Synaptic signaling in an active central network only moderately changes passive membrane properties

The membrane resistance of mammalian central neurons may be dramatically reduced by synaptic events during network activity, thereby changing their integration properties. We have used the isolated neonatal rat spinal cord to provide measurements of the effect of synaptic signaling on passive membrane properties during network activity. Synaptic signaling could take place during fictive locomotor activity with only modest (on average 35%) reduction of the input resistance (R-in) and of the cell's charging time constant (tau(in)). Individual synaptic signals, however, often introduced a peak conductance that was greater than the input conductance (G(in) = 1/R-in) without synaptic activity. The combination of moderate average synaptic conductance and large conductance of individual synaptic signals suggests that individual presynaptic neurons have large but short-lasting influence on the integration properties of postsynaptic neurons.