The resistance of the septum of the median giant axon of the earthworm

Abstract

It is generally thought that nexuses constitute low-resistance pathways between cell interiors in epithelial, neural, muscular, and even connective tissues. However, there are no reliable estimates of the specific resistance of a nexus. The reason for this is that in most cases the surfaces of nexuses between cells are geometrically complex and therefore it has been very hard to accurately estimate nexal areas. However, the septa of the median giant axon have a relatively simple shape. Moreover, in this preparation, it was possible to make a measuring current flow parallel to the axon axis so that from the voltage difference appearing between intracellular electrodes during current flow, the specific septal membrane resistance could be calculated. The average specific nexal resistance obtained was 5.9 ω cm2 if one assumes that 100% of the septum is nexus. The steady state I-V curve for the septum is linear (±10 mV). Placement of electrodes was validated by ionophoresing fluorescein into one of the cells. This allowed visualization of the septa even though the septa were found to be permeable to fluorescein and TEA. Exposure of the axon to hypertonic saline impedes the movement of fluorescein across the septa. By analogy with other tissues it is concluded that hypertonic solutions disrupt nexuses. A mathematical model was derived which predicts the steady-state transmembrahe potential vs. distance from a point source of intracellular current. When the specific nexal membrane resistance is 5.9 ω cm2, the prediction closely approximates the fall of transmembrane potential vs. distance in an ordinary infinite cable. This is commensurate with the electrophysiological behavior of this multicellular "axon". © 1977, Rockefeller University Press., All rights reserved.