In the resting muscle small fluctuations in membrane potential occur at the endplate region (Fig. 7.12). They follow much the same time course as do end-plate potentials, but are only about 0.5 mV in size, hence they are called miniature endplate potentials. They are reduced in size by curare and increased in size by anticholinesterases, and so it looks as though they are produced by the spontaneous
release of 'packets' of acetylcholine from the motor nerve ending. Kuffler and Yoshikami compared their size with those of responses to ionophoretic application of acetylcholine; they concluded that each miniature end-plate potential is produced by the action of just under 10000 molecules of acetylcholine.
An excess of magnesium ions blocks neuromuscular transmission by reducing the amount of acetylcholine released per nerve impulse. Del Castillo and Katz found that the size of the small end-plate potentials produced under these conditions fluctuates in a stepwise manner. Each step was about the size of a miniature end-plate potential. They therefore suggested that acetylcholine is released from the motor nerve terminals in discrete 'packets' or quanta. The normal end-plate potential is then the response to some hundreds of these quanta, all released at the same time following the arrival of a nerve impulse at the axon terminal. Miniature end-plate potentials are the result of spontaneous release of single quanta.
But why should acetylcholine be discharged from nerve endings in packets of nearly 10000 molecules? The axon terminals contain large numbers of synaptic vesicles about 50 nm in diameter (Fig. 7.2). Similar vesicles have been found in the presynaptic terminals at other synapses where chemical transmission occurs. Del Castillo and Katz suggested that they contain the chemical transmitter substance (acetylcholine at the neuromuscular junction), and that the discharge of the contents of one vesicle into the synaptic cleft corresponds to the release of one quantum of the transmitter.
Confirmation of this idea has since been provided by biochemical separation techniques, especially by Whittacker and his colleagues. A tissue containing a large number of nerve endings (such as brain tissue or electric organ) is first homogenized and then centrifuged. Nearly pure fractions of synaptic vesicles have been obtained from electric organs by this method, and it is found that they contain acetylcholine. They also contain some adenosine triphosphate (ATP), but the function of this is not too clear.
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