Electron microscopic serial section analysis of nodes of Ranvier in lumbar spinal roots of the cat: A morphometric study of nodal compartments in fibres of different sizes

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Abstract
Serially sectioned nodes of Ranvier from nerve fibres 2–20 μm in diameter of feline ventral and dorsal spinal roots were examined electron microscopically, reconstructed to scale and analysed morphometrically. The assumed ‘fresh-state’ value ot several structural variables, considered to be of functional significance, were calculated by the use of compensation factors. The compensated data were plotted against fibre and axon diameters. It was calculated that the membranous area of the ‘fresh-state’ nodal axon segment increased more or less exponentially from less than 5 μm2 to 30 μm2 with increasing fibre diameter (D). Most variables associated with the nodal gap and the Schwann cell initially increased rapidly withD and then levelled out or even decreased in fibres with aD value greater than 8–12 μm. The area open for communication between the nodal axolemma and the endoneurial space was 30–100 times smaller than the membrane area of the nodal axolemma. The volume of the extracellular space in the nodal gap, outside the nodal axolemma, increased linearly from less than 0.1 μm3 to about 0.6 μm3 with increasing fibre size. The Schwann cell membrane area facing the nodal gap outnumbered the membrane area of the nodal axon by 10–15 times in nerve fibres with aD value between 5 and μm. Some functional implications of the ‘fresh-state’ nodal model are discussed.