Passive cable properties of hippocampal CA3 pyramidal neurons

The passive electrical cable properties of CA3 pyramidal neurons from guinea pig hippocampal slices were investigated by applying current steps and recording the voltage transients from 25 CA3 neurons, using a single intracellular microelectrode and a 3-kHz time-share system. Two independent methods were used for estimating the equivalent electrotonic length of the dendrites, L, and the dendritic to somatic conductance ratio, p. The first method is similar to that used by Gorman and Mirolli (1972) and gave an average L of 0.96; the average p was 2.44. The second method is derived here for the first time and assumes a finite-length cable with lumped soma. It is an exact solution for L and p, using the slopes and intercepts of the first two peeled exponentials. The average L was 0.94; the average p was 1.51. The results, using both methods, are in close agreement. The average membrane time constant for all CA3 neurons was 23.6 ms. suggesting a large (23,600 omega cm2) average membrane resistivity. It is concluded that CA3 neurons are electronically short.