Action Potential

How neurons send an electrical message

Action Potential�How Neurons Communicate

• Neurons communicate by means of an electrical signal called the Action Potential
• Action Potentials are based on movements of ions between the outside and inside of the axon
• When an Action Potential occurs, a molecular message is sent to neighboring neurons
• Action Potential is an All or Nothing Process (like a gun firing)

Threshold: �Triggering Action Potential

• When a neuron is resting = balance of excitatory & inhibitory signals.
• If excitatory exceeds inhibitory threshold is reached (pull the trigger)
• If inhibitory exceeds excitatory threshold is NOT reached (safety on – can’t fire)
• When Threshold is reached it causes an electrical impulse to fire down the axon (action potential)

• How do you feel something that is intense?

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• More neurons fire, the intensity of their electric impulse always stays the same.

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• 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.

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Steps to Action Potential

Resting Potential

• At rest, the inside of the cell is at -70 microvolts
• With inputs to 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

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Axon at Resting Potential - fluid inside the axon is mostly negatively charged with positive Na on the outside (polarized)

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Step 1: Threshold is Reached

• An impulse is triggered in the neuron’s dendrite when stimulated by:
• Pressure
• Heat
• Light
• NT - chemical messenger from another neuron (synaptic transmission)
• Minimal level of stimulation that causes the axon to fire is called Stimulus Threshold

Step 2: Action Potential Begins

• When neuron fires, its axon membrane is selectively permeable.
• Gates in the axon called ion channels open allowing positive sodium ions to enter the axon while potassium ions leave causing a brief positive electrical charge in 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

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 and potassium ions back inside. (repolarized).
• This step is called the refractory period and the axon cannot fire again until it returns to resting potential (negative polarized state).

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• Why do you think the axon has to set itself back to a resting state so quickly (3 milliseconds)?
• So the neuron can fire again and send another message immediately after the last one.

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• The entire process is like falling dominoes falling (see video) all the way down the axon except these dominoes can set themselves back up as soon as they fall over.

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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

Finally, Hyperpolarization

• Repolarization leads to a voltage below the resting potential, called hyperpolarization
• Now neuron cannot produce a new action potential
• It must return a resting state
• This is the refractory period

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Action Potential Within a Neuron

STEPS TO ACTION POTENTIAL

1. Threshold is reached
2. +Na ions enter beginning of axon
3. this triggers the next Na gates to open.
4. As they open & allow in Na+,
5. previous gates begin pumping the Na+ out.
6. Before the action potential has reached the end, the beginning of the axon is back at resting potential & ready for another firing.
1. Video Review (5 min)

A Review Action Potential

DAILY

DOUBLE

How can a TOILET represent Action Potential?

• Full Toilet – Resting Potential (Polarized)
• Push Flush Lever – Threshold Stimulus triggering Action Potential.
• Fresh Water Flooding the Bowl = + Sodium Ions rushing into the Axon (Depolarization)
• Toilet Refilling/Can’t Flush – + Sodium Ions leaving (Repolarization/Refractory Period)
• Sewer Pipes – One-way communication (Toilet to Sewer) like action potential only goes from dendrite end to axon terminal end.
• Watch this quick overview if you need to review!