called the action potential or “spike.” These spikes are generated by a change in the concentration of charged ions across a neuron’s membrane, and can therefore be interpreted by neighboring cells so that the signal is carried on to a final destination. When a neuron spikes it is said to have “fired.”

In the brain, there are also groups of neurons called “inhibitory circuits” which can regulate spikes of other neurons. These circuits are directly involved in facilitating the systems that respond to signals from the sensory organs, including the olfactory system, which is responsible for interpreting smells. The team’s investigation focused on the olfactory system.

This system is a common model for scientists who study how the brain processes and responds to multiple stimuli, because of the many different and competing smells that can be picked up by the nose.

Specifically, the team observed the response of mitral cells in the olfactory bulb of the brain. These cells are unique in the olfactory pathway in that they both receive input and relay output messages.

The main methods of observation used by the researchers were slice electrophysiology and computer simulations. Slice electrophysiology measures the electrical properties of neurons in the brain and how they change over time. These measurements are taken in brain “slices,” neurons that all lie in the same horizontal plane.

Their findings, reported in the online journal Proceedings of the National Academy of Sciences earlier this month, show a time-dependant relationship between the effects of the inhibitory circuit and the method by which the message generated by the spike is relayed.

On a short timescale, several milliseconds, the olfactory bulb’s inhibitory circuit causes the mitral cells to spike in synchrony. The spikes are then said to be “correlated.” The result is a much stronger signal than would be achieved if only one cell fired, comparable to a crowd of sport spectators all shouting in support of their team at the same time.

The results were different for messages relayed on a longer timescale of about one second. Signals that are transmitted by neurons on short timescales are simple ones, quite different from those that take a longer time to be transmitted. These sometimes require a larger number of neurons to fire, or require the message to be sent to several parts of the body, or even require multiple messages to be sent.

For these more complex messages, the inhibitory circuit generates a form of competition between the mitral cells, so that neurons with larger spikes effectively silence those with smaller spikes. This method decreases cooperation between the cells, and is comparable to a group of people with every person speaking. People who speak quietly or less forcefully would not be able to get their message across because of the comparable loudness and forcefulness of other people’s voices.

Nathan Urban, a professor of life sciences at Carnegie Mellon and one of the authors of the results, said in an interview with www.futuriy.org that the desire to have your message heard is not unique to neurons, but can be found across many fields of biology.

“The solution we found in neuroscience,” he said, “can be applied to other systems to try to understand how they manage competing demands.”