Scivra
Pro 🔒~18 min

Neuron & Action Potential

The electrical language of the nervous system

How it works

Neurons maintain a resting membrane potential of −70 mV due to the Na⁺/K⁺ ATPase pump (3 Na⁺ out, 2 K⁺ in) and selective ion permeability. An action potential is triggered when a stimulus depolarizes the membrane to threshold (≈−55 mV). Voltage-gated Na⁺ channels open rapidly → Na⁺ rushes in → membrane depolarizes to +40 mV. Na⁺ channels then inactivate while K⁺ channels open → K⁺ rushes out → repolarization and brief hyperpolarization (refractory period). The action potential propagates down the axon; myelination creates saltatory conduction (jumping between nodes of Ranvier), dramatically increasing speed (up to 120 m/s in myelinated vs 0.5 m/s in unmyelinated). Multiple sclerosis involves myelin sheath degradation.

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Step-by-step

  1. Set stimulus strength and click 'Stimulate'.
  2. Watch the voltage trace build in the oscilloscope-style graph.
  3. If the stimulus exceeds threshold, an action potential fires — observe the Na⁺ and K⁺ channel animations.
  4. Reduce Na⁺ concentration to see how the amplitude changes.
  5. Toggle myelination (Pro) to compare conduction velocities.

Key formulas

  • Em=−70 mV (resting)→+40 mV (peak)→−90 mV (hyperpol.)E_m = -70 \text{ mV (resting)} \to +40 \text{ mV (peak)} \to -90 \text{ mV (hyperpol.)}Membrane potential during action potential phases
  • ENa=RTzFln⁡[Na+]o[Na+]i≈+60 mVE_{Na} = \frac{RT}{zF} \ln\frac{[\text{Na}^+]_o}{[\text{Na}^+]_i} \approx +60 \text{ mV}Nernst equation: equilibrium potential for Na⁺
  • vmyelinated≫vunmyelinated(saltatory conduction)v_{myelinated} \gg v_{unmyelinated} \quad (\text{saltatory conduction})Myelin increases conduction velocity via saltatory conduction at nodes of Ranvier

Frequently asked questions

What is the threshold potential for a typical neuron, and what happens if a stimulus is below threshold?
Threshold ≈ −55 mV. Below threshold, a graded potential forms but decays — no action potential fires (all-or-nothing principle).
Why can't a second action potential fire immediately after the first?
The refractory period: Na⁺ channels are inactivated (absolute refractory) or the membrane is hyperpolarized (relative refractory). A stronger stimulus is needed during relative refractory period.
Tetrodotoxin (TTX) blocks voltage-gated Na⁺ channels. What effect does this have on action potential generation?
No Na⁺ influx → membrane cannot depolarize past threshold → no action potential. TTX causes paralysis and is lethal. Found in puffer fish.
In multiple sclerosis, the myelin sheath is degraded. Predict the effect on: (a) conduction velocity, (b) the type of conduction, (c) neurological symptoms.
(a) Velocity decreases dramatically, (b) saltatory conduction fails → continuous conduction or signal failure, (c) symptoms: muscle weakness, numbness, visual disturbances — due to slowed or blocked nerve signals.