SCIATIC NERVE

Action potential

Rapid reversal of membrane potential caused by opening of voltage-gated Na⁺ channels followed by K⁺ channel–mediated repolarization.

Depolarization

Membrane potential becomes less negative due to Na⁺ influx.

Repolarization

Return toward resting potential caused primarily by K⁺ leaving the cell.

Hyperpolarization

Membrane potential becomes more negative than resting potential due to excess K⁺ permeability.

Resting membrane potential

Stable negative membrane potential produced mainly by K⁺ leak channels.

How does an action potential move along an axon?

Local depolarization activates neighboring Na⁺ channels, regenerating the action potential along the membrane.

Do action potentials decrease in size as they travel?

No. They regenerate and remain the same amplitude along the axon.

Why don’t action potentials change ion gradients significantly?

Only a very small number of ions cross during each action potential.

What actually causes repolarization?

K⁺ leaving the cell through voltage-gated potassium channels, not the Na⁺/K⁺ pump.

Conduction velocity

Speed at which an action potential travels down an axon.

Two major factors affecting conduction velocity

1. Axon diameter 2. Myelination

Effect of axon diameter on conduction velocity

Larger diameter → faster conduction.

Why does myelination increase conduction velocity?

Myelin prevents current leakage and allows passive current to travel farther.

Saltatory conduction

Action potentials jump between nodes of Ranvier instead of occurring continuously along the membrane.

Nodes of Ranvier

Unmyelinated regions where Na⁺ channels are concentrated.

Why saltatory conduction is faster

Action potentials only regenerate at nodes rather than along the entire axon.

Compound Action Potential (CAP)

The summed electrical signal from many nerve fibers firing action potentials simultaneously.

Why CAP amplitude increases with stimulus intensity

Stronger stimuli recruit additional axons with higher thresholds.

Fiber recruitment

Activation of additional nerve fibers as stimulus strength increases.

Order of recruitment

Large diameter fibers → lower threshold → activated first.

Stimulus artifact

Electrical signal caused by passive spread of stimulating voltage rather than an action potential.

Why stimulus artifact occurs

Stimulus voltage spreads through surrounding conductive fluid.

How a ground electrode helps reduce artifact

It diverts passively spreading voltage away from recording electrodes.

Why action potentials are not eliminated by grounding

Action potentials regenerate actively along the membrane.

Refractory period

Time after an action potential when another action potential cannot easily occur.

Absolute refractory period

Second action potential cannot occur regardless of stimulus strength.

Cause of absolute refractory period

Voltage-gated Na⁺ channels remain inactivated.

Relative refractory period

A second action potential can occur but requires a stronger stimulus.

Why relative refractory period occurs

Some Na⁺ channels have recovered, but K⁺ permeability remains high.

How labs test refractory periods

Two stimuli are delivered with decreasing intervals between them.

How to detect absolute refractory period experimentally

No second action potential occurs even with very strong stimulus.

How to detect relative refractory period experimentally

A larger second stimulus produces a second action potential.

What happens when pulses are far apart?

Two normal action potentials occur.

What happens when pulses are moderately close?

Second AP may occur but is reduced (relative refractory).

What happens when pulses are very close?

No second AP occurs (absolute refractory).

Why large stimuli are used in the lab

To ensure neurons in the relative refractory period still fire.

Three states of voltage-gated Na⁺ channels

1. Closed 2. Open 3. Inactivated

Why Na⁺ channels cannot immediately reopen

They must first deactivate and remove inactivation during repolarization.

What happens if the membrane remains depolarized?

Na⁺ channels stay inactivated and cannot generate another AP.

Why action potentials propagate in one direction

The region behind the AP is in the refractory period.

Why increasing stimulus amplitude increases CAP amplitude but not AP amplitude

Each individual action potential is all-or-none.

What determines CAP amplitude

Number of fibers activated.

Why larger fibers conduct faster

Lower internal resistance.

AC coupling

Used for recording fast electrical signals such as action potentials.

DC coupling

Used for slow signals such as mechanical transducer outputs.

Ground electrode purpose

Removes passively spreading voltage and reduces stimulus artifact.

CAP latency

Time between the stimulus and the recorded compound action potential.

What determines CAP latency

Distance between electrodes and conduction velocity.

Why larger fibers appear first in CAP recordings

They conduct faster, so their action potentials arrive earlier.

Threshold stimulus

Minimum stimulus intensity required to trigger an action potential in a nerve fiber.

Why different fibers have different thresholds

Fiber diameter and membrane properties vary.

Extracellular recording

Recording electrical activity from outside the nerve fiber by measuring voltage differences in the surrounding fluid.