Electronic 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 cells were injected with Biocytin during whole-cell patch-clamp recordings and stained with horseradish peroxidase. 2. The somata and dendritic trees of the cells were reconstructed with a semiautomatic reconstruction system. The motoneurons had a common multipolar shape. An elliptic soma gave rise to 3-9 stem dendrites with a mean diameter of 2.5 +/- 0.9 (SD) micron terminating in 24 +/- 7 dendritic endings. The mean total dendritic path length was 3,306 +/- 1,075 microns. The mean total membrane surface area was 15,594 +/- 10,404 microns 2 with a dendritic to somatic membrane surface area ratio of 3.4 +/- 1.4 (n = 7 cells). 3. The 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 trees 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 RN of 498 +/- 374 M delta, 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 electronic length (L) calculated from tau m and the slowest voltage-clamp time constant (tau VC1) was 0.7 +/- 0.04 (n = 7 cells). 5. The specific membrane capacitance (Cm) estimated from the charge of the capacitive current during a voltage step and the total membrane surface area was 1.08 +/- 0.3 microF/cm2 (n = 6 cells). 6. Compartmental computer models were constructed of individual cells. Experimental and simulated voltage transients were matched with Cm = 1 microF/cm2, a uniform membrane resistivity (Rm) = tau m/Cm and a cytosolic resistivity (Ri) of 308 +/- 39 omega.cm (n = 3 cells). 7. The mean electrotonic length of the dendritic paths was 0.83 +/- 0.2 (n = 5 cells). The mean input resistance at the dendritic terminals (RT) was 1,413 +/- 260 M omega. Synaptic conductances were applied at all distal dendritic compartments of the model cells. The resulting synaptic currents were calculated at the input site and at the soma. The mean transient current attenuation ratio was 4.7 +/- 1.7 under idealized voltage-clamp conditions.(ABSTRACT TRUNCATED AT 400 WORDS)