Size-dependent variation of nodal properties in myelinated nerve

Although the physiological properties of myelinated nerve fibres, such as action potential duration, are known to vary with fibre size 1,2 , there has been little systematic examination of whether their biophysical properties show a similar variation (for example, in ionic conductance). Rather, differences have been identified between fibres on the basis of species 3–6 , or on whether the fibres are sensory or motor 7,8 . We report here the results of a comparison of amphibian ( Rana pipiens ) fibres of different sizes. Surprisingly, only the largest fibres (16–20 µm), usually selected for voltage-clamp study, had the large outward potassium currents described initially by Dodge and Franken-hauser 9 . Small fibres (9–11 µm) had little or no potassium conductance, and fibres of intermediate diameter had properties graded between these two extremes. This gradual loss of potassium conductance in fibres of decreasing size seemed to be due to the progressive electrical concealment of the potassium channels beneath the paranodal myelin. Thus brief treatment with the demyelinating agent lysophosphatidyl choline 10,11 induced a large potassium conductance in the smaller fibres, but had little effect on the ionic currents of large fibres. We conclude that the biophysical properties of myelinated nerve fibres can vary with fibre size.