Overview
This lecture explains the process of the action potential, which is how neurons transmit information along their axons to enable nervous system functions.
Resting State of the Neuron
- Neuron resting state is maintained by sodium-potassium pumps in the cell membrane.
- Each pump moves 3 sodium ions (Na⁺) out and 2 potassium ions (K⁺) into the neuron.
- This creates higher sodium concentration outside and higher potassium concentration inside the cell.
- The result is a negative internal charge of about -70 millivolts (mV), called the resting potential.
Initiation of the Action Potential
- Neurons receive excitatory postsynaptic potentials (EPSPs) that raise cell voltage and inhibitory postsynaptic potentials (IPSPs) that lower it.
- The action potential starts if the combined voltage reaches the threshold value of -55 mV.
Propagation of the Action Potential
- At threshold, sodium channels open at the axon's start, allowing sodium to enter and making the inside more positive.
- Voltage can rise to about +50 mV, causing sodium channels to close.
- Potassium channels open as the cell depolarizes (starting near -30 mV, fully open at +50 mV), letting potassium exit and lowering the cell's voltage.
- The voltage briefly drops below -70 mV before returning to resting state.
Transmission Along the Axon and Restoration
- Opening of sodium channels in one region triggers the next, propagating the action potential along the axon.
- The sodium-potassium pump restores resting potential after the action potential.
- A brief refractory period follows before the neuron can fire again.
Key Terms & Definitions
- Action Potential — a rapid electrical signal that travels down the axon.
- Resting Potential — the charge difference across the neuron's membrane at rest (~ -70 mV).
- Sodium-Potassium Pump — protein that moves Na⁺ out and K⁺ into the neuron to maintain resting potential.
- Threshold Value — membrane voltage (~ -55 mV) needed to trigger an action potential.
- Excitatory Postsynaptic Potential (EPSP) — signal that increases cell voltage.
- Inhibitory Postsynaptic Potential (IPSP) — signal that decreases cell voltage.
- Refractory Period — short time after an action potential when the neuron cannot fire again.
Action Items / Next Steps
- Review the process steps of action potential generation and propagation.
- Learn key terms and their definitions.
- Prepare to diagram the sequence of ion movements during the action potential.