Action Potentials Drive Nerve Impulses for Neural Communication
- A nerve impulse is a rapid electrical signal that travels along a neuron’s axon.
- This electrical nature arises from the movement of positively charged ions across the neuron’s membrane, creating a wave-like shift in the membrane potential known as an action potential.
- These signals enable neurons to communicate with each other and other cells, forming the basis of the nervous system.
Definition
Action Potential
The action potential is the rapid electrical signal generated when the neuron's membrane potential reaches a threshold.
- Action potentials are all-or-nothing events.
- They either occur fully or not at all, depending on whether a threshold potential is reached.
The Phases of an Action Potential
1. Resting Potential
- Before an action potential begins, the neuron is at resting potential.
- This is a stable state where the inside of the neuron is negatively charged compared to the outside, typically around $-70 mV$. $-70 mV$.
- This charge imbalance is maintained by:
- These pumps actively transport three sodium ions (Na⁺) out of the neuron for every two potassium ions (K⁺) pumped in, creating a net negative charge inside.
- The membrane is more permeable to K⁺ than Na⁺, allowing more K⁺ to leak out, further increasing the negative charge inside.
- Large, negatively charged proteins inside the neuron contribute to the overall negative charge.
Remember, the resting potential is maintained by active transport, which requires energy from ATP.
2. Depolarization
- When a stimulus reaches the neuron, voltage-gated sodium channels open, allowing Na⁺ ions to rush into the cell.
- This influx of positively charged ions reverses the charge imbalance, making the inside of the neuron positive relative to the outside.
- The influx of positively charged ions reverses the charge imbalance, making the inside of the neuron positive relative to the outside.
- The membrane potential rises from $-70 mV$ to about $+30 mV$.$-70 mV$ to about $+30 mV$.
- Imagine a crowd rushing into a stadium through open gates.
- The sudden influx of people changes the environment inside the stadium, just as the influx of Na⁺ ions changes the charge inside the neuron.
3. Repolarization
- Shortly after depolarization, the sodium channels close and voltage-gated potassium channels open.
- K⁺ ions diffuse out of the neuron, restoring the negative charge inside.
- The membrane potential returns to around $-70 mV$.
Think of repolarization as opening exit doors in the stadium, allowing people to leave and restore balance.
Common Mistake- Students often confuse the direction of ion flow during depolarization and repolarization.
- Remember, depolarization is caused by sodium influx, and repolarization by potassium efflux.
4. Hyperpolarization and the Refractory Period
- Sometimes, too many K⁺ ions leave the neuron, causing the membrane potential to briefly become more negative than the resting potential.
- This is called hyperpolarization.
- During this time, the neuron cannot fire another action potential. This refractory period ensures that nerve impulses travel in only one direction.
5. Return to Resting Potential:
- The sodium-potassium pump restores the ion gradients, preparing the neuron for another impulse.
- A common mistake is thinking that action potentials involve the movement of electrons, like in electrical wires.
- Instead, they rely on the movement of ions (Na⁺ and K⁺) across the membrane.
Propagation of Action Potentials
- How does an action potential travel along the axon?
- The answer lies in local currents.
- Think of an action potential as a "wave" traveling through a stadium.
- The wave itself doesn’t physically move people; instead, people stand up and sit down in sequence.
- Similarly, the signal moves along the axon without the ions themselves traveling the entire distance.
Local Currents and Propagation
- Depolarization in one part of the axon causes Na⁺ ions to diffuse into the neighboring region.
- This raises the membrane potential in the neighboring region, triggering voltage-gated sodium channels to open.
- As a result, the action potential is propagated along the axon in a wave-like manner.
Imagine a row of dominoes. When one domino falls, it triggers the next one to fall, creating a chain reaction. Similarly, the depolarization of one part of the axon triggers depolarization in the next part.
Why Do Action Potentials Move in One Direction?
- Action potentials always move from the cell body to the axon terminal.
- This is because of the refractory period.
- After an action potential passes, the sodium channels in that region become temporarily inactive, preventing the signal from moving backward.
The refractory period ensures that nerve impulses are unidirectional, allowing precise communication within the nervous system.
Why Are Nerve Impulses Electrical?
- Nerve impulses are electrical because they involve the movement of positively charged ions (Na⁺ and K⁺) across the neuron's membrane.
- This movement creates changes in the membrane potential, which are detected as electrical signals.
- Although nerve impulses are electrical, they do not involve the movement of electrons, as in metal wires.
- Instead, they rely on the flow of ions across the cell membrane.
- How does the concept of action potentials challenge our understanding of "electricity"?
- In what ways does this biological process differ from the electricity used in human-made systems?
- What are the main phases of an action potential?
- How do local currents contribute to the propagation of nerve impulses?
- Why do nerve impulses travel in only one direction along an axon?
