Bioelectric sources nerves

In living tissue, important communication control is implemented by hormones and nerves. Hormones are slow broadcasting information carriers nerves are quick prewired point-to-point information carriers. Some cells are not excitable, such as adipose, connective tissue, and blood. They are passive, not under nerve control, and only weakly polarized. However, nerve, muscle, and gland cells are polarized and excitable within a 1/1000 s, such cells may react on trigger signals. The excitation of a cell is accompanied by an action potential. The action potential is the basic bioelectric event and signal source in the body. 

Parallel with the discovery of new sources of electricity, the detection of small bioelectric currents became possible. Soon after 0rsted s discovery in 1820, the first galvanometers appeared. The problem was twofold to increase sensitivity and to make the new sources follow the rapid changes of muscle and nerve currents. 

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. 

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