Overview
This lecture explains the concepts of absolute and relative refractory periods in neurons, focusing on the role of voltage-gated sodium channels during action potentials.
Voltage-Gated Sodium Channels
- Voltage-gated sodium (Na⁺) channels have three states: closed, open, or inactive.
- These channels possess an activation gate and an inactivation gate.
- When stimulated, the activation gate opens, allowing Na⁺ to enter the cell.
- After about 0.5 to 1 millisecond, the inactivation gate closes, making the channel inactive.
- Channels in open or inactive states cannot be restimulated until they return to the closed state.
Action Potential and Refractory Periods
- An action potential occurs when a stimulus raises the membrane potential above the threshold, opening Na⁺ channels.
- During depolarization, Na⁺ enters the neuron, making the inside more positive.
Absolute Refractory Period
- The absolute refractory period occurs when Na⁺ channels are open or inactive after an action potential.
- During this time, no new action potential can be triggered regardless of stimulus strength.
Relative Refractory Period
- As the neuron repolarizes (membrane potential returns toward resting), potassium (K⁺) channels open and K⁺ exits the cell.
- Na⁺ channels begin resetting from inactive to closed state during repolarization.
- A new action potential is possible, but only if a much stronger stimulus is applied because few Na⁺ channels are closed and ready.
- The relative refractory period ends when all Na⁺ channels have reset to the closed state.
Key Terms & Definitions
- Voltage-gated sodium channel — a protein in the neuron membrane that opens or closes in response to voltage changes to allow Na⁺ ions through.
- Activation gate — part of the Na⁺ channel that opens with stimulus.
- Inactivation gate — part of the Na⁺ channel that closes after activation, making the channel inactive.
- Depolarization — the process by which the membrane potential becomes less negative (more positive).
- Repolarization — the return of the membrane potential toward its resting value, mainly due to K⁺ exiting the cell.
- Absolute refractory period — the time during which no new action potential can be initiated.
- Relative refractory period — the time following the absolute refractory period when a stronger-than-normal stimulus can cause another action potential.
Action Items / Next Steps
- Review the episode on depolarization for a deeper understanding.
- Prepare for the next lecture by revisiting key points about voltage-gated ion channels.