Letter to the Editor

We read with great interest the article written by Peter A. Watterson and Graham M. Nicholson: ‘Nerve–muscle activation by rotating permanent magnet configurations’ (Watterson & Nicholson, 2016). Our understanding is that the authors produced a non-pulsating, time-varying, sustained magnetic field by the rotation of permanent magnets. This magnetic field flux change is adequate enough to induce an electric current in a very closely (couple of millimetres) placed isolated frog’s (Bufo marinus) peripheral nerve and muscle with an end effect of muscle activation. Existing coil systems generate a timevarying magnetic field able to induce an electric current in a nerve by passing an electrical current through the coil. The authors describe a very interesting concept, and a way to induce an electrical current in an excitable tissue, quite different from the existing and clinically used method. In their article the authors speculate on the potential to develop a system capable of inducing a current in the brain transcranially and activating nerves or muscles that lie close to the skin. The authors explain that the depth of penetration of the magnetic field scales with the device dimension (implying that adequate, deeper if necessary penetration can be achieved by using larger magnets). The trade-off, when using larger magnets, is using lower frequency to maintain the same stress in the magnet containment sleeve. It should be noted that using larger magnets would probably lead to a decreased level of focality which would reduce the possible precision. Another major trade-off is that this kind of rotating magnet set-up, compared to the coil system, uses a magnetic field pattern with constant amplitude which, a priori, disables the control of magnetic induction duration (because of sustained sinusoidal excitation). This is the most likely cause of the fatigue that the authors observed due to the depletion of the transmitter at the neuromuscular junction. Furthermore, the authors say that this fatigue behaviour is beyond the scope of their paper and that they focused on the activation threshold. These kind of results, which are characteristic of this procedure, need to be addressed before the conclusions about the usability of this kind of set-up can be generally accepted. Beside the authors’ enthusiasm regarding this kind of set-up, technologically it seems unlikely that adequate controlled stimulation of excitable tissue could be achieved for two reasons: (a) the distance from the skull to the brain is too long and a rotating permanent magnet cannot generate a strong enough field with sufficient focality to induce a precise electric current in the brain, and (b) the rotating permanent magnets generate a magnetic field with constant amplitude and it will be close to impossible to control them and make short trains of pulses, which is the standard methodology for excitable tissue stimulation.