AXONAL CONSTRICTION AT RANVIERS NODE INCREASES DURING DEVELOPMENT

We have studied the ratio between the nodal and the intermodal diamete r (the d(n)/d(in) ratio) of large myelinated axons in the L(7) ventral spinal root of the cat during pre- and postnatal development using li ght and electron microscopy. A substantial nodal constriction, d(n)/d( in) = 0.6, was found at the beginning of myelination, about 2 weeks be fore birth The ratio decreased during the subsequent 10 weeks and appr oached the adult value of 0.47 (SE 0.01, N = 45) in the 8 weeks old ki tten. The observations are discussed with respect to the maturation of the nodal region and to our earlier idea that the constricted nodal a xon segments of large peripheral myelinated nerve fibres of adult cats and kittens 2 months and more of age are sites capable of interacting with and perhaps even controlling the passage of axonally transported materials.