The Journal of Neurophysiology Vol. 79 No. 5 May 1998, pp. 2485-2502
Copyright ©1998 The American Physiological Society

Passive Electrical Properties of Ventral Horn Neurons in Rat Spinal Cord Slices

David Thurbon¹, Hans-R. Lüscher^{1, 2}, Thomas Hofstetter², and Stephen J. Redman¹

¹ Division of Neuroscience, John Curtin School of Medical Research, Australian National University, Canberra 2600, Australia; and ² Department of Physiology, University of Bern, CH-3012 Bern, Switzerland

Thurbon, David, Hans-R. Lüscher, Thomas Hofstetter, and Stephen J. Redman. Passive electrical properties of ventral horn neurons in rat spinal cord slices. J. Neurophysiol. 79: 2485-2502, 1998. 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 µF/cm², R_m = 5.3 ± 0.9 k/cm², and R_i = 87 ± 22 /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 . 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 µF/cm²) implies much greater dendritic attenuation of fast synaptic potentials, and a much enhanced integrative response of motoneurons to synaptic potentials.

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