ACTION-POTENTIAL PROPAGATION THROUGH EMBRYONIC DORSAL-ROOT GANGLION-CELLS IN CULTURE .1. INFLUENCE OF THE CELL MORPHOLOGY ON PROPAGATION PROPERTIES

1. In this and the companion paper the reliability of action potential (AP) propagation through dorsal root ganglion (DRG) cells was investig ated. Experimental data were collected from DRG cells of embryonic rat slice cultures of the spinal cord. A field stimulation electrode was used to elicit an AP in the axon. The propagated AP or, in case of con duction block, its electronic residue (ER), was measured intracellular ly in the soma of the DRG cell. 2. The morphological and electrophysio logical data combined with published data from voltage-clamp studies w ere taken to implement a compartmental computer model, which allows a precise description of the propagating AP and the channel kinetics at any point along the axon. 3. The safety factor for conduction was foun d to be low. Thus failures of AP invasion of the DRG cell soma could o ccur at sites of impedance mismatch when a hyperpolarizing current was applied, a second stimulus felt into the relative refractory period o f the first, or when the axon was repetitively stimulated.4. The ERs o f the failed APs had discrete amplitude levels, suggesting that the fa ilures were always caused at the same site along the axon. These sites of low safety factor were found to be the branch point in the unipola r DRG cell and the entrance of the stem piece into the soma in both ce ll types, the bipolar as well as the unipolar. 5. A systematic compari son of bipolar and unipolar DRG cells showed that the AP conduction th rough the latter is more reliable. For large cell bodies, the unipolar configuration is needed for save conduction. 6. Conduction through un ipolar DRG cells is faster than through bipolar cells because the elec trical load of the soma is masked by the high-resistive stem piece. Th e length of this stem piece is correlated inversely to the delay cause d at the branch point, as the electrical load of the soma is more effi ciently masked by a long stem piece.