Electrical equivalent circuit for myelinated nerve fibers

Source:The Axon, structure and function and pathophysiology. 1995
Edited by Stephen G. Waxman, Jeffery D. Kocsis, Peter K. Stys
Ch 4. Physiology of axons by J. Murdoch Ritchie
extract from page 79

The right-hand side shows the passive values for an entire node and internode for a fiber with an external diameter of 15 um and an internodal length of 1.5 um (<=probably 1.5 mm?)

Fig 4-13. Electrical equivalent circuit for myelinated nerve fibers.

We add some fragmentary data
On p 86 the following information is found (this is not an exact quotation)

Waxman and Bennett (1972) have shown that myelinated fibers can have diameters down to 0.2 um in the CNS (central nervous system) and that the ratio of the axon diameter and the diameter of axon plus myelin is close to 0.6 in most CNS myelinated axons.

An approximate relation between conduction velocity and fiber diameter for myelinated fibers is v/D~6m/s/um for diameters down to 2 um. For a diameter of 15 um this results in v~90m/s

A theoretical relation for nonmyelinated axons is v~2.3 m/s(D/1.1um)1/2

Internodal distance appears to be determined by the size of the cells enwrapping the axon.

Values as short as 10um are commonly observed in the CNS (Hildebrand et al., 1985b; Waxman, 1978). The length of mature oligodendrocytes is 10-15 um which in the Hursh-Rushton relation corresponds to a diameter of about 0.2 um

In the PNS (peripheral nervous system) internodal distance is determined by the size of Schwann cells and therefore there is a lower limit of 0.15-0.5 mm

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