What everyone should know about action potential

- As basic as it is, it is easy to forget what you have learned, let alone what you never knew. To begin with, how is action potential generated?

- The action potential is an elementary neural phenomenon, an electrical impulse that is considered a key element in the transmission of neural information. They are fast running, but strong, high amplitude electrical signals that travel along nerve fibres, allowing information to be transmitted from one nerve cell to another. Such a neural action potential can be broken down into two main stages. There is an ascending phase, during which the cell membrane voltage rises, and then, after reaching a peak, the descending phase, during which the membrane voltage returns to the ground state. 

- The Little Prince could understand this if you drew a pointed straw-hat shape. . . 

- What is important is that these stages are controlled by the well separated and coordinated action of different ion channels in time. The up-phase is caused by the rapid opening of voltage-dependent sodium channels, while in the down-phase, potassium channels pull the membrane voltage back to the resting range. It should be emphasized that previous observations have shown that the strength of the neuronal connections is closely related to the shape of the action potential. Specifically, the slower the action potential decays, the stronger the connection becomes. Since brain functioning is largely based on the strength and dynamics of these connections, it is essential to understand what factors and regularities influence the shape of the action potential.

- Explained in this way, it is remarkably simple. It just had to be discovered.

How long has the law describing the relationship between the change in the shape of the action potential and the spatial morphological properties of a given phase been known?

- Since the 1970s, it has been assumed that the shape and propagation of the action potential in nerve fibres depend to a large extent on the local size variations of the axon. Indeed, one of the characteristics of nerve fibres is that they are extremely variable in shape. The nerve endings on them and the sections that connect them show a pattern that can be compared to a string of pearls. However, a review of the scientific results describing the relationship between nerve cell morphology and the action potential curve shows that the assumptions are mostly supported by theoretical considerations or computer simulations. The reason for this is simple: most nerve fibres in the brain are extremely thin, often less than a thousandth of a millimetre in diameter, and have so far been virtually inaccessible to direct electrical measurement techniques. Direct measurement is crucial to ensure the right signal-to-noise ratio and the necessary high temporal resolution.

- So it's up to you to discover how to make measurements. Why now?

- In the lab, we have been successfully performing electrophysiological measurements directly from individual nerve cell extensions smaller than the cell body for a very long time. For example, we have already paid particular attention to the study of giant terminals in the moharosts of hippocampal granule cells, but we have also done research in which we have made direct measurements from small diameter dendrites of granule cells. Building on these foundations, we decided that the next step would be to investigate the electrical properties of extremely thin cortical nerve fibres, which had previously been considered inaccessible! I would say that we had both the necessary expertise and the ideal instrumentation to be able to carry out this difficult experiment with persistent fine-tuning of the technique. 

- And you have done it!

- Finally. Science rarely runs smoothly, and as we pushed the technical boundaries, a new kind of difficulty came to the fore. In electrophysiological measurements, the cell under test is connected to the entire measuring system through a tiny glass capillary, which can distort the electrical activity of the cell. When measuring whole nerve cells, this distortion is usually negligible when using appropriate compensation circuits. However, when measurements are made on such small cell extensions, this signal distortion suddenly becomes significant. Recognising this, we have started further developments to complement and confirm our results. One approach we took was to develop a complex computer model that helped to clean the voltage signals from these distortions. The other approach was to optically measure the axonal stress signals using a special fluorescent dye. This dye is embedded in the cell membrane and indicates membrane voltage by changes in the intensity of the light emitted. The dye works extremely fast, it can accurately track the fastest voltage changes and so we were able to visualise the action potential along the axon using a fast and sensitive camera. 

- So the puzzle had all the pieces, but no one before you had thought to put it together. Congratulations. But what could have "forced" the nerve cells to try to maintain its running properties through the trenches?

- That's a great question! It is perhaps worth pointing out that the action potential is the signal that triggers communication between neurons, and its course is crucial for neural connections. To use a metaphor, the role of the action potential can best be thought of as that of a starting gun in an Olympic running race. When it is fired, all the connections of a given neuron are activated simultaneously. If the shape of the action potential depends on the local size of the nerve fibre, according to computer simulations, then the start signal can vary from point to point along the axon. It may weaken in some places and strengthen in others. In other words, the action potential would not be a single, digital signal, but would vary analogously according to the size of the axon. To use the starting gun example, a foot race would start where instead of a single gunshot, some would barely hear a gunshot, and others would hear a cannon firing. This would mean that an unrelated property - the local diameter of the nerve fibre - would have a significant effect on the strength of each synaptic connection! It is also easy to imagine that this would lead to a functional disorder in the network of neurons. However, we have observed that nerve cells can maintain the digital character of the electrical signal propagating along the axon by varying the distribution of voltage-dependent ion channels as a function of local diameter. 

- Truth, as it always does, has finally triumphed over theory! But! In the introductory section on the formation and evolution of the action potential, sodium and potassium ions were discussed, and in this regulation only the density of potassium ion channels changes. Why?

- In axons, the distribution of sodium channels is also very precisely regulated, but these channels mainly control the action potential uptake, so they play a role in reliable signal transmission rather than in regulating the strength of communication. In terms of synaptic connections, however, the duration of the action potential is the more dominant property, and this is controlled by the potassium channels, so we have focused on the potassium channels, i.e. we have studied the potassium currents. Indeed, electrophysiological measurements allow us to record the voltage changes generated by the operation of the channels and the underlying ionic currents.

- How?

- One of our methods was to take small pieces of nerve fibre cell membrane and measure the current of ion channels in the membrane on these samples. This technique allowed us to map the distribution of the ion channels very accurately, because we knew exactly the size of the detached piece of membrane and also from which part of the axon it originated. The experiment showed that the small diameter axon sections had stronger potassium currents than the large diameter axon terminals, suggesting that there were more potassium channels. We then used targeted inhibitors to find that of the different potassium channels, Kv1-type channels are more abundant in small diameter axon terminals. 

- Could we influence the distribution of these channels, which regulate the strength of synaptic connections, for clinical - therapeutic purposes?

- Investigating the distribution of the channels would have required a different set of experiments, so we did not investigate this now. However, it is known that the exact spatial distribution of Kv1 channels is regulated by a specific signal transduction mechanism. Elements of this pathway may play a role, for example, in certain hereditary epileptic conditions, and gene knockout of some components of the pathway can lead to severe seizures in experimental animals. Although gene knockout does indeed alter the distribution of potassium channels in the membrane, disruption of the pathway has much more complex effects. Among other things, genetic manipulation results in altered levels of glutamate receptors and has clear developmental consequences.

- How general can your findings be considered?

- We were also deeply interested in this question, so we included other types of nerve fibres in the study. To do this, we randomly measured the electrical behaviour of different axons and subsequently determined the type of axon by detailed anatomical analysis. Among the fibres examined were those that were extensions of stimulating cells from other cortical regions, as well as those that were derived from a particular type of inhibitory cell. We also examined an axon type that originated in the hippocampus from a subcortical region. 

We found that, although the shape of the action potential differed between different fiber types, it was independent of the local size of the axon within a given fiber type. This suggests that our finding is generalizable to axons.