Overview
This lecture explains the mechanisms behind resting membrane potentials, graded potentials, and action potentials in neurons, including their ionic basis and key events.
Resting Membrane Potential
- Resting membrane potential is the voltage difference across the neuronal cell membrane at rest, typically around –70 mV (range: –70 to –90 mV).
- All cells possess a resting membrane potential, not just neurons.
- The sodium-potassium ATPase pump moves 3 Na⁺ out and 2 K⁺ into the cell, generating a small negative charge and creating Na⁺/K⁺ gradients.
- Leaky potassium channels allow K⁺ to exit the cell, making the inside negative (approaching –90 mV).
- Leaky sodium channels allow some Na⁺ in, slightly offsetting negativity (resting potential settles near –70 mV).
- The cell membrane is much more permeable to K⁺ than Na⁺.
Calculating Equilibrium Potentials (Nernst Equation)
- The Nernst equation calculates the equilibrium potential for an ion where chemical and electrical forces balance.
- Nernst equation: Eₓ = (61.5/z) × log([X]ₒ/[X]ᵢ), where z is the ion charge, [X]ₒ is outside, [X]ᵢ is inside.
- Potassium equilibrium potential ≈ –90 mV; sodium ≈ +70 mV.
- The actual resting potential is closer to K⁺ potential due to higher permeability.
Graded Potentials
- Graded potentials are small changes in membrane potential that move the neuron towards or away from threshold.
- EPSP (Excitatory Postsynaptic Potential): depolarizes membrane, brings it closer to threshold (e.g., caused by glutamate/Na⁺ influx).
- IPSP (Inhibitory Postsynaptic Potential): hyperpolarizes membrane, moves it further from threshold (e.g., caused by GABA/Cl⁻ influx or K⁺ efflux).
- Ligand-gated ion channels mediate EPSPs and IPSPs via neurotransmitter binding.
- Temporal summation: one presynaptic neuron fires repeatedly to add up EPSPs.
- Spatial summation: multiple presynaptic neurons fire simultaneously to add up EPSPs.
- More EPSPs than IPSPs are needed to reach threshold (usually –55 mV).
Action Potentials
- Action potential is triggered when membrane potential reaches threshold (–55 mV) at the axon hillock (trigger zone).
- Voltage-gated sodium channels open: Na⁺ enters, depolarizing membrane to +30 mV.
- At +30 mV, Na⁺ channels inactivate and voltage-gated K⁺ channels open: K⁺ exits, repolarizing and hyperpolarizing membrane to about –90 mV.
- Sodium-potassium pumps and leaky channels restore resting potential.
- Depolarization: membrane becomes more positive.
- Repolarization: returns to negative resting potential.
- Hyperpolarization: membrane becomes even more negative than rest.
Refractory Periods
- Absolute refractory period: cannot trigger another action potential (Na⁺ channels inactivated, until resting potential restored).
- Relative refractory period: possible to trigger action potential, but requires stronger stimulus (membrane is hyperpolarized).
Key Terms & Definitions
- Resting membrane potential — voltage across a cell membrane at rest.
- Sodium-potassium ATPase — pump that moves 3 Na⁺ out and 2 K⁺ in, using ATP.
- Leaky potassium/sodium channels — always-open channels allowing K⁺ out or Na⁺ in.
- Nernst potential — equilibrium potential for a specific ion.
- EPSP — depolarizing graded potential moving membrane closer to threshold.
- IPSP — hyperpolarizing graded potential moving membrane further from threshold.
- Ligand-gated channel — opens when a neurotransmitter binds.
- Voltage-gated channel — opens/closes in response to membrane voltage changes.
- Action potential — rapid, all-or-none depolarization and repolarization of membrane.
- Threshold — membrane potential at which an action potential is triggered (≈–55 mV).
- Refractory period — time after action potential when cell cannot (absolute) or is less likely (relative) to fire again.
Action Items / Next Steps
- Review the mechanisms and ions involved in resting, graded, and action potentials.
- Practice calculating Nernst potentials with sample ion concentrations.
- Memorize the phases and refractory periods of the action potential.