Nerve impulses occur through a chain reaction of chemical events within neurons. When a neurotransmitter binds to a neuron, it causes depolarization by allowing sodium ions to enter the axon. This is followed by repolarization where potassium ions exit to regain the charge imbalance. This cycle of depolarization and repolarization propagates down the axon until the neurotransmitter is released at the axon terminal to stimulate the next neuron. For a nerve impulse to continue, sodium and potassium pumps must actively transport ions back to their resting positions inside and outside the axon membrane.

1. ANATOMY
Unit 2 Notes:
Nerve
Impulses

2. (1) What is a Nerve Impulse?
• Chain reaction of chemical events, allowing
your neurons to send “messages”.
• Synonymous Terms:
– Neuron Message
– Nervous System Response
– Depolarization & Repolarization

4. (2) The Basic Steps
1. Neurotransmitter binds to neuron.
2. Depolarization.
3. Repolarization.
4. Repeat #2 and 3 until reaching axon terminal.
5. Release of another neurotransmitter from axon
terminal.
6. Regain resting potential.

5. Neurotransmitter
Binds
Depolarization & Repolarization
Down The Axon
Release a neurotransmitter
+
Stimulate next move

6. (3) Resting Potential
• “Resting Potential”:
The natural state of a neuron when it’s not
doing anything.
• At Rest There is…
– Potassium (K+) on the inside of axon.
– Sodium (Na+) on the outside of axon.
– More cations on the outside than inside.
*This means that the outside is naturally more
positive than the inside (making the axon polar).

8. (4) Depolarization
• De = without
Polar = uneven charge
• Depolarization:
To remove the imbalance of charge.
• What Happens?
– Sodium pumps open.
– Sodium ions fly thru channels into axon.
– Temporarily increases positive charge inside
axon, removing polarization.

10. (5) Repolarization
• Re = repeat
Polar = uneven charge
• Repolarization:
To regain the charge imbalance.
• What Happens?
– Potassium pumps open.
– Potassium ions fly thru channels out of axon.
– Temporarily increases positive charge back on
the outside of the axon, regaining polarization.

12. (6) Reaching the Terminals
• A neurotransmitter will be released.
• This neurotransmitter will:
– Bind to another neuron or organ.
– Make something else happen (muscle
contraction or another nerve impulse).

14. (7) Regaining Resting Potential
• Even though the axon is polarized at the
end, the ions are on the wrong sides of the
axon.
• Must actively transport ions:
– Sodium must go back out.
– Potassium must go back in.
• Requires ATP.

16. …Let’s Review All That Again…

17. (8) Myelin Sheath
• Where Is It?
– Covers the outside of the axon.
– Wraps around Na/K pumps.
• What Is Its Purpose?
– Block some of the Na/K pumps.
– Increases the speed of nerve impulse.
• How Does It Work?
– Just like main-roads versus the freeway.
– Main Roads = Axon without myelin sheath.
– Freeway = Axon with myelin sheath.