Increase in conduction velocity in myelinated nerves due to stretch - An experimental verification

Abstract

Background: Based on published experimental evidence, a recent publication revealed an anomalous phenomenon in nerve conduction: for myelinated nerves the nerve conduction velocity (NCV) increases with stretch, which should have been the opposite according to existing concepts and theories since the diameter decreases on stretching. To resolve the anomaly, a new conduction mechanism for myelinated nerves was proposed based on physiological changes in the nodal region, introducing a new electrical resistance at the node. The earlier experimental measurements of NCV were performed on the ulnar nerve at different angles of flexion, focusing at the elbow region, but left some uncertainty for not reporting the lengths of nerve segments involved so that the magnitudes of stretch could not be estimated.

Aims: The aim of the present study was to relate NCV of myelinated nerves with different magnitudes of stretch through careful measurements.

Method: Essentially, we duplicated the earlier published NCV measurements on ulnar nerves at different angles of flexion but recording appropriate distances between nerve stimulation points on the skin carefully and assuming that the lengths of the underlying nerve segment undergoes the same percentages of changes as that on the skin outside.

Results: We found that the percentage of nerve stretch across the elbow is directly proportional to the angle of flexion and that the percentage increase in NCV is directly proportional to the percentage increase in nerve stretch. Page's L Trend test also supported the above trends of changes through obtained p values.

Discussion: Our experimental findings on myelinated nerves agree with those of some recent publications which measured changes in CV of single fibres, both myelinated and unmyelinated, on stretch. Analyzing all the observed results, we may infer that the new conduction mechanism based on the nodal resistance and proposed by the recent publication mentioned above is the most plausible one to explain the increase in CV with nerve stretch. Furthermore, interpreting the experimental results in the light of the new mechanism, we may suggest that the ulnar nerve at the forearm is always under a mild stretch, with slightly increased NCV of the myelinated nerves.

Keywords: elbow flexion; myelinated nerve; nerve conduction velocity; nerve stretch; nodes of Ranvier; ulnar nerve.

Figure 1

Simplified electrical model of a myelinated nerve fibre. (A) traditional model (B) proposed model (Rabbani, 2018).

Figure 2

Homemade gadget to measure NCV due to stretching at different angles of elbow flexion.

Figure 3

Schematic diagram of elbow position during experiment. (A) relaxed position, (B) flexed position (stretched ulnar nerve).

Figure 4

Schematic diagram of electrode arrangement for the measurement of NCV of the ulnar nerve between different nerve segments.

Figure 5

Experimental changes of average values of conduction velocities. (A) NCV_BE-W, (B) NCV_AE-W and (C) NCV_AE-BE corresponding to different angles of elbow flexion from 44 nerves of both hands of 22 subjects. Average values and standard errors of means are shown for each experimental point.

Figure 6

Percentage of nerve stretch with flexion angle for all subjects (44 nerves). Average values and standard errors of means are shown for each experimental point. A fitted straight line is also shown.

Figure 7

Variation of conduction velocity at the AE-BE segment. (A) The change of NCV_AE-BE plotted against the percentage of nerve stretch due to elbow flexion, the references of both being the corresponding values at 0° angle of flexion. (B) The percentage change of NCV_AE-BE plotted against the percentage of nerve stretch due to elbow flexion, the reference of the former being the corresponding value at 0% stretch. Fitted straight lines are also shown.