Some of you might be thinking I chose a rather odd name for this blog. What is an axon? What’s a squid got to do with science? And if squids have an axon, do we have one too? If you have asked yourself any of these questions then read on, because I’ll be telling you the story of how squids have helped us unveil some of the most fundamental secrets about the nervous system.
Squids, like us, have a nervous system that is made up of cells called neurons. What makes neurons special compared to most other cells is that they are electrically excitable. This means that, like a battery, they can generate electrical impulses. Every neuron has an axon, which is a long and thin filament projecting out of the cell’s body. The axon has the function of propagating the electrical impulses, also known as action potentials, in a similar way to what a wire or a cable would do.
Squids are equipped with one particularly long and chunky axon, called the giant axon because of its size. It runs from the head to the tail of the animal and its purpose is to control tail movements, enabling the squid to quickly move its body away from predators. Its size can be of up to 1mm in diameter for large animals (the average size of axons in our brain is 1µm, that is one-thousandth time smaller than the giant axon of the squid!), which makes it the perfect preparation to conduct experiments on.
By the end of the 1940s scientists Alan Hodgkin and Andrew Huxley started investigating the electrical properties of neurons on the squid’s axon. At that time the only thing that was known about neurons was that the voltage inside the cell is lower than it is on the outside. So, basically, there are less positive charges inside the cell than there are outside, and the two environments are separated by the cell membrane. This difference between charges is called membrane potential and, when the cell is resting, it is around -70mV. They also knew that neurons could generate very short electrical impulses known as action potentials, and when this happened the membrane potential quickly reversed its sign, going from -70mV to +30mV and back to normal in just a few milliseconds. Since an electrical current is caused by movements of charges and, in biological systems, these charges are ions (most commonly sodium, potassium, and chloride), the way an action potential is generated in a neuron has to be linked to the movement of one or more of these ions across the membrane.
Therefore, Hodgkin and Huxley designed a series of very clever experiments using the new voltage-clamp technique to work out exactly what currents flow through the membrane and what ions are involved in the generation of the action potentials. They then developed a mathematical model to describe how the excitability of neurons is determined by the movements of sodium and potassium through the membrane. Their work is even more impressive if we consider that, at the time they carried out these experiments, electron microscopy hadn’t been invented and no one even knew about the existence of channels selective for sodium and potassium on the cell’s membrane. Their model was simple and elegant yet remarkably accurate, and their contribution to the understanding of neuroscience had their names and that of Sir John Eccles added to the list of Nobel prize winners in 1963.
Oh mighty axon of the squid, we have to thank you and the minds of these remarkable scientists if today we know what we know about the brain. Which arguably isn’t that much, but still, a lot more than we did 50 years ago. And don’t you worry, the squid was only the starter, the main course is yet to come!