Download Action potentials travel along the axons of neurons.

Action Potentials
A nerve is usually composed of a bundle of neurons with Glial cells and blood vessel to supply needed
materials.
Neurons are not connected directly to one another – there are gaps (synapses) between the neurons.
Action potentials travel along the axons of neurons.
The cell membrane of neurons have an uneven distribution of charges, with the inside more negative
than the outside (Resting potential – 10mV)
The balance is maintained by the Na/K pump – where Na+ is more concentrated outside the cell, and K+
is more concentrated inside the cell. The cell membrane allows K+ to pass more easily.
A change in this balance by moving ions causes an electrical charge along the length of the cell  this is
an Action potential.
Action potentials are very quick “Flips” of the resting potential. Na & K gates open to allow the ions to
cross and reverse positions (Na+ inside ... K+ outside). The reason positive ions move is because the
negative ions are too large
Action potentials start at one specific area and then spread of Propagate along the length of the axon.
Action potentials are the nerve impulses that occur along axons.
If a stimulus (sound, light, touch, etc) occurs that is strong enough to reach the Threshold level ... An
action potential results. There is no difference in the strength of the action potential. They are “All or
Nothing” event.
Neural impulses are essentially Digital signals (1 or 0) since they happen or they don’t. An ACTION
POTENTIAL is the same strength each time.
Different strengths of stimulus are felt by:
-
The frequency of which the impulses go
The amount of neurons sending the stimulus or impulse
*Picture from the blog of the action potential graph*
A stimulus causes Na+ leak channels to open in the cell membrane in a small area.
This depolarizes a small area of the axon as Na+ rush into the cell (Graded potential). If this reaches
Threshold the voltage gated Na+ channels open causing more Na+ to flood into the cell.
Positive Feedback mechanisms cause more and more Na+ gated channels to open causing the
membrane potential to become more and more positive.
The amount of Na+ in exceeds the amount of K+ going out.
Conformation shifts occur in the Na+ channels that allow Na+ ions to enter the cell
A.
B.
C.
D.
At rest: activation gate closed (AG) ... Inactivation gate open (IG)
Activation: Both Gates Open
Inactivation: Inactivation gate closed (IG) ... Activation gate open (AG)
Refractory Period: Both gates closed
Once the membrane potential inside the cell becomes positive the voltage gated Na+ channels snap
shut and there is no influx of Na+ into the cell.
This is the ‘peak’ of the action potential
Gates for K+ ions open that allow K+ to leak across the cell membrane
This is a negative feedback mechanism
Eventually the membrane returns to resting potential (-70mV)
However... the concentrations of Na+ and K+ aren’t where they were before the action potential.
The concentrations have been reversed!
We now have more Na+ in the cell and more K+ outside the cell.
The Na/K pump kicks in and restores the original concentrations.
Until this occurs the cell is hyperpolarized and cannot fire another action potential  Refractory Period
Question: How fast does an action potential travel?
Roughly a rate of 2m/s
How can we increase the speed?
1. The diameter of the axon is proportional to the speed of the action potential.
2. Use Myelin
The myelin sheaths secreted by Schwann cells insulate portions of the axon,