What Is an Axon? Experts Offer New Insights on the Role of Axon Design to Signaling Efficiency

The brain is one of the most complicated organs in the human body, composed of almost 100 billion nerve cells or neurons. Such a vast number of cells are interconnected to send signals to each other.

The connections between neurons are made possible through thin fibers called axons. As the threadlike part of the nerve cell that carries out impulses, axons demonstrate flexibility and strength, which make them mysterious to biologists.

What is an Axon, and How Does It Look Like?

Throughout the nervous system, axons act like transmission cables that allow the passage of electrical signals within the regions of the brain and between the brain and the rest of the body. Every neuron has only one axon, which can come in various sizes and lengths.

Some axons are as small as one millimeter, while some are almost one meter long. The longest axons are those of the sciatic nerve, which runs from the base of the spinal cord down to the end of the big toe. It is believed that the larger the diameter of the axon is, the more quickly it can transmit nerve signals.

An axon is usually straighter and smoother than the dendrites or the part of the neuron that receives the input signals. They also contain bundles of microtubules, neurofilaments, and scattered mitochondria.

Investigating the Design of Axons

Neuroscientists have long been puzzled about why axons are designed to be long and spindly. This mystery was solved by a team of bioengineers from the University of California San Diego with their leader Francesca Puppo.

In this study, the median refraction ratio value of almost 12,000 axonal branches was 0.92, which is quite close to the theoretically assumed balance. Using the information from the NeuroMorpho database, the research team investigated a particular kind of neuron called basket cells.

Puppo reconstructs a graph-based model of the axon branches using the 3D morphological data. Then the conduction velocity along the axon is calculated given the diameter at various points.

Finally, the conduction velocity and the length of every axon branch were used in computing the propagation delay.

It was found that all kinds of axons - long, short, straight, and curvy - had a refraction ratio that approached one. When the axons grow with a long and curved shape, the neuron is specifically designed to slow down the action potential of signals. Meanwhile, neurons not signaling at this ratio tend to damage the efficiency of information flow between cells.

In short, axons are designed the way they are to balance the speed of information flow into the neuron relative to the time it takes for the neuron to process that information. Puppo and her team are confident that the new insights they gained from their study can help better understand the complex nature of neurological disorders. It can also revolutionize the way scientists measure the signal flow in the brain and even have a large impact on artificial neural networks used in machine learning.