Cation accessibility of the peripheral nervous system in Limulus polyphemus--an electrophysiological study.


The nervous systems of many vertebrates, and of some invertebrate phyla, are known to possess a 'blood-brain barrier' conferring a selective impermeability to certain ionic and molecular constituents of the body fluids and thus protecting the excitable membranes within a relatively constant micro-environment; this subject has been recently reviewed by Abbott & Treherne (1977). In vertebrates the barrier may be a property of the tight junctions between capillary endothelial cells (Brightman & Reese, 1969). Similarly in insects, at least to the level of the large peripheral nerves, electrophysiological evidence (Treherne et al. 1970; Treherne, 1974) and finestructural studies (Lane & Treherne, 1972; Lane, 1972) indicate the existence of a barrier preventing free diffusion, residing in the tight junction complexes of the perineurium. However, investigations of crustacean nervous systems (Abbott, 1970; Abbott, Moreton & Pichon, 1975) show rapid access of ions to the peripheral nerves, but a delayed entry to the central nervous tissue that may be related to gap junctions and/or to maculae occludentes between the perineurial sheath cells (Lane & Abbott, 1975). An analogous situation may occur in some gastropod molluscs, where a partial 'neurone-haemolymph barrier' has been described (Reinecke, 1976), though in many members of this phylum the nervous tissue is freely accessible to ions (Sattelle & Lane, 1972; Willmer, 1978). This communication considers the peripheral nerves of a marine Xiphosuran arthropod, applying similar techniques to those used in other invertebrate systems to assess the accessibility of the tissue to physiologically important cations. A parallel study will consider the structural correlates of accessibility in both central and peripheral nerves of Limulus (J. B. Harrison & N. J. Lane, in preparation). Electrophysiological records were obtained from isolated ligatured leg nerves, by a modified sucrose-gap technique; results are therefore expressed throughout using relative amplitude scales. The effects of reduced sodium concentrations are summarized in Fig. la. The action potential was progressively reduced as [Na]0 declined, and the effects are consistent with a predominantly sodium-dependent signal; the simple logarithmic relation of spike amplitude to [Na]0 indicates a conventional Nernst relationship, although the calculation of a Nernst gradient is not appropriate with extracellular

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@article{Willmer1979CationAO, title={Cation accessibility of the peripheral nervous system in Limulus polyphemus--an electrophysiological study.}, author={P G Willmer and Jason Harrison}, journal={The Journal of experimental biology}, year={1979}, volume={82}, pages={373-6} }