An analysis of the cable properties of spinal motoneurones using a brief intracellular current pulse

1. A brief intracellular current pulse, with duration less than 500 musec, has been applied to lumbosacral motoneurones in anaesthetized cats. The resulting voltage transients have been analysed by the procedure suggested in Jack & Redman (1971b) to obtain the cable parameters for each motoneurone.2. Forty-three motoneurone responses were analysed. In all cases the voltage response indicated that the dendrites could be represented as uniform, finite length cables, with either a sealed distal end, or at least a high resistance distal termination. The electrical length of the equivalent uniform dendritic cable ranged from 1.0 to 2.1 space constants, with a mean value of 1.5.3. The initial decay of the membrane potential following the removal of the current pulse was more rapid than was predicted by the Rall model for the motoneurone, in approximately two thirds of the responses. Consequently a value of dendritic to soma conductance ratio could not be obtained for these motoneurones.4. The explanation given for the departure from the theoretical response to a brief current pulse is that the specific resistivity of the soma membrane is lower than the specific resistivity of the dendritic membrane. This explanation is complicated by the possibility of the electrode tip not lodging in the isopotential soma region. The contribution that each of these effects has on the early decay phase of the current pulse response has been assessed.5. It is concluded that the specific resistivity of the soma membrane could be as low as one third of the dendritic membrane resistivity. Tonic inhibitory activity restricted to the soma is suggested as an explanation.