Electrical activity and structural correlates of giant nerve fibers in Kuruma shrimp (Penaeus japonicus)

Morphology and recordings of electrical activity of Kuruma shrimp (Penaeus japonicus) giant medullated nerve fibers were carried out. A pair of giant fibers with external diameter of about 120 μ and 10 μ in myelin thickness were found in the ventral nerve cord. The diameter of the axon is about 10 μ. Thus there is a wide gap between the axon and the external myelin sheath. Each axon is doubly coated directly by Schwann cells and indirectly by the myelin sheath layer which is produced by those Schwann cells. Impulse conduction velocities of these giant fibers showed a range between 90–210 m/sec at about 22°C. Large action potentials (up to 113 mV, rise time of 0.16–0.3 msec, maximum rate of rise of 650–1250 V/sec, half decay time of 0.2–0.3 msec, maximum rate of fall of 250–450 V/sec and total duration of less than 1.5 msec) could be obtained by inserting microelectrodes or by longitudinal insertion of 25 μ diameter capillary electrodes into the gap but no DC-potential difference was observed across the myelin sheath. Transmyelin electrical parameters were very favorable for fast impulse conduction: myelin resistance of 3 × 10⁴ Ω cm²; time constant of 0.38 msec; myelin capacitance of 1.35 × 10^?8 F/cm²; gap fluid resistivity of 23 Ω cm. The existence of nodes of Ranvier could not be demonstrated morphologically, but electrophysiological evidence suggests that a type of saltatory conduction occurs in these giant fibers.