BiologyNeural control and coordinationClass 11Class 12

Action potential & synaptic transmission: interactive simulation

Neurons are excitable cells because their membranes are in a polarized state. In resting state, the axonal membrane is more permeable to K+ and nearly impermeable to Na+, establishing a resting potential of -70 mV.

Useful for: NEET, AIIMS, CBSE Boards

Try this

When a threshold stimulus arrives, rapid influx of Na+ reverses polarity to +30 mV (Action Potential). At the axon terminal, Ca²⁺ influx causes synaptic vesicles to release neurotransmitters (e.g. Acetylcholine) across the synaptic cleft to stimulate the postsynaptic neuron.

  1. 1

    Click "Stimulate Action Potential!" and follow the red marker on the graph.

    What you should see

    Watch the rapid voltage spike to +30 mV as Na+ channels open, followed by immediate K+ efflux bringing the voltage back down.

  2. 2

    Look at the Chemical Synapse in the bottom panel.

    What you should see

    Ca²⁺ channels open at the presynaptic membrane, causing synaptic vesicles to fuse and release yellow Acetylcholine molecules into the 20 nm synaptic cleft.

Formulas behind it

Resting Membrane Potential

Vrest=−70 mV(Inside negative)V_{\text{rest}} = -70\text{ mV} \quad (\text{Inside negative})

Threshold Stimulus

Vthresh=−55 mV(All-or-None Law)V_{\text{thresh}} = -55\text{ mV} \quad (\text{All-or-None Law})

Peak Depolarization

Vpeak=+30 mV(Reversal of polarity)V_{\text{peak}} = +30\text{ mV} \quad (\text{Reversal of polarity})

Na+/K+ ATPase Stoichiometry

3 Na+ pumped OUT:2 K+ pumped IN(ATP consumed)3\,\text{Na}^+\text{ pumped OUT} : 2\,\text{K}^+\text{ pumped IN} \quad (\text{ATP consumed})

Common mistakes

  • Assuming the inside of an axon is positive at rest. The axoplasm is negatively charged (-70 mV) relative to the extracellular fluid.
  • Thinking Na+/K+ pump causes depolarization. The pump MAINTAINS the resting gradient; depolarization is caused by passive influx of Na+ through voltage-gated channels.
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FAQ

Why is the resting membrane negative inside?

Due to active pumping of 3 Na+ out for every 2 K+ in, combined with high resting permeability to K+ and the presence of negatively charged trapped proteins in the axoplasm.

What is the All-or-None principle?

If a stimulus fails to reach threshold (-55 mV), no action potential fires. Once threshold is reached, a full-amplitude (+30 mV) action potential fires regardless of stimulus strength.

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