Cable Properties of Neurons
Objective 6: Summarize cable properties of neurons.
Many schoolchildren in the United States are familiar with the completion of the Transcontinental Railroad in 1869 and how it transformed US history. Fewer are familiar with an event that happened about the same time: the successful deployment of a transatlantic telegraph cable between the United States and United Kingdom in 1866.
The Scottish scientist William Thomson was one of the greatest scientists of the 19th century. Along with Michael Faraday, whose work we met when we discussed the Nernst (Equilibrium) potential and the incorporation of the Faraday constant, Thomson made many key discoveries in thermodynamics, electricity, and magnetism. Among the most important, and yet least-known, of his works was making a 3000 km-long undersea telegraph cable feasible.
Several technological problems confronted the scientists and engineers who tried to lay the first transatlantic cable. Unlike telegraph cables that were surrounded by insulating air, undersea cables were surrounded by an electrically conductive medium (sea water) and this presented several technical problems. (Most of these are irrelevant for right now, but are detailed in this article.)
We have already seen the theoretical basis for one of Thomson’s greatest achievements: the telegraph cable equivalent of the node of Ranvier. Remember that even the best insulator has some leakage. This leakage, combined with a physical property called impedence, means that the electrical signal traveling down an axon gets smaller and smaller as it travels along the axon. The nervous system solves this problem by occasionally removing the myelin sheath at the node of Ranvier, where voltage-gated sodium and potassium channels “boost” the now-barely-above-threshold signal into a new action potential.
Thomson’s equivalent of the node of Ranvier was an instrument called the mirror galvanometer, which boosted a small telegraph signal periodically so that telegraph cable operators could avoid using huge voltages which would burn out the cable. For this achievement, and his other work on the transatlantic cable, Queen Victoria knighted Thomson in 1866. His record of achievement continued, and by the end of his life was so great he was made the first British scientist to join the House of Lords, in 1892.
Now we know him as Lord Kelvin, and the absolute temperature scale is named after him. Temperatures in Kelvin have the same increment as the Celsius scale, but the zero is set to absolute zero instead of the freezing point of water. (–273°C = 0 K; human body temperature of 37°C = 310 K, which was the number we used in deriving the Nernst equation.)
Much of the physics involved in deploying and operating the undersea telegraph cable applies to neurons as well. One has a cable (the cytoplasm of the axon, called the axoplasm for short); an insulating sheath (gutta-percha for the transatlantic cable, myelin for the axon); the cable/axo