Action potentials triggered by EPSPs have a time course of 0.1 msec/div and an amplitude of 5 mV. Figure 6.7 illustrates the properties of EPSPs. A fast EPSP has a shorter duration compared to the underlying EPSC. However, in some cases, the EPSC can have a fairly extensive tail. Intracellular recordings of EPSPs at different levels of depolarization in motor neurons show that at initial membrane potentials of -42 and -60 mV, the EPSP triggers an action potential. At more depolarized levels, Na+ channels are inactivated, preventing the occurrence of spikes. To determine the EPSP reversal potential, the initial membrane potential is plotted against the size of the EPSP (ΔV), which is found to be -7 mV. IPSPs, like EPSPs, are triggered by the binding of an action potential to neurotransmitter receptors on the postsynaptic membrane. They increase the permeability of the membrane by opening ligand-gated channels, resulting in an outward flow of current. IPSP channels are permeable to either Cl- or K+ ions, and their reversal potential is equal to the Nernst potential of the respective ion. Despite the fact that IPSPs depolarize the membrane, they are still considered inhibitory because they decrease the probability of spiking. In contrast, EPSPs are always depolarizing. In summary, EPSPs are depolarizing, IPSPs can be either depolarizing or hyperpolarizing, and a hyperpolarizing potential is always an IPSP. The key distinction between inhibitory and excitatory synapses, as well as between IPSPs and EPSPs, lies in how they affect the probability of spiking.
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