Action PotentialHow neurons send an electrical

message

How Neurons Communicate

• Action Potential is the electrical process that neurons use to communicate with each other

• 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)

Threshold: 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).

Resting Potential

• 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 (depolarized).

• 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.

Repolarization follows

• 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

DAILY

DOUBLE

How can a toilet represent Action Potential?

• Full Toilet – Resting Potential• Push Flush Lever – Threshold Stimulus

triggering Action Potential.• Toilet Refilling/Can’t Flush –

Repolarization/Refractory Period• Sewer Pipes – One-way communication

like action potential only goes from dendrite end to axon terminal end.