The response to acetylcholine

We have seen that stimulation of the motor nerve causes the release of acetylcholine. Is the end-plate potential a direct result of the action of this acetylcholine on the postsynaptic membrane? Clearly a good way to test this idea is to apply acetylcholine to the postsynaptic membrane and see if it produces a depolarization. If it does not, we shall have to think again, but if it does, then the idea becomes much better established as a result of passing this test.

The best way of applying acetylcholine to the postsynaptic membrane is by means of a technique known as ionophoresis or iontophoresis. Acetylcholine is a positively charged ion (Fig. 8.1), and so it will move in an electric field. Hence it can be ejected from a small pipette by passing current through the pipette, as is shown in Fig. 7.6. Thus a brief, highly localized 'pulse' of acetylcholine can be applied to the postsynaptic membrane. Such a pulse produces a depolarization of the muscle fibre which is very similar to the end-plate potential (Fig. 7.6b). The pharmacological properties of the response are also

Fig. 7.7. Measurement of the end-plate potential and end-plate current in a curarized frog muscle fibre. Trace a shows the EPP, recorded in the usual manner. Trace b shows the EPC recorded from the same end-plate with the membrane potential clamped at its resting level. From Takeuchi and Takeuchi (1959).

Fig. 7.7. Measurement of the end-plate potential and end-plate current in a curarized frog muscle fibre. Trace a shows the EPP, recorded in the usual manner. Trace b shows the EPC recorded from the same end-plate with the membrane potential clamped at its resting level. From Takeuchi and Takeuchi (1959).

similar to those of the end-plate potential; it is reduced in the presence of curare, for example. So it seems very reasonable to conclude that the end-plate potential is indeed produced by the acetylcholine which is released from the motor nerve ending.

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