An anatomical basis for the resistance and capacitance in series with the excitable membrane of the squid giant axon

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Abstract
To correlate periaxonal tissue layer resistance with Schwann cell layer anatomy, cross and longitudinal sections of giant axons ofLoligo pealei were examined by transmission electron microscopy. Measurements were made of the width and frequency of mesaxonal clefts entering the Schwann cell layer from the periaxonal space and leaving the cell layer adjacent to the basal lamina. The average mesaxonal cleft width is 10.5 nm. One cm2 of the giant axon surface is enclosed by a single cell layer containing about 690 000 Schwann cells. One cm2 of axon surface has a sheath mesaxonal area of 0.002 cm2 at the periaxonal surface and 0.016 cm2 at the basal lamina, the mesaxons branching frequently as they cross the sheath. The volume of the Schwann cell layer extracellular space was estimated to be roughly 1% of the Schwann cell layer volume. Several models were used to predict the resistance,R, across the Schwann cell layer. Assuming the mesaxonal clefts contain seawater, and can be lumped into volume conductors having simple geometries, then (normalized for one cm2 of axon surface)R was estimated to be between 0.4 and 0.9 Ω cm2. This value compares favourably with electrophysiological estimates of the periaxonal tissue resistance (current clamp value = 0.9 Ω cm2 and the voltage clamp value = 1.4 Ω cm2) as these electrically measured values include the resistance across the outer connective tissue layer as well as the Schwann cell layer. The value of the Schwann cell membrane capacity was estimated to be approximately 0.7 μF/cm2.