What is a tonic current?

Answer 1

Not all the electrical signalling in the nervous

system is by way of action potentials, or impulses.

Indeed it could be argued that some of the most

important, if not the most important, of the central

nervous system's communications depend upon

non-impulse signalling. These signals, which are at

least one order of magnitude and sometimes two or

more orders of magnitude, weaker than action

potentials have been termed electrotonic potentials.

They are small depolarisations of a nerve process's

membrane and are caused by the essentially passive

spread of electrical current through the conducting

fluids inside and outside nerve cells and their

processes. Nonetheless, however small electrotonic

potentials may be, they can have very considerable

effect on the physiology of neuronal membranes

and thus on the large-scale functioning of the brain.

cited from - Elements of Molecular Neurobiology 3rd ed C. U. M. Smith