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Balancing neuronal activity
Specialized sites within neurons are responsible for a rapid balancing of the amount of activity in the brain.

A pyramidal cell from the brain (in magenta) on which the axon initial segment (in green) was visualized by immunostaining.

4th January 2021

To process information in our brains, neurons produce brief electrical impulses, called action potentials, triggered from one highly specialized region at the cell’s axon, called the axon initial segment (AIS). Scientists from the Institute of Neuroanatomy at the Mannheim Center for Translational Neuroscience (MCTN), together with researchers from the Netherlands Institute for Neuroscience in Amsterdam and the University of Göttingen now show that the AIS surprisingly changes with experience: It shrinks with increasing experience and, vice versa, elongates when less input arrives in the brain. This reversible effect could contribute to balancing all neuronal activity generated within a distinct functional network in the brain.

Exploring the environment

Rodents learn about their environment by moving their highly sensitive whiskers, with which they touch objects and, for example, identify food sources. To examine whether neurons change with the number of sensory experiences, researchers placed mice in an environment in which many new objects were present, with variable textures, shapes, and possibilities to explore. As a result of such a rich environment, neurons actively shortened their AIS in a very short time frame. With the shortening, neurons also showed a lower rate by which action potentials were generated, effectively reducing their own excitability. In contrast, when sensory stimuli were not able to reach the brain because the whiskers were impaired, the AIS elongated and produced more electrical impulses. In essence, neurons shaped their own excitability in direct response to the synaptic input they received through the surrounding network.

Structural changes of neurons, which directly affect a single cell’s input and output parameters, are a phenomenon termed ‘plasticity’. Neuronal plasticity is the basis of why we keep learning during our entire life and can adapt to an ever-changing world. Previously, it was thought that these anatomical changes in neurons are primarily occurring at synapses. However, the new study shows that plasticity also occurs at the trigger sites for action potentials, which may be important to balance the amount of neuronal activity and prevent overexcitation. Now the focus is shifting on the molecular machinery that makes such plasticity possible.