title: Axolemmal and septal conduction in the impedance of the earthworm medial giant nerve fiber.
abstract: Ionic conduction in the axolemmal and septal membranes of the medial giant fiber (MGF) of the earthworm (EW) Lumbricus terrestris was assessed by impedance spectroscopy in the frequency range 2.5-1000 Hz. Impedance loci in the complex plane were described by two semi-circular arcs, one at a lower characteristic frequency (100 Hz) and the other at a higher frequency (500 Hz). The lower frequency arc had a chord resistance of 53 k omega and was not affected by membrane potential changes or ion channel blockers [tetrodotoxin (TTX), 3,4-diaminopyridine (3,4-DAP), 4-aminopyridine (4-AP), and tetraethylammonium (TEA)]. The higher frequency arc had a chord resistance of 274 k omega at resting potential, was voltage-dependent, and was affected by the addition of TTX, 3,4-DAP, 4-AP, and TEA to the physiological EW salines. When all four blockers were added to the bathing solution, the impedance locus was described by two voltage-independent arcs. Considering the effects of these and other (i.e., Cd and Ni) ion channel blockers, we conclude that: 1) the higher frequency locus reflects conduction by voltage-sensitive ion channels in the axolemmal membrane, which contains at least four ion channels selective for sodium, calcium, and potassium (delayed rectifier and calcium-dependent), and 2) the lower frequency locus reflects voltage-insensitive channels in the septal membrane, which separates adjacent MGFs.
authors.affiliation: Department of Zoology, College of Pharmacy, University of Texas, Austin 78712.
authors.forename: T L
authors.initials: TL
authors.lastname: Krause
authors.forename: H M
authors.initials: HM
authors.lastname: Fishman
authors.forename: G D
authors.initials: GD
authors.lastname: Bittner
mesh_terms: D015761:4-Aminopyridine / Q000031:analogs & derivatives / Q000494:pharmacology; D000077770:Amifampridine; D000818:Animals; D001369:Axons / Q000502:physiology; D004558:Electric Stimulation; D066298:In Vitro Techniques; D007473:Ion Channels / Q000037:antagonists & inhibitors; D008564:Membrane Potentials / Q000187:drug effects; D008959:Models, Neurological; D008962:Models, Theoretical; D009412:Nerve Fibers / Q000187:drug effects / Q000502:physiology*; D009431:Neural Conduction / Q000187:drug effects; D009835:Oligochaeta; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology
chemical_list: D007473:Ion Channels; D013757:Tetraethylammonium Compounds; D013779:Tetrodotoxin; D019789:Tetraethylammonium; D015761:4-Aminopyridine; D000077770:Amifampridine