Permeability and Excitation of Insect Muscle

Abstract

THE membrane of most nerve and muscle cells is assumed to be more permeable to potassium ions than to sodium ions in the resting condition and the potential observed approaches that of a potassium electrode as predicted by the Nernst equation. The plasma membrane becomes much more permeable to sodium ions during excitation, and the potential across the membrane reverses, producing a positive overshoot of the action potential. The skeletal muscles of herbivorous insects, however, have unusual bioelectric properties and have been studied by Wood^1–4 and Huddart^5–7. Analysis of the haemolymph and myoplasm of Orthoptera, Dictyoptera and Lepidoptera reveals an unusual ionic distribution⁸ and it is suggested that the observations of the resting and action potentials fail to agree with the values predicted by the Nernst equation and with the classical studies of excitable cells. For example, frequently no positive overshoot to the action potential has been recorded. It has been suggested that active transport of chloride ions may contribute to the resting potential⁴, and that an inward movement of sodium⁹ or magnesium ions^1,9 may participate in the action potential. Our present knowledge in this field has been reviewed by Aidley⁹.