Fig. 6.5 ). Moreover, small spontaneous depolarizations of the postsynaptic membrane, termed miniature end plate potentials (mEPPs), are observable. The amplitude of the mEPP (≤1 mV) corresponds to that of the smallest EPP evoked under low [Ca++], and the amplitudes of other EPPs were shown to be integral multiples of the mEPP amplitude; thus, it was proposed that each mEPP corresponded
SynthesizedandpackagedinthenerveterminalSynthesizedandpackagedinthecellbody;transportedtothenerveterminalbyfastaxonaltransportSynthesizedinactiveformActivepeptideformedwhenitiscleavedfromamuchlargerpolypeptidethatcontainsseveralneuropeptidesUsuallypresentinsmallclearvesiclesUsuallypresentinlargeelectron-densevesiclesReleasedintoasynapticcleftMaybereleasedsomedistancefromthepostsynapticcellTheremaybenowell-definedsynapticstructureActionofmanyterminatedbecauseofuptakebypresynapticActionterminatedbyproteolysisorbythepeptidediffusingawayterminalsviaNa+-poweredactivetransportTypically,actionhasshortlatencyandshortduration(msec)Actionmayhavelonglatencyandmaypersistformanyseconds •Fig. 6.4 PresynapticCa++ currentanditsrelationshiptothepostsynapticresponse.A, Schematicofasquidgiantsynapsepreparation.Electrodes1and2areusedtovoltage-clampthepresynapticterminalandrecorditsvoltageandcurrent.(NotethattetrodotoxinandtetraethylammoniumwerepresenttoblockNa+ andK+ conductancetoisolatetheCa++ conductance.)Electrode3recordsthemembranepotentialofthepostsynapticaxon.Thepresynapticterminalwasvoltage-clampedtoincreasinglymoredepolarizedlevels(blue traces). Withasmalldepolarization(B), asmallCa++ currentstartsshortlyafterthevoltagestep,continuestogrowforthedurationofthestep(oncurrent),andthendecaysexponentiallyafteritstermination(offortailcurrent).Alargervoltagestep(C) increasesboththeonandtheoffcomponentsoftheCa++ current,andnowdistinctonandoffresponsesareobservedinthepostsynapticresponse.D, ThevoltagestepistotheNernstpotentialforCa++ ,sothereisnoCa++ currentduringthestep,butalargetailcurrentandoffresponseareobserved.(BasedondataofLlinasRetal.Biophys J 1981;33:323.) to the release of transmitter from a single vesicle and that EPPs represented the combined simultaneous release of transmitter from many vesicles.
This linking of mEPPs and vesicles implies that each mEPP is caused by the action of many molecules of neurotransmitter binding to postsynaptic receptors. The alternative that each mEPP could be caused by a single transmitter molecule binding to and opening a single postsynaptic receptor was rejected, in part because responses smaller in amplitude than mEPPs could be generated experimentally by directly applying dilute solutions of acetylcholine to the muscle. In fact, mEPPs were calculated to be caused by the action of approximately 10,000 molecules, which corresponds well to estimates of the number of neurotransmitter molecules contained within a single vesicle.
Many additional studies have confirmed the vesicle hypothesis of neurotransmitter release. For example, biochemical studies have shown that neurotransmitter is concentrated in vesicles, and fusion of vesicles to the plasma membrane and their depletion in the terminal cytoplasm after action potentials have been shown with electron microscopic techniques.