The perineurium was found to form the principal barrier to diffusion across the blood-brain barrier system of the cockroach, Periplaneta americana, since the resistance across this layer was much greater than that across the underlying neuroglia. An equivalent electrical circuit of the perineurium was then used to analyse recordings made in apparent perineurial cells and the interstitial system. Trans-perineurial resistance was at least 900 ωcm2, while the ratio between basolateral and apical membrane resistances was 11:1, indicating that the apical membrane had an area much greater than that of the basolateral membrane. Raising the potassium concentration in the saline produced changes in potential difference (p.d.) and resistance that were interpreted as due to the effect of potassium upon the basolateral membrane. Analysis indicated that the resting electromotive force (e.m.f.) generated by the basolateral membrane was less than that generated by the apical, although the K level in the saline was near that considered to be in the interstitial system. The analysis also yielded a value of 9 for the ratio of shunt resistance to apical resistance. Most changes in recorded values following the K elevation could be simulated by use of the estimated parameters, and an estimation of a change in interstitial K level. From these results, the shunt can be calculated to be an important contributor to the resistance across the perineurium, having a resistance about 0.9 times that of the transcellular resistance.

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