--- title: "Channel Kinetics and the Ionic Basis of the Spike" description: "In a typical sodium-based neuronal action potential, sodium channels open rapidly and then inactivate, while delayed potassium channels open more slowly. Voltage-clamp recordings and tests that isolat" canonical: https://lightmysky.com/learn/science/channel-kinetics-and-the-ionic-basis-of-the-spike-mt_47-LDzgcKp source: https://lightmysky.com/learn/science/channel-kinetics-and-the-ionic-basis-of-the-spike-mt_47-LDzgcKp.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Channel Kinetics and the Ionic Basis of the Spike In a typical sodium-based neuronal action potential, sodium channels open rapidly and then inactivate, while delayed potassium channels open more slowly. Voltage-clamp recordings and tests that isolate ionic currents reveal these different timings. Subject: Science · Area: The Human Body · Ages 18 to 19 Page: https://lightmysky.com/learn/science/channel-kinetics-and-the-ionic-basis-of-the-spike-mt_47-LDzgcKp ## Ready when they can - Assigns each phase of the spike to the channel population responsible. - Distinguishes closed-available sodium channels from inactivated ones, and explains absolute and relative refractory periods. - Explains what voltage-clamp current reveals and why additional tests are needed to distinguish ionic currents. ## Lesson: The spike in two currents You have linked resting voltage to ion gradients and permeability. In a typical sodium-based neuronal action potential, or spike, sodium channels open rapidly and sodium entry drives the rise. Sodium channels then inactivate as delayed potassium channels open. Potassium exit drives repolarization, the fall. These events overlap. Potassium channels can stay open after the fall, pulling voltage below rest toward potassium's equilibrium value. *(drawing: Read the phases in order. Channel activity overlaps, and the cells do not show exact durations.)* Threshold marks the start of a self-amplifying rise. Sodium entry depolarizes the membrane, opening more available sodium channels and allowing more sodium entry. Below threshold, this feedback is too weak to sustain a spike. Under the same conditions, reaching threshold triggers an all-or-none spike. A stronger input does not produce a proportionally taller spike. A closed-available sodium channel passes no sodium but can open with depolarization. An inactivated channel also passes no sodium and must recover first. Repolarization allows recovery. In the absolute refractory period, channel availability is too low for another spike, even with a strong stimulus. In the relative period, enough channels have recovered to allow a spike, but remaining inactivation and elevated potassium conductance mean a stronger stimulus is usually needed. *(drawing: Depolarization opens an available channel. Inactivation stops flow. Recovery makes the channel available again.)* A voltage clamp injects current to hold a chosen membrane voltage. Once membrane-charging current is accounted for, the injected current balances the net ionic current. A depolarizing step below sodium's equilibrium voltage but above potassium's can show early inward current and delayed outward current. Selective blockers or ion changes help identify the components. Voltage clamp alone does not separate them. **Example.** Suppose a blocker reduces delayed potassium current while sodium-channel function and ion gradients stay unchanged. Less positive charge leaves through those potassium channels, so repolarization is slower. The spike can last longer, and its undershoot may also change. Blocking one channel type need not affect only one phase, because channel activity overlaps. **Recap.** Sodium entry drives the rise. Sodium inactivation and delayed potassium exit drive the fall. Channel recovery and potassium conductance shape the refractory periods. *Original lesson, questions and flashcards written for this dataset.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Equilibrium potentials and what sets the resting voltage](https://lightmysky.com/learn/science/equilibrium-potentials-and-what-sets-the-resting-voltage-mt_mRT_5vDEO1) ## Opens up - [Ion Channels: Selectivity, Gating and the Evidence from Single Channels](https://lightmysky.com/learn/science/ion-channels-selectivity-gating-and-the-evidence-from-single-channels-mt_2xPzPeXZZ_) - [Cable Properties and Dendritic Integration](https://lightmysky.com/learn/science/cable-properties-and-dendritic-integration-mt_bbPl-eJUFF) - [Pacemaker Potentials, Conduction and Reading an ECG](https://lightmysky.com/learn/science/pacemaker-potentials-conduction-and-reading-an-ecg-mt_D-WKFc7BZm)