Neuron at Rest vs. Action Potential: Ion Channels, Membrane Potential, and Synaptic Transmission

Resting Neuron

• Resting Membrane Potential (RMP): ~−70 mV (inside negative).
• Ion Distribution: High K⁺ inside, high Na⁺/Cl⁻ outside (maintained by Na⁺/K⁺-ATPase).
• Permeability: High K⁺ leak through open K⁺ channels; low Na⁺ permeability (voltage-gated Na⁺ channels closed).
• Equilibrium: RMP near E_K (−90 mV) due to K⁺ dominance.
• Synapses: No neurotransmitter release.

Activated Neuron: The Action Potential

1. Depolarization: Stimulus opens voltage-gated Na⁺ channels → Na⁺ influx → Vₘ rises to +30 mV.
2. Repolarization: Na⁺ channels inactivate; voltage-gated K⁺ channels open → K⁺ efflux → Vₘ returns negative.
3. Hyperpolarization (Undershoot): K⁺ channels close slowly → Vₘ transiently drops below −70 mV.
4. Refractory Periods: Absolute (Na⁺ channels inactivated); relative (K⁺ conductance high).

Synaptic Transmission

Action potential reaches the axon terminal → voltage-gated Ca²⁺ channels open → Ca²⁺ influx → vesicle fusion → neurotransmitter release into synaptic cleft → binds postsynaptic receptors → generates EPSP or IPSP.

Summary

At rest, K⁺ permeability dominates. During activation, a transient Na⁺ permeability surge is followed by K⁺ efflux. This electrical signal triggers chemical communication at the synapse.