Electrotonic potential

In physiology, electrotonus refers to the passive spread of charge inside a neuron and between cardiac muscle cells or smooth muscle cells. Passive means that voltage-dependent changes in membrane conductance do not contribute. Neurons and other excitable cells produce two types of electrical potential:

• Electrotonic potential (or graded potential), a non-propagated local potential, resulting from a local change in ionic conductance (e.g. synaptic or sensory that engenders a local current). When it spreads along a stretch of membrane, it becomes exponentially smaller (decrement).
• Action potential, a propagated impulse.

Electrotonic potentials represent changes to the neuron's membrane potential that do not lead to the generation of new current by action potentials.^[1] However, all action potentials are begun by electrotonic potentials depolarizing the membrane above the threshold potential which converts the electrotonic potential into an action potential.^[2] Neurons which are small in relation to their length, such as some neurons in the brain, have only electrotonic potentials (starburst amacrine cells in the retina are believed to have these properties); longer neurons utilize electrotonic potentials to trigger the action potential.

Electrotonic potentials have an amplitude that is usually 5-20 mV and they can last from 1 ms up to several seconds long.^[3] In order to quantify the behavior of electrotonic potentials there are two constants that are commonly used: the membrane time constant τ, and the membrane length constant λ. The membrane time constant measures the amount of time for an electrotonic potential to passively fall to 1/e or 37% of its maximum. A typical value for neurons can be from 1 to 20 ms. The membrane length constant measures how far it takes for an electrotonic potential to fall to 1/e or 37% of its amplitude at the place where it began. Common values for the length constant of dendrites are from .1 to 1 mm.^[2] Electrotonic potentials are conducted faster than action potentials, but attenuate rapidly so are unsuitable for long-distance signaling. The phenomenon was first discovered by Eduard Pflüger.