A magnetic evaluation of peripheral nerve regeneration: I. The discrepancy between magnetic and histologic data from the proximal segment.

Histologic techniques can quantify the number of axons in a nerve, but give no information about electrical conductibility. The number of functional myelinated neuronal units in a nerve can be quantified based on a magnetic recording technique. When studying reconstructed peripheral nerves a significant difference between the results found with these two techniques can be observed. A comparison was made between the long-term changes in the number of histologically and magnetoneurophysiologically measured neuronal units proximal to a nerve reconstruction. This study was performed on 6 New Zealand White rabbits, 20 weeks after the peroneal nerve had been reconstructed. The contralateral nerves were used as a control. Histologic examination demonstrates a statistically significant decrease of approximately 5% in the number of myelinated fibers. The magnetoneurophysiological results demonstrate a decrease which is estimated to be caused by the loss of approximately 50% of the functional myelinated neuronal units in the nerve. Therefore we conclude that of the initially available myelinated neuronal units, 5% degenerate completely, 45% are vital but lose their signal conducting capability, and the remaining 50% are vital and continue to conduct signals. Apparently, only this latter group of 50% of the initially available functional neuronal units appears to remain available for functional recovery.

A magnetic evaluation of peripheral nerve regeneration: II. The signal amplitude in the distal segment in relation to functional recovery.

Motor and sensory function in a healthy nerve is strongly related to the number of neuronal units connecting to the distal target organs. In the regenerating nerve the amplitudes of magnetically recorded nerve compound action currents (NCACs) seem to relate to the number of functional neuronal units with larger diameters regenerating across the lesion. The goal of this experiment was to compare the signal amplitudes recorded from the distal segment of a reconstructed nerve to functional recovery. To this end, the peroneal nerves of 30 rabbits were unilaterally transected and reconstructed. After 6, 8, 12, 20, and 36 weeks of regeneration time the functional recovery was studied based on the toe-spread test, and the nerve regeneration based on the magnetically recorded NCACs. The results demonstrate that the signal amplitudes recorded magnetically from the reconstructed nerves increase in the first 12 weeks from 0% to 21% of the amplitudes recorded from the control nerves and from 21% to 25% in the following 23 weeks. The functional recovery increases from absent to good between the 8th and the 20th week after the reconstruction. A statistically significant relation was demonstrated between the signal amplitude and the functional recovery (P < 0.001). It is concluded that the magnetic recording technique can be used to evaluate the quality of a peripheral nerve reconstruction and seems to be able to predict, shortly after the reconstruction, the eventual functional recovery.

Loss of viable neuronal units in the proximal stump as possible cause for poor function recovery following nerve reconstructions.

Function recovery after nerve reconstructions is often poor. Could this be caused by a loss of viable neuronal units proximal to the nerve reconstruction? The number of neuronal units (i.e., a motor or sensory neuron, including its axon and axonal branches) in the proximal segments of reconstructed peripheral nerves were studied using a novel magnetic recording technique. In five rabbits a common personal nerve was transected and microsurgically reconstructed. After 8 weeks regeneration time the nerve compound action signals were recorded magnetically from the reconstructed as well as from the healthy contralateral peroneal nerve and from peroneal nerves of five unoperated control animals. The amplitudes of the recorded signals were compared and the diameter distribution histograms were calculated. These calculations were based on the conduction distance between the stimulator and the sensor and the conduction velocities of 30 different axon diameter classes ranging from 3 to 18 microns. Our results indicate that there is a reduction of approximately 50% in the number of viable neuronal units at 10 mm proximal to a simple nerve reconstruction after 8 weeks regeneration time. The number of neuronal units innervating a hand is strongly correlated with clinical function in a healthy hand. The reduction in viable neuronal units after a reconstruction, demonstrated in our experiments, corresponds with a frequently clinically observed decrease in function after nerve reconstructions. Therefore, we suggest that the number of viable neuronal units may be a good indicator of final functional recovery.

A comparison of electric and magnetic compound action signals as quantitative assays of peripheral nerve regeneration.

The evaluation of peripheral nerve regeneration is of great interest in clinical as well as in experimental situations. However, there are few techniques that give early and quantitative information on the status of the regeneration process. If quantitative assays would be available, different surgical techniques and medications could be evaluated more accurately in relation to axonal ingrowth and functional recovery. The purpose of this study was to investigate the merits of nerve compound action signals (NCASs) recorded electrically and signals recorded with a novel magnetic recording technique. We compared the two techniques in the rabbit peroneal nerve, 2, 4, 6, and 8 weeks after a nerve reconstruction. Our conclusions are that the signals recorded with the magnetic sensor are far more reproducible and less prone to stimulus artifact than the electrically recorded signals. Furthermore, the magnetic recording shows that the number of axons that have regenerated increases with time. Previously, this could only be determined with histological studies. Other ingrowth parameters that can be quantified are the average ingrowth distance, and the variation between axons in ingrowth velocity.

The biomagnetic signature of a crushed axon. A comparison of theory and experiment.

The response of a crayfish medial giant axon to a nerve crush is examined with a biomagnetic current probe. The experimental data is interpreted with a theoretical model that incorporates both radial and axial ionic transport and membrane kinetics similar to those in the Hodgkin/Huxley model. Our experiments show that the effects of the crush are manifested statically as an elevation of the resting potential and dynamically as a reduction in the amplitude of the action current and potential, and are observable up to 10 mm from the crush. In addition, the normally biphasic action current becomes monophasic near the crush. The model reflects these observations accurately, and based on the experimental data, it predicts that the crush seals with a time constant of 45 s. The injury current density entering the axon through the crush is calculated to be initially on the order of 0.1 mA/mm2 and may last until the crush seals or until the concentration gradients between the intra- and extracellular spaces equilibrate.

A model for axonal propagation incorporating both radial and axial ionic transport.

We present an axonal model that explicitly includes ionic diffusion in the intracellular, periaxonal, and extracellular spaces and that incorporates a Hodgkin-Huxley membrane, extended with potassium channel inactivation and active ion transport. Although ionic concentration changes may not be significant in the time course of one action potential, they are important when considering the long-term behavior (seconds to minutes) of an axon. We demonstrate this point with simulations of transected axons where ions are moving between the intra- and extracellular spaces through an opening that is sealing with time. The model predicts that sealing must occur within a critical time interval after the initial injury to prevent the entire axon from becoming permanently depolarized. This critical time interval becomes considerably shorter when active ion transport is disabled. Furthermore, the model can be used to study the effects of sodium and potassium channel inactivation; e.g., sodium inactivation must be almost complete (within 0.02%) to obtain simulation results that are realistic.

Cellular magnetic fields: fundamental and applied measurements on nerve axons, peripheral nerve bundles, and skeletal muscle.

We review the fundamental origins of biomagnetic fields in terms of ionic currents flowing at the cellular level. Mathematical models provide the link between the macroscopic fields and the microscopic electrophysiological sources. The single cell view is then expanded to include more complex systems such as nerve and muscle bundles. We provide an overview of the two most promising methods to measure the magnetic fields from these systems, we discuss the capabilities and limitations of the techniques based on comparisons with conventional electric methods, and we show that the direct measurement of action currents and the ability to scan along nonuniform samples are of prime importance. We present a number of interesting applications for basic research under laboratory conditions, including measurements of the time course of electrophysiological changes following a crush injury to a nerve and the spatial and temporal dependence of action currents as they propagate away from the motor endplate zone of a single motor unit in skeletal muscle. We conclude by discussing the potential applications in the clinic, including the intraoperative assessment of neuroma-in-continuity and the long-term monitoring of nerve regeneration and degenerative neuromuscular disorders.

Magnetic field of a single muscle fiber. First measurements and a core conductor model.

We present the first measurements of the magnetic field from a single muscle fiber of the frog gastrocnemius, obtained by using a toroidal pickup coil coupled to a room-temperature, low-noise amplifier. The axial currents associated with the magnetic fields of single fibers were biphasic and had peak-to-peak amplitudes ranging between 50 and 100 nA, depending primarily on the fiber radius. With an intracellular microelectrode, we measured the action potential of the same fiber, which allowed us to determine that the intracellular conductivity of the muscle fiber in the core conductor approximation was 0.20 +/- 0.09 S/m. Similarly, we found that the effective membrane capacitance was 0.030 +/- 0.011 F/m2. These results were not significantly affected by the anisotropic conductivity of the muscle bundle. We demonstrate how our magnetic technique can be used to determine the transmembrane action potential without penetrating the membrane with a microelectrode, thereby offering a reliable, stable, and atraumatic method for studying contracting muscle fibers.