Bioelectric events, detection

However, there are still electrode-detectable bioelectric fields from myelinated nerves during action events, since volume currents flow, as shown in Fig. 17.13. These bioelectric currents are dipolar in nature, meaning that ionic currents flow spatially from the source at the leading edge of depolarization to a sink at the repolarization edge. Separation distances between these nodes in mammals are on the order of a millimeter. 

Complex, random-looking electromyographic (EMG) waveforms are detected by electrodes placed near skeletal muscle. These waveforms result from the time and spatial superposition of thousands of motor unit events. During a strong contraction of a large skeletal muscle, monitored skin bioelectric waveforms can reach millivolt amplitudes. 

The propagated bioelectric response is detected with skin bioelectrodes some distance away from the stimulus. Nerve action events are transient and not synchronous with other fibers within a nerve bundle, so these action currents do not sum together in the same way as they do, for example, in the heart. Detection of the skin surface potential created by an action event from a single nerve fibril within a larger bundle is very difficult, pertiaps impossible. 

---