Localization of nerve depolarization with magnetic stimulation

Transcranial magnetic stimulation of the central and peripheral motor pathways to the lingual muscles in cats

OBJECTIVE

We have assessed a technique to stimulate the intracranial hypoglossal nerve with ease and reproducibility by using magnetic coils (MCs) and to detect a reliable site of excitation in animal experiments in order to establish a method to evaluate the motor pathway to lingual muscles.

METHODS

We recorded the motor responses from the lingual muscles of 5 adult cats under general anesthesia by magnetic and electrical stimulation of the intracranial hypoglossal nerves. Figure of 8 and round MCs were used to investigate the optimal position and direction to evoke the motor responses.

RESULTS

The round MC was useful for cortical stimulation. The figure of 8 coil, positioned in the back of the head of the examined side, parallel to the cervical spine, was essential for stimulation of the intracranial hypoglossal nerve. Analysis of the latencies, and the observation that the motor responses disappeared after transection of the nerves at the exit of the hypoglossal canal, demonstrated that the site of the excitation is at the exit of the hypoglossal canal.

CONCLUSION

Magnetic stimulation using a figure of 8 coil can elicit tongue motor responses with ease and reliable reproducibility, stimulating the hypoglossal nerve at the exit of the hypoglossal canal.

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.