Nerve Impulse: Action Potential Along an Axon

Real-World Phenomenon · Yale–New Haven Hospital uses nerve conduction velocity (NCV) tests to diagnose multiple sclerosis — a disease that destroys myelin and slows neural signals from ~50 m/s to <2 m/s · NGSS HS-LS1-2

How can a single nerve cell relay a signal from your fingertip to your spinal cord in milliseconds? The answer spans four levels of biological organization: individual ion channels (molecular) open and close to shift charge across the membrane (cellular), creating a self-propagating wave of electrical activity along the axon (tissue), which ultimately delivers a signal through the peripheral nervous system (organ system). This Hodgkin-Huxley cable model lets you manipulate each level and see how changes cascade upward.

NGSS HS-LS1-2: Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms — from ion channels to the action potential to whole-nerve conduction.

Recording electrode at compartment 9 · axon length: 40 mm Na⁺ in K⁺ out

🔬 Before You Click "Fire!"

The stimulus slider is set at 25 µA/cm². Fire the stimulus and observe. Now reset and try 10 µA/cm². What changes? Find the threshold value — the minimum strength that produces a full action potential. Why does the all-or-nothing principle mean there is such a sharp boundary?

📐 Hierarchical Organization (HS-LS1-2)

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Molecular: Voltage-gated Na⁺ & K⁺ ion channels open/close in response to membrane voltage — the mechanism driving everything above.

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Cellular: Ion flows shift the neuron's membrane potential, generating the characteristic action potential spike (+40 mV peak).

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Tissue: Cable coupling between compartments propagates the AP as a traveling wave along the axon — speed measurable in m/s.

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Organ System: Networks of myelinated axons in peripheral nerves carry sensory/motor signals; NCV tests detect demyelination (MS).

Data & Analysis

Vm at electrode

−65.0 mV

Status

Resting

Conduction Velocity

—

Vm vs Time at Electrode

−55 mV threshold

Axon Colour Scale

−90 mV−65 (rest)+50 mV

📌 Did You Know?

Lidocaine (used in dental anesthesia) works by blocking voltage-gated Na⁺ channels — the exact same mechanism shown by the "Na⁺ Channel Block" slider. Moving the slider to 100% simulates complete anesthesia: no AP can form.

📊 Analysis Questions

Set stimulus strength to 10 µA/cm², fire, and observe. Then set it to 25 µA/cm² and fire again (resetting between trials). Why does the second stimulus produce a full action potential but the first does not, even though it still depolarises the membrane slightly? How does this illustrate the all-or-nothing principle?
With no Na⁺ block, fire a stimulus and record the conduction velocity. Now increase the Na⁺ block to 60%, reset, and fire again. What happens to the AP waveform and the velocity? At what block percentage does the AP fail entirely? This models how lidocaine (dental anesthetic) silences pain signals by blocking Na⁺ channels.
Toggle myelination on. Fire a stimulus and compare the conduction velocity with the unmyelinated case. How does the visual appearance of the propagating signal differ (continuous vs. saltatory/jumping pattern)? Why does myelin increase velocity? How does multiple sclerosis — which destroys myelin — reduce conduction velocity?
Trace the causal chain across the hierarchy: starting from a single ion channel opening (molecular), explain how each level (cellular → tissue → organ system) depends on the level below to transmit a neural signal. Use your velocity measurements as quantitative evidence.