Diffusion Barriers in the Squid Nerve Fiber

Abstract

The [Venezuelan] squid [Doryteuthis plei] nerve barriers are formed by (a) the axolemma (membrane of the axon proper), a membrane 80 A thick perforated by cylindrical pores 4.0 to 4.5 A radius, and (b) the Schwann layer, constituted of numerous cells forming a layer one cell thick, crossed by 60 A wide slit channels. If a molecule present in the axoplasm has to reach the extraneural space, it has to pass (a) the pores, and (b) the channels, in series, and the diffusion rate will depend on the effective diffusion areas per unit path length, Apd/[DELTA] x for the axolemma, and Acd/[DELTA] x for the Schwann layer. By addition, And/[DELTA] x, the transneural effective area for diffusion per unit path length is obtained. The diffusion rates of C14-ethylene glycol (2.2 A radius), and C14-glycerol (2.8 A radius) were measured. The diffusion rate of H3-labeled water (1.5 A radius) has been previously determined. The results expressed in terms of And/ x (mean values [plus or minus] SD, referred to 1 cm2 of nerve surface) are 5.3 [plus or minus] 1.4 cm for water, 2.5 [plus or minus] 0.4 cm for ethylene glycol, and 0.29 [plus or minus] 0.03 cm for glycerol. Theoretical values for And/[DELTA] x of 2.5 and 0.83 cm for ethylene glycol and glycerol have been calculated. The agreement between the theoretical values for And/[DELTA] and the experimental ones supports the diffusion barrier model described above.