ELECTROTONIC STRUCTURE OF MOTONEURONS IN SPINAL-CORD SLICE CULTURES -A COMPARISON OF COMPARTMENTAL AND EQUIVALENT CYLINDER MODELS

1. Voltage-clamp, current-clamp, and morphological data were obtained from visually identified motoneurons in organotypic cocultures of rat embryonic spinal cord, dorsal root ganglia, and skeletal muscle. The c ells were injected with Biocytin during whole-cell patch-clamp recordi ngs and stained with horseradish peroxidase. 2. The somata and dendrit ic trees of the cells were reconstructed with a semiautomatic reconstr uction system. The motoneurons had a common multipolar shape. An ellip tic soma gave rise to 3-9 stem dendrites with a mean diameter of 2.5 /- 0.9 (SD) mu m terminating in 24 +/- 7 dendritic endings. The mean t otal dendritic path length was 3,306 +/- 1,075 mu m. The mean total me mbrane surface area was 15,594 +/- 10,404 mu m(2) with a dendritic to somatic membrane surface area ratio of 3.4 +/- 1.4 (n = 7 cells). 3. T he ratio between the sum of the diameters of the two daughter branches and the diameter of the parental branch each raised to the 3/2 power at all branch points was 1.3 +/- 0.28 (n = 8 cells). The dendritic tre es of the cells tapered continuously from the soma to the distal ends. The mean normalized dendritic trunk parameter of all cells was 0.62 /- 0.22. 4. The motoneurons had a mean input resistance R(N) of 498 +/ - 374 M Omega, a mean membrane time constant (tau(m)) of 22 +/- 4.6 ms , and a mean dendritic dominance (rho) of 2.7 +/- 0.86 (n = 5 cells). The mean electrotonic length (L) calculated from tau(m), and the slowe st voltage-clamp time constant (tau(VC1)) was 0.7 +/- 0.04 (n = 7 cell s). 5. The specific membrane capacitance (C-m) estimated from the char ge of the capacitive current during a voltage step and the total membr ane surface area was 1.08 +/- 0.3 mu F/cm(2)(n = 6 cells). 6. Compartm ental computer models were constructed of individual cells. Experiment al and simulated voltage transients were matched with C-m = 1 mu F/cm( 2), a uniform membrane resistivity (R(m)) = tau(m)/C-m and a cytosolic resistivity (R(i)) of 308 +/- 39 Omega.cm (n = 3 cells). 7. The mean electrotonic length of the dendritic paths was 0.83 +/- 0.2 (n = 5 cel ls). The mean input resistance at the dendritic terminals (R(T)) was 1 ,413 +/- 260 M Omega. Synaptic conductances were applied at air distal dendritic compartments of the model cells. The resulting synaptic cur rents were calculated at the input site and at the soma. The mean tran sient current attenuation ratio was 4.7 +/- 1.7 under idealized voltag e-clamp conditions. The mean transient current attenuation ratio calcu lated for the same cells assuming an equivalent cylinder model was 5.2 +/- 1.2. 8. We conclude that the geometric constraints by which a den dritic tree can be reduced to an equivalent cylinder are not met by mo toneurons in spinal cord slice cultures. Nevertheless the cable parame ters correspond well to an average electrotonic structure of the cells .