6.8A single action potential in axon 2 versus 1). This is because the cell membrane is leaky and synaptic currents are generated locally at the synapse, so even if two synapses generate a local EPSC of the same size, less of the initial current will arrive at the initial segment from the more distal synapse than from the more proximal one, thereby resulting in the generation of a smaller EPSP at the initial segment by the distal synapse (see discussion of length constant in ). Thus, the synapse’s spatial location in the dendritic tree is an important determinant of its efficacy. However, as already mentioned, EPSPs generated by most CNS synapses, even those in favorable positions (i.e., close to the initial segment), are too small by themselves to reach •Fig. 6.8 SynapticintegrationofEPSPsrecordedattheaxonhillockadjacenttotheinitialsegment.A, ComparisonofEPSPsevokedbyproximalversusdistalsynapses(2versus1).B, Temporalsummation.EPSPsinresponsetotwospikesinthesameaxonoccurringinrapidsuccession(axon2).C, Spatialsummation.Responsesevokedbysynapsesthatareelectricallydistantfromeachother(1and3).D, Sublinearsummationoftwosynapseslocatedneareachotherbecauseofshunting(2and4).
the spiking threshold in the postsynaptic cell, as illustrated in
Fig. 6.8A , where an action potential in either axon 1 (distal) or 2 (proximal) both produce EPSPs that are too small to trigger a spike. Thus, generally the summed EPSPs from multiple synapses are required to reach threshold and trigger a spike.
The requirement for multiple EPSPs to summate in order to trigger a spike is what makes the relatively long duration of EPSPs so important. Temporal summation refers to the fact that EPSPs that are separated by a latency less than their duration can sum. This is illustrated in
Fig.
6.8B , where the same synapse is activated multiple times in rapid succession (axons can fire action potentials at rates well over 100 Hz); in this situation, successive EPSPs will be less than 10 msec apart and therefore overlap and sum. Note the higher amplitude of the second peak.
Spatial summation refers to the fact that synaptic potentials generated by different synapses can interact. For example, in
Fig. 6.8 , suppose axon 1 and 3 each fire an action potential but at widely separated times. Each produces an EPSP that depolarizes the cell but is too small to reach threshold (see
Fig. 6.8C , EPSP1, EPSP3). Instead, if both axons fire within a short enough time of each other, their effect can be additive, as shown in
Fig. 6.8C (EPSP 1+3). The combined EPSP amplitude may then reach threshold and lead to spiking of the cell. If the EPSPs generated by axons 1 and 3 were simultaneous, then we would have an example of pure spatial summation. In the example shown, however, the times of the two EPSPs were slightly separated, thus we have both spatial and temporal summation present. The fact that EPSPs have a long time course (when compared with action potentials or the underlying EPSCs) facilitates both types of synaptic integration.
In the foregoing example, the combined EPSP was approximately the linear summation of the two individual EPSPs evoked by action potentials in axons 1 and 3. This is the case when two synapses are far apart. If the two synapses are close together, such as for axons 2 and 4 (see