Passive electrical properties of ventral horn neurons in rat spinal cord slices

Recordings were made from large neurons located in the ventral horn of transverse spinal cord slices from young rats (7-15 days). Whole cell recordings were made simultaneously with two electrodes from the soma of these neurons, visualized using infra-red differential interference contrast optics. Positive identification of motoneurons could not always be achieved. The response of a neuron to a brief pulse of current delivered by one electrode, and recorded by the other electrode, were matched optimally to responses of a compartmental model of the same neuron with an identical current pulse as input. The compartmental model was based on a reconstruction of the neuron, using Biocytin staining. The compartmental model had three free parameters: specific membrane capacitance (C(m)), membrane resistivity (R(m)), and cytoplasmatic resistivity (R(i)), all assumed to be uniform throughout the neuron. The experimental and model responses could be matched unequivocally for four neurons, giving C(m) = 2.4 +/- 0.5 mu F/cm(2), R(m) = 5.3 +/- 0.9 k Ohm/cm(2), and R(i) = 87 +/- 22 Ohm/cm. No somatic shunt was required. For the remaining six neurons, a less perfect fit (but still within 95% confidence limits) was indicative of nonhomogeneous membrane properties. The electrotonic length of uncut dendrites was 0.85 +/- 0.14 lambda. The results resolve the issue of a somatic shunt conductance for motoneurons, relegating it to a microelectrode impalement artifact. They are consistent with previous reports on the electrical compactness of motoneurons to steady state currents and voltages. However, the much higher value of C(m) (than the previously assumed 1 mu F/cm(2)) implies much greater dendritic attenuation of fast synaptic potentials, and a much enhanced integrative response of motoneurons to synaptic potentials.