Asymmetry of the node of ranvier

Abstract

ABSTRACT The nodal constrictions of Ranvier in normal limb nerves of mammals are bounded by asymmetric paranodal bulbs, the proximal bulb (nearer the cell body) being larger in all dimensions. This investigation was undertaken to determine whether the asymmetry results from partial damming of proximo-distally flowing axoplasm at the nodal constriction, or whether other features of local growth patterns are more relevant. The degree of asymmetry was estimated on teased, osmicated fibres from normal immature and mature limb nerves to skin and muscle and on ventral nerve roots. Estimates were also made on the central processes of dorsal root ganglionic cells with their contrasting direction of flow, after alteration of growth patterns in regenerates of crushed immature and mature nerves, and in the recurrent laryngeal nerve which pursues an exceptional course in relation to surrounding tissues. A polarization of asymmetry with larger proximal bulbs was found in uncomplicated limb nerves and after simultaneous regeneration and limb growth following crushing of immature nerves. Mixed populations (that is, with no preferred direction of asymmetry and often symmetrical bulbs) were found in dorsal and ventral nerve roots and the apical recurved segment of the recurrent laryngeal, whilst the mature regenerates closely approached the symmetrical condition. The descending and ascending limbs of the recurrent laryngeal nerve showed a reversal of polarization with respect to the cell body, but similar with respect to the cephalo-caudal body axis. It was concluded that damming of directionally flowing axoplasm was not causally related to the formation of asymmetric bulbs. The overall interstitial growth pattern of limb nerve elongation as revealed by intemodal distance studies contrasts with the differential growth and maturation gradients shown by the various limb segments through which the nerve passes. These differences result in a relative movement between the myelinating and elongating Schwann cell and the surrounding limb tissues. The movements are considerable and unidirectional throughout most of the limb, the Schwann cells becoming relatively further removed from the limb apex, whilst the movements minimize at the limb root and apex. It is suggested that the altered mechanical conditions operating at the ‘advancing’ and ‘trailing’ ends of the cell are related to the dimensional differences of the paranodal apparatus comprising the bulb at each end of the cell, and secondarily result in asymmetric nodes of Ranvier.