Early Reactive Changes at Myelin Sheaths Gaps (nodes of Ranvier) of Nerve Fibers (a supravital study)

and functional characteristics of single nerve fibers have provided an understanding of some of the electrophysiological parameters of fibers [1, 2]. Previous studies of gaps (nodus interruptionis myelini, nodes of Ranvier) using voltage clamping have demonstrated the absence of ion channels (K+ channels) within them [5, 10]. Further studies showed that potassium currents appear 15–60 min after stretching of fibers, exposure to hypotonic Ringer’s solution or lysolecithin, and other treatments applied to paranodal areas [6, 9]. This region undergoes changes reminiscent of shifts or displacements of the myelin sheath away from gaps (Fig. 1). By gaps, physiologists often refer only to the myelin–free node clefts in fibers [15], though morphologically, gaps show a cleft, a myelin cone, and a gap bulb (Sotnikov’s terminology, see Fig. 2). These changes are often termed local demyelination, which refers to the loss of myelin and exposure of the axolemma in the paranodal region in addition to the axolemma not sheathed by myelin in the node cleft. It was hypothesized that K+ channels are initially located beneath the myelin in the axolemma of the paranodal region of the fiber outside the node cleft zone [8, 16] and possibly also along the internodal segment of the axon [7]. The distribution of K+ channels is associated with axoglial membrane contacts in the paranodal area [11, 12], suggesting that they are attached there by adhesion moleNeuroscience and Behavioral Physiology, Vol. 42, No. 7, September, 2012