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Unformatted text preview: 109 J.Physiol. (I952), II7, I09-I28 SPONTANEOUS SUBTHRESHOLD ACTIVITY AT MOTOR NERVE ENDINGS BY P. FATT AND B. KATZ From the Biophysics and PhysiologyDepartments, UniversityCollege, London (Received5December1951) Thepresentstudyarosefromthechanceobservationthat end-plates of resting musclefibresaretheseatofspontaneouselectric discharges whichhavethe characterofminiature end-platepotentials. The occurrence ofspontaneous subthreshold activity at an apparently normal synapse isofsome general interest,and afull descriptionwillbe givenhereof observationswhichhave beenbrieflyreportedelsewhere (Fatt & Katz,1950a). METHOD Theintracellularrecordingtechnique (Ling & Gerard,1949;Nastuk & Hodgkin,1950)was used inmostexperiments,confirmatoryevidencebeingobtainedwithexternalrecordinginafewcases. The adaptation ofthe microelectrode technique to a studyofthe motor end-platehas been describedindetailina previouspaper(Fatt& Katz,1951). Thepreparationwas them. ext. longusdig. IV andm. sartoriusofthefrog. Inafew experi- ments, limb and abdominal muscles oflizards (Lacerta dugesii) and tortoises(Testudo graeca) were used. RESULTS Preliminaryobservations Inthecourseofsomeearlier work, while recording fromthesurfaceofisolated musclefibres,we occasionally noticedaspontaneous discharge ofsmallmono- phasicactionpotentials. The potentials variedsomewhatinsize,buthad a very consistenttimecourse, risingrapidlyin 1-2 msec, and decliningmore slowly,toone-halfinabout 3-4msec. Theywere localizedat one regionofthe fibre,andintheir shape and spatialspread resembledtheend-platepotential (e.p.p.)(cf.Eccles,Katz & Kuffler, 1941;Fatt& Katz, 1951). Moreover,the dischargedisappearedwhen a moderate doseofcurarine was appliedtothe muscle. Not much attentionwas paidtothephenomenon atthetime,and itwas suspectedtobeduetoalocalinjurydischargeatthenerveendings,theaxon havingbeencutclosetothemusclefibre. Inmore recentexperiments,how- P. FATT AND B. KATZ ever,we observedthesame phenomenon innerve-musclepreparationswhich seemedtobeabove any suchsuspicion,andthisinducedustoexaminethe natureofthe dischargeindetail. Localizationofthespontaneousdischargeattheend-plateregion Theformalresemblanceofthespontaneousdischargestothee.p.p.suggests thattheyoriginateatthenerve-musclejunction.Thisisnotbyitselfcritical evidence,foranybriefdisturbanceofthemusclemembrane-anywherealong thefibre-wouldbefollowed byalocalpotentialchangesimilarintimecourse andspatialspreadtothee.p.p.(seeEcclesetal.1941;Katz,1948;Fatt& Katz, 1951). A more decisive point was thatthe discharge couldnever beseenat Fig. 1. Localizationof spontaneousactivity. M. ext. 1. dig.IV. Intracellularrecording. PartA was recorded at the end-plate,part B at a distance2m m away, inthesame muscle fibre....
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