Cellular Mechanisms of Transcranial Magnetic Stimulation in Climbing Fibers and Purkinje Neurons in the Cerebellum

Although transcranial magnetic stimulation (TMS) is widely used for brain stimulation, fundamental issues about its underlying mechanisms remain unresolved. We investigated some of these issues experimentally using an intact isolated turtle cerebellum in vitro, employing a novel chamber designed to deliver precisely calibrated induced electric fields along cortical depth. Our results show that single-pulse TMS can directly activate Purkinje cells and climbing fibers, and synaptically activate Purkinje cells via climbing fibers -- all within the first 1.2 ms. Specifically, current source density analysis showed that TMS directly (non-synaptically) activated (1) climbing fibers near the bend w