The principal objective of the proposed project is to design and test coils that can produce localized magnetic fields for neural excitation. Magnetic stimulation is a fairly recent method to activate excitable tissue in the peripheral and the central nervous system non-invasively. Despite the potential usefulness of this technique, the most important problem with magnetic stimulation to date is the large spread of the induced field produced by coils compared to the size of neuronal structures. Localization of induced fields as proposed here refers to a reduction of the spatial extent of the induced fields produced by magnetic coils. Such a reduction would. provide concentrated regions of higher flux linkage and thereby improve selectivity and efficiency of magnetic stimulation. Further, some of the basic mechanisms underlying activation of peripheral nerves and neural tissue in the brain are still unclear. In particular, elements excited and sites of excitation are still not well known in both peripheral nerves and the CNS. Another problem is the absence of accurate methods to predict threshold strengths based on information about the induced fields. The knowledge of the above mentioned basic mechanisms would enable design of better coils for use in magnetic stimulation. Moreover, it is now known that tissue inhomogeneities in excitable tissue (e.g.. branching, bending and termination) would localize excitation sites to anatomical structures. However, the exact mechanisms of effects of such inhomogeneities with localized fields are unknown. Therefore, we propose to develop methods to predict threshold strengths during magnetic stimulation and to analyze effects of tissue inhomogeneities using theoretical models, computer simulations and in vitro experiments. We also propose to design optimal coils such that both energies required for threshold excitation of neuronal.structures and the spatial extents of the fields are minimized. Localization of magnetic fields will firstly be used in in vitro hippocampal slice preparations to measure the effects of magnetically-induced fields in the CNS. This preparation is particularly useful since the neurons are directly accessible for intra- and extracellular recording and will allow direct measurement of excitation mechanisms. It is important to use coils that produce localized fields for this preparation because the spatial extent of the fields from regular solenoidal coils are large compared to the small size of the tissue. Hence, concentration of flux linkage is required for efficient excitation. Finally, we propose to test localized coils using in vivo experiments for bladder contraction in dogs. Since stimulation of the nerves to the bladder using large coils stimulates nerves to the sphincter and other abdominal and pelvic structures, we intend to use coils that produce localized fields for selective contraction of the bladder. This would allow magnetic stimulation to be used not only as a diagnostic tool for non-invasive excitation of the nervous system but also as a functional tool for patients with impaired bowel and bladder function. This combination of coil design and optimization based on theoretical models, computer simulations with related experiments should generate useful and important information about the mechanisms of magnetic stimulation for excitation of the nervous system and thereby increase its functionality.
