Membrane currents evoked by afferent fiber stimulation in rat piriform cortex. II. Analysis with a system model

1. The detailed visualization of membrane currents over time and depth provided by current source-density (CSD) analysis was used as the basis for development of a system model that reproduces the response of piriform cortex to afferent fiber stimulation. This model has allowed the testing and substantial revision of previous hypotheses concerning the sequence of neuronal events underlying this response, has enabled net membrane currents visualized by CSD analysis to be separated into active and passive components, and has generated predictions for important axonal and synaptic parameters as well as for the behavior of piriform cortex as a system. 2. The model was developed in three steps. Activity in excitatory fiber systems was first represented with continuous distributions. The "population conductances" due to the activation of excitatory fiber systems were then computed from the distribution of action-potential arrival times and the conductance waveform for excitatory synapses. Finally, these temporally dispersed excitatory conductances and locally mediated inhibitory conductances were introduced at appropriate locations on a compartmentalized cable that simulated the passive response of the pyramidal cell population. 3. After the simulation of membrane currents at one site, all parameters in the model were fixed so that it could be used to predict the variation in the time course of membrane currents at additional recording sites; comparison with the results of CSD analysis at these sites provided the primary validation of the model. Additional validation included the simulation of membrane potentials derived by intracellular recording, including the effects of manipulating somatic potential with current injection. 4. Several conclusions have emerged from the mathematical description of activity in fiber systems. Propagation of activity in both afferent and association (corticocortical) fiber systems is "dispersive" as a result of a wide spectrum of axon conduction velocities. The characteristically different time courses of afferent and association fiber-mediated responses are largely determined by the focal, shock-evoked origin of the volley in afferent fibers as opposed to the spatially distributed disynaptic origin of activity in association fibers. Conduction velocity distributions for afferent and association fiber systems are skewed and can be approximated with lognormal distributions. 5. General solutions, which relate an arbitrary conduction velocity distribution to arrival time and spatial distributions of action potentials, were used to generate specific solutions describing the effects of dispersive propagation.(ABSTRACT TRUNCATED AT 400 WORDS)