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Axon Test Chamber

This axon test chamber, not used in physiology anymore, was the apparatus used by Drs Hodgkin and Huxley for their prize-winning work in describing the action potential on a giant squid axon.

The device includes two compartments, a lower section filled with oil and a larger upper section filled with seawater or paraffin. A glass panel in between prevents the oil from the bottom compartment from rising to the surface. The compartments themselves sit on the vertical plate of a micromanipulator of an optical microscope. The microscope has a horizontal optical axis and the plate can be moved vertically to adjust to the optimal position to observe the axon.

A diagram of the axon test chamber used by doctors Hodgkin and Huxley. The test chamber is a cube shaped tank with two compartments. The top compartment contains seawater. The bottom compartment contains oil. The compartments are separated by glass plates which have a small gap between them. The fibre goes between these glass plates. The fibre is connected to an electrode in the lower compartment and to the microelectrode inside the cannula. The cannula is above the test chamber and held by a rod. The test chamber can be moved up and down by a movable mechanical stage.

Figure 1: A diagram of the axon test chamber used by Drs Hodgkin and Huxley. A - fixed plate of mechanical stage microscope. B - a movable plate of the mechanical stage. C - shelf attached to B. D - the upper compartment of recording cell normally filled with seawater. E - lower compartment normally filled with oil. F - glass tubes conveying leads to stimulating electrodes. G - fibre. H - cannula. I - microelectrode. J - glass plates forming a partition between compartments. The fibre passes through a hole formed by notches that have been ground in the edges of the plates. K - handle for closing partition. L - holder for cannula. An adjustable joint at the base of the holder is not shown. The external electrode in the upper compartment and the arrangement of mirrors have been omitted.

The freshly dissected axon is attached so that one end is hanging in the higher compartment, and the other end is sitting in the lower compartment. Multiple recording and stimulating electrodes are then connected to the axon.

These electrodes, usually a glass capillary surrounding a silver wire, permit the use specific currents (current clamp mode) or specific voltages (voltage clamp mode).