How It Works:

Got all that! Good. Now let's put it all together.  We'll start with the neurotransmitter.  When neurotransmitter crosses the synaptic cleft, it bumps into the receptor sites on the postsynaptic dendrite, and temporarily binds to those sites, activating them.  Neurotransmitter is then released from the receptor sites, back into the synaptic cleft, where it is either destroyed by enzymes and carried away as waste, or taken back up into the presynaptic terminal button, to be used again.  Note that the neurotransmitter does not enter the postsynaptic neuron.  Instead, it works like a key going into a lock, that may unlock or open the door, but may not enter the room.

Once the receptor sites have been stimulated by neurotransmitter, one of two things can happen, depending on the type of synapse and/or the type of neurotransmitter.  1) an EPSP, or 2) an IPSP. Let's assume it's an EPSP this time.  This would cause chemically gated Na+ channels to open up for a fraction of a second. Na+ would rapidly flow in, depolarizing the cell and bringing it closer to action (exciting it).  If enough other EPSPs are collected, it is possible that the threshold potential (-50mV) could be reached. If it is, then voltage-gated channels to Na+ open up, causing more Na+ to flow in. If this is not counteracted by IPSPs and spreads to the axon hillock, the threshold potential may be reached at the axon hillock, which would cause the cell to fire. This means that voltage-gated Na+ channels will open up on the axon, near the axon hillock, causing Na+ to rush in and change the -70mV difference to at least a 0mV difference.  This starts an action potential. The cell is now "in action."  The myelin insulates the axon, so that the 0mV difference caused by the opening of voltage-gated channels at the axon hillock spreads down the axon a bit, to the first node of Ranvier.  At the node, more voltage-gated channels open (then close quickly), bringing in more Na+, and changing the -70mV difference there to at least 0mV.  This will open more channels a bit further down the axon (at the next node of Ranvier), changing the -70mV difference there to at least 0mV. This will open more channels a bit further down the axon (at the next node of Ranvier), changing the -70mV difference there to at least 0mV. This will open more channels a bit further down the axon (at the next node of Ranvier), changing the -70mV difference there to at least 0mV. (OK, enough of that…) This continues all the way down the axon and spreads into the terminal branches. It is important to remember that the channels open only very briefly, then close again.  Meanwhile, right behind this wave of opening and closing channels, voltage-gated K+ channels are also open, causing K+ to flow out, down the K+ gradient, and allowing the cell to regain its resting potential. So the action potential appears sort of like a wave of action going down the axon. Nothing actually moves through the axon, though. Energy is transferred, but that's it. 

Activities

Diagrams