Ionic current flow during the endplate potential

Fatt and Katz suggested that the end-plate potential was produced by a general increase in the ionic permeability of the postsynaptic membrane. A closer look at the problem was provided by A. and N. Takeuchi, who used a voltage-clamp technique on frog muscle fibres to examine the postsynaptic current flow during the response to a nerve impulse. They found that the duration of the end-plate current flow is briefer than the end-plate potential, as is shown in Fig. 7.7. This is because the 'tail' of the end-plate potential is caused by a recharging of the membrane capacitance, which does not occur when the membrane potential is clamped.

The membrane potential can be clamped at different values. When this is done it is found that the amplitude of the end-plate current varies linearly with membrane potential (Fig. 7.8). This linear relation between current and voltage is just what we would expect from Ohm's law: it indicates that the conductance of the membrane at the peak of the end-plate current is

Fig. 7.8. Results of a voltage clamp experiment on a curarized frog muscle fibre, showing that the peak end-plate current varies linearly with membrane potential. From Takeuchi and Takeuchi (1960).

constant and not affected by the membrane potential. This is in marked contrast to the situation in the nerve axon, where the sodium and potassium permeabilities of the membrane are most strongly altered by changes in membrane potential.

The point at which the line through the experimental points in Fig. 7.8 crosses the voltage axis is called the reversal potential. If the membrane potential is made more positive than this we would expect the end-plate current to flow in the opposite direction, as indeed it does.

The reversal potential is the membrane potential at which the net ionic current is zero. If only one type of ion were flowing, the reversal potential would be at the equilibrium potential for that ion; for example, it would be about +50 mV if only sodium ions were flowing. But if more than one type of ion flows during the end-plate current, then the reversal potential will be somewhere between the various equilibrium potentials for the different ions. The actual reversal potential, at about —15 mV in the Takeuchis' experiments, is compatible with the idea that both sodium and potassium ions flow during the end-plate current. If we alter the equilibrium potential for one of the ions involved, then the reversal potential will also alter. The Takeuchis did just this by changing the ionic concentrations in the external solutions. They found that alterations in sodium and potassium ion concentrations both altered the reversal potential, whereas alterations in chloride ion concentration did not. This means that the end-plate current consists of a flow of sodium and potassium ions. To put it another way, acetylcholine increases the permeability of the postsynaptic membrane to both sodium and potassium ions simultaneously.

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