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Results from parameter variations, of the sort just mentioned, led to the question of which ionic currents were most responsible for each of the response properties of the Purkinje cell. In fact, the parameter sensitivity of model output to ion channel densities followed directly from an analysis of the role of the different currents in the Purkinje cell firing behavior. In some cases the relationship between ion currents and response properties was straightforward; in other cases the response of the- cell was a result of the interaction between different currents. Sometimes this was further complicated by the fact that the types of channels and their densities varied between the three different regions of the model. The following sections first describe the spatial distribution of the patterns of activity and then consider in detail the currents that cause each component of Purkinje cell responses to current injection in the model. Figure 10 shows images representing the membrane potential distribution and calcium concentration in all parts of the model during a somatic action potential and a dcndritic spike.
Figure 11 shows the change in membrane potential, submembrane Ca2+ concentration, and amplitude of all the ionic currents at four representative locations in the model during a current injection in the soma. The respective contribution of different channels to the somatic spikes, dendritic spikes, and the corresponding depolarizing spike bursts in the soma can be determined from these figures. In each case the data shown were obtained long after the initiation of the current injection so that the repetitive firing properties of the model had settled into a steady state.