PHYSIOLOGY, MORPHOLOGY AND DETAILED PASSIVE MODELS OF GUINEA-PIG CEREBELLAR PURKINJE-CELLS

1. Purkinje cells (PCs) from guinea-pig cerebellar slices were physiol ogically characterized using intracellular techniques. Extracellular c aesium ions were used to linearize the membrane properties of PCs near the resting potential. Under these conditions the average input resis tance, R(N), was 29 M Omega, the average system time constant, tau(0), was 82 ms and the average cable length, L(N), was 0.59. 2. Three PCs were fully reconstructed following physiological measurements and stai ning with horseradish peroxidase. Assuming that each spine has an area of 1 mu m(2) and that the spine density over the spiny dendrites is t en spines per micrometre length, the total membrane area of each PC is similar to 150000 mu m(2), of which similar to 100000 mu m(2) is in t he spines. 3. Detailed passive cable and compartmental models were bui lt for each of the three reconstructed PCs. Computational methods were devised to incorporate globally the huge number of spines into these models. In all three cells the models predict that the specific membra ne resistivity, R(m), of the soma is much lower than the dendritic R(m ) (similar to 500 and similar to 100000 Omega cm(2) respectively). The specific membrane capacitance, C-m, is estimated to be 1.5-2 mu F cm( -2) and the specific cytoplasm resistivity, R(i), is 250 Omega cm. 4. The average cable length of the dendrites according to the model is 0. 13 lambda, suggesting that under caesium conditions PCs are electrical ly very compact. Brief somatic spikes, however, are expected to attenu ate 30-fold when spreading passively into the dendritic terminals. A s imulated 200 Hz train of fast, 90 mV somatic spikes produced a smooth 12 mV steady depolarization at the dendritic terminals. 5. A transient synaptic conductance increase, with a 1 nS peak at 0.5 ms and a drivi ng force of 60 mV, is expected to produce similar to 20 mV peak depola rization at the spine head membrane. This EPSP then attenuates between 200- and 900-fold into the soma. Approximately 800 randomly distribut ed and synchronously activated spiny inputs are required to fire the s oma. 6. The passive model of the PC predicts a poor resolution of the spatio-temporal pattern of the parallel fibre input. An equally sized, randomly distributed group of similar to 1% of the parallel fibres, a ctivated within a time window of a few milliseconds, would result in a pproximately the same composite EPSP at the soma.