Channel Kinetics and the Ionic Basis of the Spike
The action potential comes from two channel populations with different speeds: sodium channels that open fast and then inactivate, and potassium channels that open late. Clamping the voltage and measuring the current is what separated those two currents.
What a learner can do afterwards
- Assigns each phase of the spike to the channel population responsible.
- Distinguishes inactivation from closing, and links inactivation to the refractory period.
- Explains what holding the voltage fixed allows an experimenter to measure.
1 · Read
The spike follows a fixed script in two currents. First fast sodium channels fly open and sodium floods in, driving voltage up. Then sodium channels inactivate while late potassium channels open, so potassium leaves and voltage falls.
An inactivated sodium channel is not a closed one. A closed channel can open at the next nudge, but an inactivated one must reset first. That forced pause is the refractory period, and it caps how fast spikes can repeat.
Holding the voltage fixed is what separated the two currents. With voltage clamped, the experimenter watches current alone: a fast inward rush first, then a late outward flow. Without the clamp, voltage and current chase each other and blur together.
Give each phase its owner: rise to sodium entry, fall to potassium exit. Threshold is the point where sodium entry outruns potassium exit, so the rise feeds itself and the spike is all or none.
Fast sodium in, then late potassium out; inactivation writes the pause between spikes.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.