FIG. 9.

Excitatory and inhibitory postsynaptic potential (EPSP and IPSP) transmission. A: EPSPs shown at the left were generated by activating 2 excitatory synaptic inputs (dendrite 1 and 2). To represent a more natural condition, the traces at the right show model responses to a combination of excitatory (dendrite 1 and 2) and inhibitory (dendrite 2) synaptic inputs. The inhibitory synapse is activated 4 ms after the excitatory synapses to respect the delay in the feed-forward Golgi cell–granule cell loop. Both in the absence and presence of inhibition, the model depolarization in dendrite 2 is almost indistinguishable from that in dendrite 4 (that was inactive) and in the soma, hillock and axonal compartments. Superimposition of traces demonstrates their remarkable similarity and absence of relevant electrotonic decay. B: EPSPs were elicited by activating 2 excitatory synapses. Selective conductance switch-off shows that EPSPs are indeed amplified and protracted by Na⁺ and N-methyl-d-aspartate currents activating in the immediate subthreshold region. However, EPSP amplification was of little effect on potential transmission into the axon.