Action Potentials..........

http://www.psych.ualberta.ca/~ITL/ap/ap.swf

I). Action Potential

A.  What is an Action Potential?

An action potential is a temporary reversal of the polarity across the membrane of a muscle cell or nerve fiber.

B.  Terms

1. Resting membrane potenial
2. Depolarization
3. Threshold
4.  Propagation
5.  Refractory period

C). Polarity of cell membrane.

Active transport creates a concentration gradient

	Na+ is actively moved from the inside of the cell (low concentration) to the outside of the cell (high concentration)
	K+ is actively moved from the outside of the cell (low concentration) to the inside of the cell (high concentration)

There are more Na+ outside the cell than there are K+ inside of the cell.  As a result there is a negative charge across the membrane

 Positive outside Negative inside

D). Action Potential Stages

Step 1). Resting State Polarized State

	The outside of the membrane is positive and contains Na+ ions.
	The inside of the membrane is negative and contains K+ ions
	A stimulus is released: neurotransmitter or the depolarization of another part of the membrane

Step 2). Depolarization

	Na+ Channels open (facilitated diffusion) and Na+ rushes across the membrane.
	There is a decrease in the resting potential and the interior of the cell becomes positive

Step 3). Propagation of the Action Potential

The positive patch in the membrane changes the adjacent patch of the membrane.

Thus depolarization spreads.

Step 4). Repolarization

	Immediately after the action potential passes the membrane permeability changes again.
	Na+ channels close and K+ channels open.
	K+ rushes out of the cell.

This restores electrical conditions not the ionic condition

 Step 5). Na+/K+ pump restores ionic conditions (active transport) by pumping Na+ out and K+ in.

   

Refractory Period: The cell cannot be stimulated again until repolarization is complete.