Trends in Neurosciences
ReviewFrom overshoot to voltage clamp
Section snippets
Finding the overshoot
We had been brought up on the theory of Bernstein [1], according to which the action potential is due to the membrane suddenly becoming permeable to all ions, so that the potential difference across the membrane would fall from its resting value to near zero. This permeability increase had been confirmed experimentally by Kacy Cole and Howard Curtis [2] (Fig. 1). Hodgkin had a hint, from experiments on single nerve fibres of crabs and lobsters, that the action potential might be larger than the
The Na+ idea
Hodgkin and I joined forces again in Cambridge in January 1946. We had already begun to discuss the possibility that Bernstein's increase in permeability during the action potential might be specific for Na+, so that the membrane potential would approach the equilibrium potential for Na+, perhaps +60 mV inside. It was known that excitable tissues lose K+ when active [5] and in 1946 Hodgkin and I used an indirect method to estimate the amount of K+ lost per impulse by a nerve fibre. We found
The voltage clamp
Hodgkin realized that measuring the current–voltage properties of the membrane would require a ‘voltage clamp’ – that is, a feed-back system to control the potential difference across the membrane. Without such a device, the instability that causes the explosive character of the action potential would make such measurements impossible. Cole had also realized this, and in 1947 he used an apparatus of this kind on squid fibres [8]. He showed that there was no discontinuity in the current–voltage
Penetration as free ions
At first, we made calculations on the assumption that free K+ and Na+ would enter the lipid phase of the membrane from one side and dissociate from the other. It was of course known that inorganic ions are insoluble in the bulk phase of lipids but it seemed possible that an appreciable number might cross the bimolecular lipid layer that forms the cell membrane. However, we did not see any prospect of finding a basis for a major effect of membrane potential on the rate of penetration, or of
Penetration in combination with a carrier anion
We therefore switched to considering the possibility that ions (e.g. Na+) cross the membrane in combination with a lipid-soluble molecule that has a large dipole moment. In the resting state, the membrane potential would hold its negative charge at the outer surface of the membrane but, if the internal potential was raised by a stimulus or by an approaching action potential, it would become free to turn round. Na+ is more concentrated in the external fluid than inside the fibre so the rate of
Computing the action potential
The computations include eleven ‘membrane action potentials’ – that is, action potentials in which the whole of the membrane area undergoes the same potential changes simultaneously – and three propagated action potentials. The equations for a membrane action potential are ordinary differential equations and I solved them using a hand-operated calculating machine, first by Adams’ method [16] and later by the method of Hartree [17]. The main equation for a propagated action potential is a
Results
Table 1 summarizes the membrane properties assumed for each run and the outcome of the computations. In the first group of action potentials (runs 1 to 4), the membrane potential did not recover to its resting value unless we assumed both inactivation of the Na+ current and a lag in the rise of the K+ current (run 4). I then computed a propagated action potential (runs 5 to 11) using the same parameters as in run 4. I did not carry the computation further than about half-way down the falling
Discussion
The action potentials computed before our voltage-clamp experiments showed that we could obtain an action potential with roughly the right characteristics by assuming that the Na+ current underwent inactivation, with or without a delay in the K+ current – although when we did not include a delay the falling phase was much too slow. These conclusions certainly helped us in interpreting our voltage-clamp records. The action potential computed after our first voltage-clamp experiments showed that
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