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The presence of a conductance like an A current (KA) in the Purkinje cell was shown by [6]. We derived our equations for KA from the original reports on A currents [2, 3] modified to fit the data in the two reports on Purkinje cell A currents that were available at that time. The whole cell voltage-clamp study of cultured Purkinje cells by [5] (their Fig. 9) provided data about the activation and inactivation time constants and the steady-state inactivation, and a single-electrode voltage-clamp study in slice by [7] supplied steady-state activation data.
Recently [9] published a more complete report on a single-electrode voltage-clamp study of the Purkinje cell. The average threshold for activation they report is lower than the value used in our model, but there seems to be a large natural variability in the activation curves (compare their Fig. 1 with their Table 1). The steady-state inactivation curve of [9] is similar to ours and to the kinetics of A currents in other systems [2, 8] but they report a much slower inactivation time constant than [5].
One point of contention in the literature about the distribution of ionic channels involves the presence or absence of an A current in the distal dendrites [1, 6]. One argument for a more extensive distribution is that a depolarizing bias current, which would inactivate A currents, also causes the dendrite to fire spikes more easily. However, a similar result would be expected if the depolarization also activated a plateau current, as shown in Fig. 11. Moreover, in contrast to the expected 4-aminopyridine (4-AP) sensitivity of A currents [8], the outward current described by [6] was not blocked by 4-AP. In addition, voltage-clamp data [9] do not support a distal dendritic location of the A current. Patch-clamp studies of cultured Purkinje cells [4] have shown the A current to be present in both somatic and dendritic membrane, but it is likely that patches were obtained from smooth dendrites only. In the current model an A current was present in the soma and main dendrite. However, dendritic spiking was more influenced by the K2 Ca2+ -activated K+ current, which allowed a finer control of dendritic excitability than the A current, which inactivated quickly during long current injections.
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