Excitation of the muscle fibre

In the absence of curare the end-plate potential is its normal size, and the response recorded at the end plate is more complicated in form, as is shown in Fig. 7.5a. The muscle cell membrane is electrically excitable so that it will carry all-or-nothing propagated action potentials just as in the nerve axon. In the absence of curare the end-plate potential is large enough to cross the threshold for electrical excitation of the muscle cell membrane, so that an

Recording circuit lonophoresis circuit

Recording circuit lonophoresis circuit

Fig. 7.6. The ionophoresis technique applied to a frog muscle fibre (a), and the response to a pulse of acetylcholine applied by this method (b). From del Castillo and Katz (1955).

action potential arises from it and propagates along the length of the muscle fibre. The record in Fig. 7.5a is thus a combination of end-plate potential and action potential. At some distance from the end-plate, a 'pure' action potential, free from the complicating effects of the end-plate potential, can be seen, as in Fig. 7.5b.

The ionic basis of the muscle action potential is much the same as in the nerve axon. It is reduced in size or blocked in low sodium ion concentrations or in the presence of tetrodotoxin. So we can assume that the cell membrane contains separate channels for sodium and potassium ions, both types being opened by a suitable change in membrane potential. The muscle action potential triggers the contraction of the muscle, as we shall see in Chapter 10.

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