Giant smooth muscle cells of beroë : Ultrastructure, innervation, and electrical properties

Abstract

Beroë muscle fibers are single cells which may be 20-40 µm in diameter in mature specimens. Longitudinal muscles may be 6 cm or more long. There is no striation pattern and the muscles were observed to contract in a tonic fashion when stretched. They are innervated by a nerve net, and external recording revealed what are probably nerve net impulses. Intracellular stimulation of the muscles themselves was found to initiate large propagating action potentials which were recorded intracellularly. The action potentials were insensitive to tetrodotoxin (10-5 g/ml), tetraethylammonium ions (50 mM), MnCl2 (25 mM), and low concentrations of verapamil (2 X 10-6 g/ml). Full-size action potentials were recorded in sodium or calcium-deficient salines, but were small and graded in salines deficient in both sodium and calcium. Cable analysis yielded mean values for λ(1.95 mm), Ri (154 Ωcm), Rm (9,253 Ωcm2), and rm (13.9 ms). The conduction velocity depended primarily on fiber diameter and maximum rate of rise of the action potential and could be predicted from the theoretical analysis of Hunter et al. (1975 Prog. Biophys. Mol. Biol. 3(h 99-144). The calculated membrane capacity (<2 µF/cm2) indicates little infolding of the surface membrane, a conclusion which is in agreement with anatomical studies. © 1980, Rockefeller University Press., All rights reserved.