• DOI:10.1113/jphysiol.1953.sp004863
• Corpus ID: 33962959

The mobility and diffusion coefficient of potassium in giant axons from Sepia

@article{Hodgkin1953TheMA,
  title={The mobility and diffusion coefficient of potassium in giant axons from Sepia},
  author={Alan Lloyd Hodgkin and Richard Darwin Keynes},
  journal={The Journal of Physiology},
  year={1953},
  volume={119}
}

• A. Hodgkin, R. Keynes
• Published 30 March 1953
• Biology
• The Journal of Physiology

The object of the experiments described here is to measure the mobility and diffusion coefficient of the potassium ions inside the giant axons of Sepia officinalis. The principle of the method is to make a short length of axoplasm radioactive by allowing internal potassium to exchange with externally applied 42K in a restricted region. If a fibre which has been treated in this way is washed with sea water and placed in oil, most of the 42K remains in the axoplasm since the extracellular space is relatively small and the external potassium concentration is low compared to the internal concentration. The movement of the radioactive potassium inside the nerve fibre under the influence of diffusion or an electric field can then be studied by moving the fibre horizontally over a suitably screened Geiger counter. In the theoretical section it is shown that the mobility can be obtained from the velocity with which the radioactive patch drifts along the fibre under the influence of an applied voltage gradient, while the diffusion coefficient can be calculated from the extent to which the patch broadens during the experiment. The voltage gradient along the axis cylinder may be measured with an external electrode since internal and external gradients should be equal near the middle of a sufficiently long interpolar region (Hodgkin & Rushton, 1946). A preliminary account of this work was given at the XVIIIth International Physiological Congress (Hodgkin & Keynes, 1950). The discussion contains a brief account of further experiments of the type described by Keynes (1951) and Keynes & Lewis (1951 a). These suggest that at least 90% of the total potassium is free to exchange with radioactive potassium, and therefore provide evidence that the results of the present study apply to the bulk of the potassium inside the axoplasm.