1. The electrical characteristics of individual rat dorsal root ganglion neurones were studied and related to the peripheral axon conduction velocity and morpho- logical cell type. Neurones were divided into four groups based on the conduction velocity of their peripheral axons (AX, 30-55 m/s; Aft, 14-30 m/s; Ad, 2-2-8 m/s and C < 1-4 m/s). 2. Electrophysiological parameters examined included membrane potential, action potential amplitude and duration, after-potential height and duration, input resistance and the occurrence of time-dependent rectification. 3. The mean duration ofthe somatic action potentials was found to be characteristic for each of the conduction velocity groupings. However, there was considerable overlap between groups. The fast-conducting (Ax) and slowly conducting (As) myelinated fibres had short-duration action potentials, within the ranges 0-49-135 and 0-5-1-7 ms at the base respectively. The Af and C cells had somatic action potentials with durations in the ranges of 0-6-2-9 and 0-6-7-4 ms respectively. The longer action potential durations could be related to the presence of an inflexion on the repolarizing phase seen in a third of Aft neurones (called Aft, neurones) and in all C neurones. 4. The action potential overshoot was larger in C neurones and Aft1 neurones than in the other neurone groups. 5. The mean duration of the after-hyperpolarization was several times greater in C neurones than in A neurones. AA neurones displayed the shortest and greatest amplitude after-hyperpolarizations. Large, long-lasting after-hyperpolarizations were not limited to neurones displaying an inflexion. 6. The electrophysiological properties of the soma membrane of AA neurones closely resembled those ofAA neurones, while in several respects those of C neurones resembled the Aft1 neuronal properties. 7. The input resistance was found to be much greater in C than in A cells, although there was no significant difference between specific membrane resistance values calculated for the different groups. A number of A cells exhibited time-dependent rectification. 8. The data are related to the size of the cell body and its probable morphological type. The results are discussed with reference to other reports on primary afferent electrophysiology.
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