Delayed Effects of Anoxia Upon the Electrical Properties of an Identified Cockroach Motoneurone

Abstract

ABSTRACT In nerve preparations isolated from cockroaches that had survived exposure to carbon dioxide or nitrogen for 1–3 h, the cell body of the metathoracic ‘fast’ coxal depressor motoneurone (Df) gained the ability to support all-or-none action potentials. Such responses were observed between 10 h and 5 days after the anoxic period. Recordings from isolated cell bodies indicated that the action potentials were generated in the cell body itself rather than in adjacent regions of the neurone. Action potentials were abolished in a sodium-free medium or in the presence of tetrodotoxin, but persisted in saline containing manganese ions, indicating that sodium ions are the major ions involved in the rising phase of these action potentials. Elevation of the intracellular H+ concentration by acid injection or by bathing the preparation in saline containing 40 mmol 1−1 ammonium chloride caused a marked depolarization of the membrane potential and action potentials could not be elicited. This indicates that a fall in intracellular pH during anoxia could not itself directly produce the enhancement in excitability reported here. Histological examination of the neurone, after periods of anoxia sufficient to enable the cell body to generate the action potentials reported here, showed no detectable change in the perinuclear distribution of RNA and hence did not indicate any alteration in protein synthesis.