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Joe Legris jalegris at xympatico.ca
Sat Nov 22 08:15:23 EST 2003

Mark Zarella wrote:
>> The propagation of action potentials in neurons involves ionic 
>> currents  AND electrical conduction.
> Care to explain the difference?  I think this conversation is awfully 
> close to bordering on semantics.  So I suppose a definition of 
> electrical current and ionic current is in order.  I think we all agree 
> that ionic current can simply be characterized as the net flow of charge 
>  due to translational motion of ions.  So is "electrical current" that 
> current due to electron flow only, or can it be characterized simply by 
> net flow of charge?

An electrical current might involve only electrons, as in metals or in a 
vacuum, or both electrons and ions.

But I think the confusion lies in the distinction between current and 
potential. Electrical current (measured in Amps) is defined as the 
amount of charge passing through a particular area per unit time. The 
charge carriers may be electrons or other charged particles, such as 
ions. Electrons are very light and mobile and effectively respond only 
to electrical forces. Ions are relatively heavy and sluggish and respond 
to both electrical forces and chemical forces, such as concentration 

Electric potential (measured in Volts) may be viewed as the rate at 
which a charged particle will acquire (or lose) energy when moving 
through an electric field. Electric fields occur around static charge 
concentrations and across conducting media such as resistors, ionic 
solutions, etc.

An action potential is a wave of electrical potential resulting from the 
  flow on ions in and around the neuron. The crest of the wave is the 
point where the membrane potential is at a maximum - the result of ion 
channels opening and allowing positively charged sodium ions to flow in. 
This process in relatively slow - the ion channels take time to open and 
the ions move slowly compared to electrons. All told it takes a 
millisecond or so.

During an action potential, there is also an ionic current flow from the 
crest of the wave along the axon in both directions. This is associated 
with an electrical field and a potential which fades the further you get 
away from the crest of the action potential. Even though the ions move 
quite slowly along the axon, the field moves at nearly the speed of 
light. The electrical field distributes the action potential locally 
along the axon, which responds as fast as the ion channels will open. 
This is associated with local ionic current along the axon, but in the 
time it takes the electric field to propagate, the ions that conduct the 
current have drifted only very slightly.

Joe Legris

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