Replacement of the Axoplasm of Giant Nerve Fibres with Artificial Solutions

When discussing the electrophysiology of nerve and muscle fibers in Section 6.1 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I write

The axoplasm has been squeezed out of squid giant axons and replaced by an electrolyte solution without altering appreciably the propagation of the impulses—for a while, until the ion concentrations change significantly.

Really? The axoplasm can be squeezed out of an axon like toothpaste? Who does that?

Their methods section (with my color coding in brackets) states

A cannula [red] filled with perfusion fluid [baby blue] was tied [green] into the distal end of a giant axon [black] of length 6-8 cm. The axon was placed on a rubber pad [dark blue] and axoplasm [yellow] was extruded by passing a rubber-covered roller [purple] over it in a series of sweeps…

I like how a little mound of axoplasm piles up at the end of the fiber (yellow, right). They continue

After an axon had been extruded and perfused it was tied at the lower end, filled with perfusion fluid and impaled with an internal electrode by almost exactly the same method as that used with an intact axon…

One might suppose that this would be disastrous and axons were occasionally damaged by the internal electrode. However, in many instances we recorded action potentials of 100-110 mV for several hours.

This experiment is a tour de force. I can think of no better way to demonstrate that the action potential is a property of the nerve membrane, not the axoplasm.

You may already know Hodgkin, but who is Baker?

Hodgkin was coauthor of an obituary of Peter Frederick Baker (1939-1987), published in the Biographical Memoirs of Fellows of the Royal Society. After describing Baker’s childhood, Hodgkin wrote that he met the undergraduate Baker

when he had just obtained a first class in biochemistry part II. Partly at the suggestion of Professor F. G. Young, Peter decided that he would like to join Hodgkin’s group in the Physiological Laboratory in Cambridge. He also welcomed the suggestion that he should divide his time between Cambridge and the Laboratory of the Marine Biological Association at Plymouth, where there were many experiments to be done on the giant nerve fibres of the squid.

Hodgkin then describes Baker’s experiments on internal perfusion of nerve axons.

Peter started work at Plymouth with Trevor Shaw in September 1960 and almost immediately the pair struck gold by showing that after the protoplasm had been squeezed out of a giant nerve fibre, conduction of impulses could be restored by perfusing the remaining membrane and sheath with an appropriate solution… Later, Baker, Hodgkin and Shaw… spent some time working out the best method of changing internal solutions while recording electrical phenomena with an internal electrode. It turned out that it did not matter much what solution was inside the nerve fibre as long as it contained potassium and little sodium. Provided that this condition is satisfied, a perfused nerve fibre is able to conduct nearly a million impulses without the intervention of any biochemical process. ATP is needed to pump out sodium and reabsorb potassium but not for the actual conduction of impulse.

There were also several unexpected findings of which perhaps the most interesting was that reducing the internal ionic strength caused a dramatic shift in the operating voltage characteristic of the membrane… This effect, which finds a straightforward explanation in terms of the potential gradients generated by charged groups on the inside of the membrane, helped to explain several unexpected results that were sometimes thought to be inconsistent with the ionic theory of nerve conduction.

Baker went on to perform an impressive list of research projects (his obituary cites nearly 200 publications). Unfortunately, he died young. Hodgkin concludes

Peter Baker’s sudden death from a heart attack at the early age of 47 has deprived British science of one of its most gifted and versatile biologists. He was at the height of his scientific powers and had many ideas for new lines of research, particularly in the borderland between molecular biology and physiology.

Both Baker and Hodgkin appear in this video. They are demonstrating voltage clamping, not internal perfusion.