The endplate potential

Stimulation of the motor nerve produces electrical responses in the muscle fibre. Our understanding of the nature of these events was greatly increased when P. Fatt and B. Katz used intracellular electrodes to study the problem in 1951. Fig. 7.3 shows the experimental arrangement which they used. The glass micropipette electrode, filled with a concentrated solution of potassium chloride, is inserted into the muscle fibre in the end-plate region. A suitable amplifier then measures the voltage between the tip of that electrode and another electrode in the external solution, so giving the electrical potential difference across the membrane. The muscle may be treated with curare (an arrow poison used by South American Indians, which causes paralysis) so as to partly block the transmission process. The nerve can be stimulated via a couple of silver wire electrodes.

Fig. 7.4 shows the response seen in the presence of a moderate amount of curare. There is a rapid depolarization of a few millivolts, followed by a rather slower return to the resting membrane potential. This response is not seen if the microelectrode is inserted at some distance from the end-plate, and hence

Fig. 7.4. The end-plate potential of a frog muscle fibre in the presence of curare.
The End Plate Potential

Fig. 7.5. Action potentials produced in a frog muscle fibre by stimulation of its motor nerve axon. In trace a the microelectrode was positioned at the end-plate region, so that the response includes an end-plate potential plus the action potential which it gives rise to. In trace b the microelectrode was positioned at some distance from the end-plate so that no end-plate potential component is recorded. Based on Fatt and Katz (1951).

Fig. 7.5. Action potentials produced in a frog muscle fibre by stimulation of its motor nerve axon. In trace a the microelectrode was positioned at the end-plate region, so that the response includes an end-plate potential plus the action potential which it gives rise to. In trace b the microelectrode was positioned at some distance from the end-plate so that no end-plate potential component is recorded. Based on Fatt and Katz (1951).

it is called the end-plate potential. Increasing the curare concentration reduces the size of the end-plate potential.

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