Talk is silver, silence is golden: effects of refractoriness on neuronal signalling
Click to enlarge. See image caption below.
Neurons communicate by sending pulse-like signals, so-called action potentials, to each other. However, the time at which neurons in the cortex of the brain send these action potentials (or, shorter, ‘spikes’) seems random to the observer. Lacking the knowledge of how the cells are individually connected, there is no way to say which course a signal has taken through the network. Therefore, scientists have often modelled the series of spike times of a neuron as a random process. But even though the time of the next spike appears to be random, there is some regularity to it: For example, it is very unlikely that a neuron produces two successive spikes in a very short time. This effect is called ‘effective refractoriness’, meaning that after each spike, the neuron remains silent for a short while. One might think that refractoriness is unfavourable for information transmission, since the neuron cannot convey any message while it is refractory. Actually, quite the opposite is the case.
In a recent study that has been published in the August issue of Physical Review E, scientists from the Bernstein Center Freiburg, the RIKEN Brain Science Institute in Japan and the Max-Planck Institute for Mathematics in the Sciences in Leipzig, present the results of their investigation on the effects of refractoriness (Deger et al., PRE 82, 021129). Deger and his colleagues found through mathematical analysis of the behaviour observed in neurons that refractoriness enables a population of neurons to amplify changes in the input. The scientists could show that a rapid increase of the input signal leads to a brief overshoot of the activity in the population. On the other hand, a decrease in the input leads to a momentary undershoot. This mechanism inherent to nerve cells all over the brain can be used to very efficiently detect changes in the environment. For example, a sudden change in light intensity might signal to a small animal that a predator is approaching. Having the nervous system responding to sudden changes with an especially strong signal is therefore advantageous.
Taken together, the study describes the effects of neuronal refractoriness in a versatile mathematical framework. The theoretical tools developed here can help scientists to explore and to understand other, more complex features of the brain's signal processing in the future – by appreciating the constraints of neuronal functioning.
Refractoriness leads to an amplification of changes in input signals - a mechanism useful for signal detection.
Contact:
Moritz Deger
deger@bcf.uni-freiburg.de
ph.: +49 (0)761 203 9503
fax: +49 (0)761 203 9559
Bernstein Center Freiburg
Hansastr. 9A
79104 Freiburg, Germany