Which single feature best discriminates the hyperpolarization (afterhyperpolarization) phase from the threshold phase in the action potential sequence?

## Hyperpolarization vs. Threshold Phase ### Threshold Phase **Key Point:** Threshold is the membrane potential (typically −55 mV) at which voltage-gated Na+ channels open sufficiently to generate an inward Na+ current that exceeds outward K+ current, triggering the action potential. - Membrane potential: ~−55 mV (critical value) - Voltage-gated Na+ channels begin to open - Regenerative depolarization begins - Once threshold is reached, an all-or-none action potential fires ### Hyperpolarization (Afterhyperpolarization) Phase **Key Point:** Afterhyperpolarization occurs AFTER repolarization when the membrane potential becomes MORE negative than the resting potential (e.g., −80 to −90 mV). - Caused by delayed closure of voltage-gated K+ channels - K+ continues to flow out even after the membrane has repolarized - Membrane potential overshoots the RMP - Self-limiting: K+ channels eventually close and the Na+/K+-ATPase restores RMP ### Comparison Table | Feature | Threshold Phase | Afterhyperpolarization | |---------|-----------------|------------------------| | **Membrane potential** | −55 mV (critical) | −80 to −90 mV (overshoot) | | **Relative to RMP** | Depolarized | Hyperpolarized | | **Primary event** | Na+ channels opening | K+ channels slow to close | | **Ion flux direction** | Inward Na+ dominates | Outward K+ continues | | **Voltage-gated K+ channels** | Closed | Open (delayed closure) | | **Functional consequence** | Triggers AP | Reduces excitability (refractory period) | **High-Yield:** Afterhyperpolarization is a **temporal overshoot** caused by the **lag in K+ channel closure**, not a new phase of depolarization. This is why it is also called the **negative afterpotential**. **Mnemonic:** **TASK** = **T**hreshold is **A**t −55, **S**ets off **K** channels closing late (afterhyperpolarization). **Clinical Pearl:** Prolonged afterhyperpolarization increases the relative refractory period, reducing the neuron's ability to fire a second action potential immediately. This is physiologically important for preventing tetanic firing and allowing temporal summation to be meaningful. [cite:Guyton & Hall 13e Ch 5]