Before you can learn about the action potential, you have to learn about gated channels.  Gated channels are simply protein channels that only open under certain conditions.  Like a tollbooth gate only opens after you have paid, gated channels only open after something has happened.  That something could be an electrical event or a chemical event.

    If an electrical event opens a gated channel, it is called a voltage-gated channel.  If a chemical signal opens a gated channel, it is called a ligand-gated channel (since a chemical that binds to something can be called a ligand).   Let's first consider voltage-gated channels.

    Remember that image I had drawn to describe leak pores to you in Unit 10?  Here it is again:

In Unit 10, all we talked about were the three channels toward the right side of the above drawing... we never got to the two (with the question marks in them) at the left.   Those two channels are the voltage-gated channels.

    When some electrical event occurs (we will discuss what that event is, later...), that triggers the voltage-gated channels to open.  There is one voltage-gated channel for sodium ions, and a separate one for potassium ions.  The sodium ion one is the sodium voltage-gated channel (Na⁺ V-gated channel), while the potassium one is the delayed rectifier.   Both of these open when the electrical event triggers them.  The Na⁺ V-gated channel opens right away, but the delayed rectifier takes a moment (less than one millisecond) to open. 

    Once a new set of channels opens, ions can flow (by diffusion) down their concentration gradient.  No energy is necessary for that to occur.  We can talk about the new route for ion flow as a new conductance.  All that means is that a pathway exists for ions to flow, as opposed to no conductance, when ions cannot cross the membrane for lack of a pathway.

    One last point to mention:  the Na⁺ V-gated channel cannot stay open long... it has an automatic shut off.  Then, after it has shut off, it endures a period of inactivation, during which it cannot re-open.   We will go over this on the action potential page.

    Let's go over the sequence of events:

1. some electrical event triggers the voltage-gated channels to open
2. the Na⁺ V-gated channel opens immediately
3. Na⁺ ions flood into the cell (along their concentration gradient)
4. the delayed rectifier opens AND the Na⁺ V-gated channel inactivates
5. K⁺ ions flood out of the cell (along their concentration gradient)
6. the Na⁺/K⁺ pump uses ATP to try to restore the concentration gradients

These events are depicted, in sequence, in this animation:

Now go on to the action potential page to see how these gated channels cause changes in the electrical potential of the cell.

© 2006 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.