Stimulation of Neurons in the Brain: Mechanisms and Limitations

Abstract

Scientists have been using electrical stimulation to monitor and treat brain disorders for years. However, the capability to target specific brain regions marked the beginning of brain stimulation. The procedure falls under a larger neuromodulation therapy category, including deep brain stimulation (DBS), transcranial magnetic stimulation (TMS), microcoil recently, and others. These therapies can target specific neurons in the brain, helping to alleviate symptoms associated with movement disorders, epilepsy, depression, and other neurodegenerative disorders. This dissertation aims to discuss different brain stimulation methods and mechanisms.The study of deep neuron stimulation in the brain involves using the Hodgkin and Huxley model when an external stimulus is applied. In response to a moderate stimulus, the membrane triggers an action potential. However, a high or low stimulus dose not excite an action potential. In other words, weak stimuli do not excite neurons, moderate strength stimuli do, but strong stimuli do not, which means you can excite deeper neurons while not exciting neurons closer to the brain surface. The electric field produced by a microcoil is too small to cause neural stimulation. The maximum value of the induced electric field due to electromagnetic induction is around 0.026 mV/m. The electric field intensity in the tissue is approximately 4 mV/m due to capacitive coupling with the same current and frequency. Therefore, the electric field produced by capacitive coupling is much larger than that produced by electromagnetic induction. Considering both large coil (TMS) and microcoil stimulation, the activating function is one source of excitation. The activating function depends on the spatial distribution of the electric field gradient in active membrane analysis and the spatial frequency in spatial-frequency analysis. Both analyses show that a microcoil (tens of microns in size) has a higher threshold than a traditional coil (tens of millimeters in size) when the spatial frequency is large, or the spatial extent of the activating function is small. Consequently, the stimulation threshold for a microcoil is much higher than that of conventional coils.