Activation of a synapse typically produces a response in the postsynaptic cell (i.e., a postsynaptic potential) that will be roughly the same each time, assuming the postsynaptic cell is in a similar state. Certain patterns of synaptic activation, however, result in changes in the response to subsequent activation of the synapse. Such use-related changes may remain for short (milliseconds) or long (minutes to days) durations and may be either a potentiation or suppression of the synapse’s strength. These changes probably underlie cognitive abilities such as learning and memory. Thus, the processes by which activity results in changes in a synapse’s efficacy are a critical feature of synaptic transmission.
When a presynaptic axon is stimulated twice in rapid succession, it is often found that the postsynaptic potential evoked by the second stimulus are larger in amplitude than the one evoked by the first (Fig. 6.9 ). This increase
A Facilitation B Facilitation C Posttetanic potentiation 3 msec 10 msec 1/sec 5/sec 10/sec 12.5/sec 31.2/sec 25/sec 1 mV 4 sec tetanic stimulation 8 sec 14 sec 27 sec Time after tetanic stimulation •Fig. 6.9 A, Facilitationataneuromuscularjunction.EPPsataneuromuscularjunctionintoadsartoriusmusclewereelicitedbysuccessiveactionpotentialsinthemotoraxon.Neuromusculartransmissionwasdepressedby5mMMg++ and2.1mMcuraresothatactionpotentialsdidnotoccur.B, EPPsatafrogneuromuscularjunctionelicitedbyrepetitivelystimulatingthemotoraxonatdifferentfrequencies.Notethatfacilitationfailedtooccuratthelowestfrequencyofstimulation(1/sec)andthatthedegreeoffacilitationincreasedwithincreasingfrequencyofstimulationintherangeoffrequencyused.Neuromusculartransmissionwasinhibitedbybathingthepreparationin12to20mMMg++ .C, Posttetanicpotentiationatafrogneuromuscularjunction.ThetoptwotracesindicatecontrolEPPsinresponsetosingleactionpotentialsinthemotoraxon.SubsequenttracesindicateEPPsinresponsetosingleactionpotentialsaftertetanicstimulation(50impulses/secfor20seconds)ofthemotorneuron.Thetimeintervalbetweentheendoftetanicstimulationandthesingleactionpotentialisshownoneachtrace.Themusclewastreatedwithtetrodotoxintopreventgenerationofactionpotentials.(A, RedrawnfromBelnaveRJ,GagePW.J Physiol 1977;266:435;B, redrawnfromMagelbyKL.J Physiol 1973;234:327;C, redrawnfromWeinrichD.J Physiol 1971;212:431.) is known as paired-pulse facilitation (PPF). Note PPF is distinct from temporal summation, in which two EPSPs overlap and sum to a larger response; with PPF the second EPSP itself is greater in size. If one plots the relative size of the two postsynaptic potentials (PSPs) as a function of the time between two stimuli, the amount of increase in the second PSP will be seen to depend on the time interval. Maximal facilitation occurs at around 20 msec, followed by a gradual reduction in facilitation as the interstimulus interval continues to increase; with intervals of several hundred milliseconds, the two PSPs are equal in amplitude and no facilitation is observed. Thus, PPF is a relatively rapid and short-lasting change in synaptic efficacy.