Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCB265

Poster Communications

A classic updated - the sodium hypothesis of action potential generation

A. M. Brown1

1. School of Life Sciences, University of Nottingham, Nottingham, Nottinghamshire, United Kingdom.


I wholeheartedly agree with the view of Stephen Kuffler that students should consult original sources (Kuffler, 1972), since by the time such experimental information has reached textbooks it has inevitably been diluted, interpreted and edited to the extent that the original intentions of the researchers may have been obscured or lost (e.g. Purves et al., 2008; Bear et al., 2015). For students of membrane excitability interested in the classic Hodgkin and Huxley era of the 1940s and 1950s, there is a rich history of such information, a considerable amount of which is freely available via The Physiological Society website. However there is one major impediment that stands in the way of students' appreciation of these papers: the conventions and the associated terminology differ from those used today, and can be confusing even for the experienced researcher. This stems from the fact that the accepted norm in physics at the time was that an inward movement of +ve charge was depicted as an upward current deflection, with the resulting change in voltage depicted as a downward deflection (Patton, 1982). Using the classic Hodgkin and Katz 1949 paper to illustrate such difficulties we find that the resting membrane potential was regarded as a positive quantity with the action potential (AP) a negative deflection, thus the AP in this, and the subsequent classic papers by Hodgkin & Huxley, was displayed as an upward deflection with the y-axis defined as -V. Modern practice deviates from this convention and shows inward current as a downward defection and membrane potential as "positive-up, negative-down". The modern convention depicts an electrode inserted into a cell submerged in a bath, with the amplifier connected to ground which is 0 mV, and a resting membrane potential (Em) of about -70 mV. The convention used by Hodgkin & Katz is based upon a reference system where changes in Em were considered relative to rest, with increases or decreases, respectively, being equivalent to hyperpolarization or depolarization, respectively. This was due to technical considerations, since the junction potentials created by switching from seawater to the test solutions in the recording cell made accurate measurements of the potential difference across the axon membrane unreliable. Another source of confusion are their references to deviations of Em from rest being variously described as ‘depression, decrease, diminution, raising, larger, reduce, lowered', each description being context dependent. Using the Hodgkin and Katz paper as a reference I describe how such papers can be updated to the modern convention without altering their meaning or content. This process involves not only substituting the appropriate modern terminology but in some instances redrawing figures and altering axes.

Where applicable, experiments conform with Society ethical requirements