0.1 msec/div Action potentials triggered by EPSP 5 mV ˜V •Fig. 6.7 PropertiesofEPSPs.A, TimecourseofafastEPSPcomparedwiththatoftheunderlyingEPSC.Inmanycases,suchasthisone,theEPSCismuchshorterthantheEPSP;however,sometimestheEPSCcanhaveafairlyextensivetail.B, IntracellularlyrecordedEPSPsatdifferentlevelsofdepolarization.EPSPswereevokedinmotorneuronsbystimulationofIaafferents.Thenumbertotheleftofeachtraceindicatesthemembranepotentialinducedbyinjectionofcurrentthroughtheelectrode.Atinitialmembranepotentialsof−42and−60mV,theEPSPtriggeredanactionpotential.Atmoredepolarizedlevels,Na+ channelsareinactivated,sonospikeoccurs.C, TodeterminetheEPSPreversalpotential,theinitialmembranepotentialisplottedagainstthesizeoftheEPSP(ΔV).ThisEPSPreversedat−7mV.(A, DatafromCurtisDR,EcclesJC.J Physiol 1959;145:529;B, datafromCoombsJSetal.J Physiol 1955;130:374.)
IPSPs. Like EPSPs, IPSPs are triggered by the binding an action potential (otherwise it would increase the prob-of neurotransmitter to receptors on the postsynaptic ability of the cell spiking and by definition be an EPSP). It membrane and typically involve an increase in membrane may seem counterintuitive that something that depolarizes permeability as a result of the opening of ligand-gated the membrane can still be considered inhibitory, but if channels. They differ in that IPSP channels are permeable it decreases the probability of spiking, then it is indeed to only a single ionic species, either Cl− or K+ . Thus, IPSPs inhibitory (a further explanation is given in the Synaptic will have a reversal potential equal to the Nernst potential Integration section). of the ion carrying the underlying current. Typically the In sum, starting from the resting membrane potential, Nernst potential for these ions is somewhat negative relative EPSPs are always depolarizing, IPSPs can be either depolarto the resting potential, so when IPSP channels open, there izing or hyperpolarizing, and a hyperpolarizing potential is is an outward flow of current through them that results in always an IPSP. Thus, the key distinction between inhibitory hyperpolarization of the membrane (see
Fig. 6.3 ). and excitatory synapses (and IPSPs and EPSPs) is how they