Part 4 – Action Potentials, Threshold, Refractory Periods & Conduction

What is an action potential (AP)?

A brief, long-distance electrical signal that travels along the axon and does not decrease in strength over distance.

Where are action potentials generated?

In axons, typically beginning at the axon hillock.

What causes an action potential?

A strong enough depolarizing stimulus that opens voltage-gated sodium and potassium channels.

What is the threshold for an action potential?

The critical level of depolarization (around –55 mV) needed to trigger the opening of voltage-gated Na⁺ channels.

What happens once threshold is reached?

Depolarization becomes self-generating as Na⁺ influx causes more Na⁺ channels to open.

Describe the first step of an action potential.

Resting state: all gated Na⁺ and K⁺ channels are closed; the resting potential is –70 mV.

What happens during depolarization?

Na⁺ channels open, allowing sodium to enter the cell, making the inside more positive.

When do Na⁺ channels inactivate?

At about +30 mV, sodium inactivation gates close to stop Na⁺ entry.

What happens during repolarization?

Voltage-gated K⁺ channels open, and potassium ions leave the cell, restoring internal negativity.

What happens during hyperpolarization?

K⁺ channels remain open longer than necessary, causing a slight overshoot beyond resting potential.

When does the sodium-potassium pump come into play?

It restores ionic balance after repolarization by pumping Na⁺ out and K⁺ back in.

Why are action potentials considered “all-or-none” events?

Because once threshold is reached, an AP will fire completely; if threshold is not reached, no AP occurs.

What is meant by stimulus intensity in neurons?

Stronger stimuli produce more frequent action potentials, not stronger ones.

What is the absolute refractory period?

The period during which a neuron cannot fire another AP, regardless of stimulus strength.

What causes the absolute refractory period?

Inactivation of Na⁺ channels immediately after depolarization.

What is the relative refractory period?

The period immediately after the absolute refractory period when a stronger-than-normal stimulus can trigger another AP.

What causes the relative refractory period?

Some K⁺ channels remain open while Na⁺ channels are resetting.

Why are refractory periods important?

They ensure one-way propagation of nerve impulses and limit firing frequency.

How does an action potential propagate along an axon?

By local currents that depolarize adjacent membrane areas, triggering new APs sequentially.

Why does the action potential move in only one direction?

Because the region behind it is in the refractory period and cannot depolarize again immediately.

What is continuous conduction?

Action potential propagation along unmyelinated axons where the impulse moves step by step along the entire membrane.

What is saltatory conduction?

Rapid impulse propagation along myelinated axons where APs “jump” from node to node.

Why is saltatory conduction faster than continuous conduction?

Because depolarization only occurs at the nodes of Ranvier, not along the entire membrane.

What are factors that affect conduction velocity?

Axon diameter and degree of myelination.

How does axon diameter affect conduction speed?

Larger diameters have lower internal resistance and conduct impulses faster.

How does myelination affect conduction speed?

Myelinated fibers conduct impulses much faster than unmyelinated ones.

What are Group A fibers?

Large-diameter, heavily myelinated fibers that conduct impulses up to 150 m/s (e.g., motor neurons).

What are Group B fibers?

Intermediate-diameter, lightly myelinated fibers conducting at about 15 m/s.

What are Group C fibers?

Small-diameter, unmyelinated fibers conducting slowly at about 1 m/s.

Which fibers carry pain signals?

Typically small, unmyelinated Group C fibers.

Which fibers carry skeletal muscle motor signals?

Large, myelinated Group A fibers.

Why is myelin essential for nerve function?

It insulates axons, increases signal velocity, and conserves energy by limiting ion exchange.

What is multiple sclerosis (MS)?

An autoimmune disease that destroys myelin in the CNS, leading to disrupted conduction and muscle weakness.

What happens to action potentials in demyelinated fibers?

They slow down or stop completely due to current leakage.

What are early symptoms of multiple sclerosis?

Visual disturbances, muscle weakness, loss of coordination, and speech problems.

How does multiple sclerosis progress?

Myelin destruction leads to hardened plaques (scleroses) and permanent neural damage.

Are there treatments for MS?

Yes, some drugs help manage symptoms and reduce the immune attack on myelin.

What happens to conduction after demyelination?

Axons may degenerate if myelin is not repaired, leading to loss of neural function.