Wednesday, March 18, 2015
The Squid Giant Axon
This post is dedicated to the squid giant axon (not the giant squid axon, although there is presumably a giant squid giant axon – and it’s really big!) These axons carry information to the muscles of a squid’s mantle when it is startled, causing them to contract and jet to safety. These axons are notable because they are so large – up to 1mm in diameter. If this doesn’t seem large to you, consider that typical axons in humans are only a few micrometers in diameter. The squid giant axon is several hundred times larger than the typical human axon. You can see the axon in question in this diagram, labeled “III” (It turns out that the axons commonly studied are the third step in the chain of large axons that carry this specific information; hence they are often referred to as “tertiary giant axons.”)
If you haven’t heard of the squid giant fiber system before, you are probably thinking “So what?” Well, I’ll tell you what. Nowadays, we have technologies that let us interact with various neurons in various ways. For example, we can use tiny glass pipettes to inject current or voltage into a neuron or record its activity. We can use arrays of electrodes to do the same thing with a large population of neurons. These procedures are rather routine in neuroscience, and are done with many different types of neurons in a great variety of animals and specific preparations.
When J. Z. Young was dissecting squid in the 1930’s, however, the techniques available to him were not so refined. He devised a way to isolate a single neuro-muscular unit from the rest of the squids anatomy and manipulate it (see The Function of the Giant Nerve Fibres of the Squid for his description of the procedure – I highly recommend this article, as he’s a great writer and it really is a classic in the history of neuroscience.) Although there were already theories of action potential conduction (notably, Bernstein’s theory that action potentials propagated due to changes in ions flowing across the cell membrane, which turned out to be correct,) Young’s preparation allowed him to directly demonstrate basic properties of single nerve cells. This allowed theories about neuronal function to be empirically tested at a whole new resolution. For example, in the paper cited above, he clearly demonstrates the all-or-none nature of action potentials (that is, when neurons are stimulated, they have a binary response: they either send an action potential down their or they don’t. There are no graded, partial responses.)
Young’s technique opened up the squid giant axon as a model system for many investigators who were trying to understand the behavior of neurons. Notably, Hodkin and Huxley developed a quantitative model of the propagation of action potentials using this preparation, in a famous series of papers that are summarized in A Quantitative Description of the Membrane Curent and its Application to Conduction and Excitation in Nerve. Essentially, the squid giant axon preparation gave researchers an incredible tool, with which they developed the basic models and techniques (for example, the development of voltage clamp by Kenneth Cole in the 1940’s, which allowed the ionic basis of action potentials to be investigated.)
In short, the basic electrophysiological techniques that are in use today almost all stem from Young’s work with the squid giant axon.
On a tangentially related note, Young spent much of the rest of his career trying to convince the scientific community that invertebrates, especially cephalopods, were good model animals with which to study neuroscience. At length, he’s convinced me, as well as (at least some) contemporary scientists, as evidenced by this recent review of the octopus as a model organism for studying memory systems (The Octopus: A Model for a Comparative Analysis of the Evolution of Learning and Memory Mechanisms ).
I have my own ideas about why it’s particularly good to study octopus; but alas, that’s for another post.
