How Squid Axons Helped Unlock the Secrets of Nerve Signals

Squid axons can be 10 to 100 times larger than human axons, giving researchers a rare way to study how electrical signals travel through neurons.

Mind

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Squid played an unexpectedly important role in the history of neuroscience because some of their axons are enormous compared with those in humans. The axon is the long part of a neuron that carries an electrical signal away from the cell body. In many human neurons, axons are tiny enough that directly measuring their electrical activity is extremely difficult. Some squid have giant axons that can reach about 1 millimeter across. That unusual size gave researchers enough physical room to insert very fine recording equipment and watch changes in electrical voltage during an action potential. An action potential is the rapid electrical event that lets a neuron send a signal along its axon. Experiments on these large squid axons helped reveal that the electrical state of a neuron changes dramatically during an action potential. Researchers were also able to study how charged particles such as sodium and potassium move through the cell membrane and contribute to the signal. A technique called voltage clamp made it possible to hold the membrane at selected voltages and examine the contribution of different ions in much greater detail. The resulting work became a major foundation for modern electrophysiology and for mathematical descriptions of how nerve signals are generated and transmitted. Later research built on these ideas to study individual ion channels and other electrically active cells. The surprising lesson is not that squid have human-like brains, but that their unusually large nerve fibers provided a practical experimental window into basic electrical processes shared by neurons.

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