Segmental cable modelling of electrotonic transfer properties of deep superior colliculus neurons in the cat

A segmental cable model of tecto-reticulo-spinal neurons of cat superior colliculus was constructed, based on detailed anatomical measurements from three neurons. The calculated membrane resistance for which the model best fitted the measured input resistance was 2,300-3,000 omega cm2. Electrotonic length of dendrites fell under 0.59-1.20 and 0.52-1.05 length constants, while the mean electrotonic length for the three cells averaged 0.91, 0.79, 0.81 and 0.80, 0.70, 0.71 (for sealed-end and open-end cable termination, respectively). Dendrite-to-soma conductance ratios averaged 16.0, 10.7, 7.5 and 21.3, 14.1, 11.4 for the two different end conditions, respectively. Synaptic efficacy was estimated by the transfer of steady-state voltage or current reaching the soma from distal dendritic locations. While voltage transfer was less than 1%, almost 60% of injected current (or charge) reached the soma. Analysis of voltage transients recorded experimentally in TRSNs demonstrated considerable difference between parameters derived from either equivalent-cylinder model or segmental cable model. The obvious deviations of TRSNs both in anatomical and electrotonic respect from the assumptions of the equivalent-cylinder model indicate that the detailed cable model will give a more appropriate description of these neurons. The significance of the estimated electrotonic parameters for the particular burst generation mechanism of TRSNs is discussed.