Concerning the phases of the action potential and the underlying ionic mechanisms, all of the following are true EXCEPT:

Explanation

## Phases of the Action Potential and Ionic Mechanisms ### The Complete Action Potential Timeline ```mermaid flowchart TD A["Resting Potential<br/>−70 mV"]:::outcome --> B["Stimulus<br/>Threshold: −55 mV"]:::decision B -->|Reached| C["Depolarization Phase<br/>Na⁺ channels open<br/>V_m → +30 mV"]:::action C --> D["Overshoot<br/>Approaches E_Na ≈ +60 mV"]:::outcome D --> E["Repolarization Phase<br/>Na⁺ inactivation<br/>K⁺ channels open"]:::action E --> F["Return to RMP<br/>−70 mV"]:::outcome F --> G["After-hyperpolarization<br/>K⁺ channels still open<br/>V_m → −80 to −90 mV"]:::outcome G -->|K⁺ channels close| H["Return to Rest<br/>−70 mV"]:::outcome ``` ### Phase-by-Phase Analysis | Phase | Ion Responsible | Mechanism | Membrane Potential | |-------|-----------------|-----------|-------------------| | **Depolarization** | Na⁺ inward | V-gated Na⁺ channels open; I_Na >> I_K | −70 → +30 mV | | **Overshoot** | Na⁺ dominates | Membrane potential approaches E_Na (+60 mV) | Peaks near +30 to +40 mV | | **Repolarization** | K⁺ outward | Na⁺ inactivation + delayed K⁺ channel opening | +30 → −70 mV | | **After-hyperpolarization** | K⁺ outward (excessive) | K⁺ channels remain open too long | −70 → −80 to −90 mV | ### Why Option 3 Is Wrong **Key Point:** The after-hyperpolarization (AHP) is NOT caused by K⁺ channels closing immediately at −70 mV. Instead, K⁺ channels remain **open too long** after the membrane potential has returned to −70 mV, allowing excessive K⁺ efflux. This drives the potential below the resting level (to −80 to −90 mV) until the K⁺ channels eventually inactivate and the Na⁺/K⁺-ATPase restores RMP. **High-Yield:** The after-hyperpolarization is a **refractory period** mechanism — it hyperpolarizes the membrane, making it harder to fire another action potential immediately (relative refractory period). This ensures unidirectional propagation along the axon and prevents tetanic firing. **Clinical Pearl:** In some neurons (e.g., cerebellar Purkinje cells), the after-hyperpolarization is prolonged, creating a long refractory period that limits firing frequency. This is clinically relevant in understanding seizure thresholds and neuronal excitability. **Mnemonic:** **AHP = After Hyperpolarization Persists** — remember, K⁺ channels stay open *too long*, not that they close at −70 mV.