The Neuron & Action Potential
Transcript The Neuron & Action Potential
How neurons send an electrical
How Neurons Communicate
• Action Potential is the electrical process
that neurons use to communicate with each
• Action Potentials are based on movements
of ions (charged particles) between the
outside and inside of the axon
• Action Potential is an All or Nothing
Process (like a gun firing)
Triggering Action Potential
Threshold is something in the environment (light, pressure,
heat, chemicals or another neuron) that tell the neuron to fire
and send an electrical signal (action potential)
How do you feel something that is intense?
More neurons fire, the intensity of their electric impulse
always stays the same.
Thresholds can tell an neuron to fire (excitory) or to
stop firing action potential (inhibitory)
Lou Gehrig’s Disease - too many inhibitory stimuli cause the
muscles to freeze up.
Parkinson’s Disease - too many excitatory stimuli cause the muscles
to move without control.
Steps to Action Potential
Step 1: Threshold is Reached
• Axon at Resting Potential - fluid inside the
axon is mostly negatively charged with
positive on the outside (polarized)
• An impulse is triggered in the neuron’s
dendrite when stimulated by pressure, heat,
light or a chemical messenger from another
neuron (stimulus threshold).
• At rest, the inside of the cell is at -70 microvolts
• With inputs from dendrites inside becomes more positive
• If resting potential rises above threshold, an action potential
starts to travel from cell body down the axon
• Figure shows resting axon being approached by an AP
Step 2: Action Potential Begins
• When neuron fires, its axon membrane is
selectively permeable (has gates that open).
• Gates in the axon called ion channels open
allowing positive sodium ions to enter the axon
giving it a brief positive electrical charge the axon
• The brief positive charge is action potential.
Depolarization Ahead of AP
• AP opens cell membrane to allow sodium (Na+) in
• Inside of cell rapidly becomes more positive than outside
• This depolarization travels down the axon as leading edge
of the AP
Step 3: Refractory Period
• As the next gates open allowing positive sodium ions in,
the previous gates close and begin to pump the positively
charged sodium ions out of the axon. (repolarized).
• This step is called the refractory period and the axon
cannot fire again until it returns to resting potential
(negative polarized state inside the axon).
• The entire process is like falling dominoes all the way
down the axon except these dominoes can set themselves
back up as soon as the fall over.
• Q: Why do you think the axon has to set itself back to a
resting state so quickly (3 milliseconds)?
• A: So the neuron can fire again and send another message
immediately after the last one.
• After depolarization potassium (K+) moves out restoring
the inside to a negative voltage
• This is called repolarization
• The rapid depolarization and repolarization produce a
pattern called a spike discharge
Action Potential Within a Neuron
How can a toilet represent Action
• Full Toilet – Resting Potential
• Push Flush Lever – Threshold Stimulus
triggering Action Potential.
• Toilet Refilling/Can’t Flush –
• Sewer Pipes – One-way
communication like action potential
only goes from dendrite end to axon