About the Importance of Paranodal Structures of the Ranvier Node for the Impulse Regeneration

Abstract

The site of the pulse regeneration in myelinated nerve is generally assumed to be the unmyelinated part of the axon membrane in the Ranvier node. To check this, a micro-irradiation technique using laser pulses (λ= 347 nm, J = 20ns) was used to produce morphological lesions of about 1 μm diameter in various regions of the Ranvier node. The electro-physiological functions were monitored parallel to the irradiation. Depending on the localization of the lesions two types of changes in these functions were observed: 1. If a definite site in the paranodal myelin sheath was damaged without affecting the axon, an action potential could no longer be elicited, although the resting potential as well as the stationary current-voltage behaviour remained unchanged. 2. A damage of the axon resulted in a break down of membrane potential and resistance. In most of the cases the excitability recovered after spontaneous or current induced restitution of the membrane resting potential and resistance. These observations indicate, that structures in the paranodal region are vital for the Na+-activation and inactivation. The membrane potential and stationary current-voltage behaviour can be attributed to the axon membrane. A new hypothesis concerning the mechanism of the Na+-activation-inactivation process is suggested. © 1975, Walter de Gruyter. All rights reserved.