Intracellular recording from vertebrate myelinated axons: mechanism of the depolarizing afterpotential

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Abstract
Electrophysiological techniques are described which allow intracellular recording from peripheral myelinated axons of lizards and frogs for up to several hours. The sciatic and intramuscular axons studied here have resting potentials of -60 to -80 mV and action potentials (evoked by stimulation of the proximal nerve trunk) of 50-90 mV. They show a prominent depolarizing afterpotential (DAP), which is present both in isolated axons and in axons still attached to their peripheral terminals. This DAP has a peak amplitude of 5-20 mV at the resting potential, and decays with a half-time of 20-100 ms. The peak amplitude of the DAP is voltage-sensitive, increasing to up to 26 mV with membrane hyperpolarization. The DAP disappears as the axon is depolarized to -60 to -45 mV, and does not appear to reverse with further depolarization. The DAP is not reduced when bath Ca is replaced by 2-10 mM divalent Mn or Ni. The DAP is not reversed when axons depleted of Cl (by prolonged exposure to Cl-deficient, SO4-enriched solutions) are bathed in Cl-rich solutions. Evidently, the DAP is not mediated by a conductance change specific for Ca2+ or Cl-. Partial substitution of tetramethylammonium for bath Na, or addition of 10-5 M-tetrodotoxin to the normal bathing solution, reduces the amplitude of both the action potential and DAP. The amplitude of the DAP is not sensitive to bath [K] over the range 1-7.5 mM, provided that all measurements are made at the same holding potential. This result suggests that the DAP is not mediated by accumulation of K outside the active axon. Treatments expected to inhibit the Na-K exchange pump (cooling from 25.degree. to 10.degree. C, or 0.15 mM ouabain) do not enlarge or prolong the DAP although they do abolish a slower hyperpolarizing afterpotential seen following repetitive stimulation. The passive voltage response of the axon to small injected pulses of depolarizing or hyperpolarizing current shows a prominent, slowly decaying component with a time course similar to that of the DAP. Depolarizing current reduces the input resistance of the axon, and increases the rate of decay of both the passive voltage response and the DAP. There is a slight conductance increase during the peak of the DAP but the same conductance increase can be produced by a comparable passive depolarization.