Quantal components of the end-plate potential.
In this paper a further study is made of the spontaneous synaptic potentials in frog muscle (Fatt & Katz, 1952a), and their relation to the end-plate response. It has been suggested that the end-plate potential (e.p.p.) at a single nerve-muscle junction is built up statistically of small all-or-none units which are identical in size with the spontaneous 'miniature e.p.p.'s'. The latter, therefore, could be regarded as the least unit, or the 'quantum', of end-plate response. A convenient picture of how hundreds of such quanta, each capable of producing a miniature potential of 0 5-1 0 mV, can build up an e.p.p. of, say, 70-80 mV is provided by the hypothesis that separate parcels of acetylcholine (ACh), released from discrete spots of the nerve endings, short-circuit the muscle membrane. The unit changes of membrane conductance produced at many parallel spots summate and lead to an intense depolarization of the muscle fibre. Although this is a plausible view, there is no direct proof that the normal e.p.p. is made up in this quantal fashion. The evidence comes from experiments in which the 'quantum content' of the e.p.p. had been reduced to a small number by lowering the external calcium concentration (Fatt & Katz, 1952 a). It was then found that the size of the end-plate response approached that of the spontaneous potential and at the same time exhibited large random fluctuations, apparently involving steps of unit size. Similar observations were made by Castillo & Engbaek (1954) on muscles treated with Mg-rich solutions. The statistical behaviour of the end-plate response under these conditions has been investigated in more detail and subjected to a quantitative analysis.