Identification of the exact stimulated site in transcranial magnetic stimulation of the facial nerve

Bony foramina facilitate magnetic stimulation: an experimental cat sciatic nerve model

We have established an experimental bony foramen model in vivo using a cat sciatic nerve, a section of skull bone and a column made from methylmethacrylate. In each model, a foramen-like slit or fissure was created. Motor responses of the cat right gastrocnemius were elicited with a figure-of-8-shaped magnetic coil. Very high intensities of magnetic stimulation were necessary to evoke motor responses with the coil placed on the thigh. However, when the bone section or methylmethacrylate column was placed under the thigh muscle layer with the sciatic nerve fitted into the foramen-like slit or fissure, motor responses could be elicited with a smaller intensity of magnetic stimulation. Despite changes in stimulation intensity or shifts in the magnetic coil, the latency of the motor responses remained constant. By comparing the latency with the electrical recording, the site of excitation was predicted to be at the exit of the foramen. Our studies have confirmed that bony structures, especially bony foramina, facilitate excitation of the nerve by magnetic stimulation and that the exit of the foramen could be the preferential site for magnetic stimulation.