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.

Vesicle accumulation and exocytosis at sites of plasma membrane disruption.

“…There is no evidence that transection evokes exocytosis in the giant axon. Rather it has been proposed that accumulated vesicles form a "plug" at the transected end, rather than contributing via exocytosis to reformation of plasma membrane continuity there (Krause et al, 1994). It is possible that the actin-based cytoskeleton, in addition to the tubulin-based cytoskeleton, participates in resealing.…”

“…It may therefore resemble morphologically, albeit on a much slower scale, the vesicle accumulation process described here for the wounded endothelial cell. Surprisingly, resealing of the squid giant axon is incomplete after 2.5 h, based on electrophysiological data (Krause et al, 1994). There is no evidence that transection evokes exocytosis in the giant axon.…”

Hydrophilic polymers enhance early functional outcomes after nerve autografting

“…Sealing of axolemmal damage normally occurs through a calcium-dependent accumulation of membranous structures that interact with nearby undamaged membrane to form a seal [14–16]. Calcium also initiates processes leading to cell death and axonal Wallerian degeneration (breakdown of the axon distal to the site of injury) within 48–96 hours after injury [9,17].…”

“…The antioxidant Methylene Blue (MB) is also added prior to adding PEG to reduce vesicle formation [5]. After adding PEG to induce PEG-fusion of open, vesicle-free axonal ends, the PEG is washed away by isotonic saline containing calcium so that vesicles form to seal any remaining holes [3–6,9,14,15]. We hypothesize that this therapy will restore nerve electrophysiology after interposition autografting leading to improved behavioral outcomes.…”

Harmonic Analysis of Neuronal Membranes and Tissue Using SQUIDs

“…Measurements have been reported of the earthworm's giant medial nerve fiber impedance [10], septum resistance with extensive modeling [11], and electrical response to nerve cord stimulation [12]. The High-SQUID-based setup [8] for harmonic response measurements of Lumbricus terrestris is shown in Fig.…”