A Secret of the Neuronal Communication Code Revealed

A key communication tool of neurons is the action potential, a brief electrical impulse that travels along the axon, a projection of neurons that rapidly transmits information to other neurons, glands, or muscles. Disruption of this process can lead to serious neurological problems, such as epilepsy or muscle weakness. However, the axon is not a simple tubular structure; rather, it resembles an uneven string of pearls with segments of varying diameters. The diameter has a significant effect on the propagation of electrical signals and, as such, should theoretically influence the course of the action potential.

Researchers at the HUN-REN Institute of Experimental Medicine (HUN-REN IEM), led by János Szabadics, used state-of-the-art technologies to study the behaviour of action potentials along axonal segments of varying diameters. Due to the small size of axons, such detailed measurements had not been possible before.

The researchers first examined the mossy fibre axons in the hippocampus. These axons contain both small terminals and unusually large giant terminals. Using electrophysiological measurements, optical techniques, and computational analyses, the researchers demonstrated that the shape of the action potential remains constant.

The research showed that the constant shape of the action potential is due to the uneven distribution of certain ion channels. A specific group of potassium channels, known as the Kv1 family, does not function uniformly along the axon. In narrower segments, where the electrical signal would naturally slow down, the Kv1 channels have a greater effect, speeding up the course of the action potential. As a result, the shape of the action potential remains the same across axonal segments of all diameters. The action potential, standardised along the axon, thus functions as a true "digital" signal.

The researchers also examined action potentials in other types of axons that do not have unusually large structures but vary in diameter, similar to the axonal projections of most known neurons. They found that while the shape of the action potential can vary between different axon types, the diameter does not influence the shape of the signal within a given axon type. The findings of the HUN-REN IEM researchers shed new light on the functioning of neurons. The regulation of ion channel density plays a key role in ensuring that uniform signals travel along axons of varying diameters.

The results, published in PLOS Biology with János Brunner as lead author, offer further insight into the biological processes of neurons.