How is the Action Potential generated and how does the Nernst Equation apply?
At rest, the inside of a neuron is electrically negative relative to the extracellular fluid (roughly $-70\text{ mV}$) maintained by the $\text{Na}^+/\text{K}^+$ ATPase pump (exporting $3\text{ Na}^+$ for every $2\text{ K}^+$ imported) and high resting $\text{K}^+$ leak permeability. The electrochemical equilibrium potential for each ion at body temperature ($37\text{ °C}$) is given by the Nernst Equation: $E_{\text{ion}} = \frac{61.5\text{ mV}}{z}\log_{10}\left(\frac{[\text{ion}]_{\text{ext}}}{[\text{ion}]_{\text{int}}}\right)$.
When a stimulus depolarizes the membrane without reaching the critical threshold ($-55\text{ mV}$), only a subthreshold local response occurs. Once the membrane potential crosses $-55\text{ mV}$, voltage-gated $\text{Na}^+$ channels open rapidly: sodium ions rush inward toward $E_{\text{Na}} \approx +61.5\text{ mV}$, reversing membrane polarity up to $+35\text{ mV}$ (depolarization phase).
Less than a millisecond later, $\text{Na}^+$ channels inactivate and delayed $\text{K}^+$ channels open, allowing potassium efflux to drive the voltage back toward $E_{\text{K}} \approx -93\text{ mV}$ (repolarization and hyperpolarization phases).
Step-by-step guided experiment
1. The All-or-None Law: Set Stimulus Depolarization to $+10\text{ mV}$ (rising from $-70\text{ mV}$ to $-60\text{ mV}$, below the $-55\text{ mV}$ threshold). Notice the small gray graded potential on the oscilloscope. Now increase it to $+18\text{ mV}$: crossing $-55\text{ mV}$ triggers a full action potential spike.
2. Hyperkalemia Effect: Increase $[\text{K}^+]_{\text{ext}}$ from $4.5\text{ mM}$ to $9.0\text{ mM}$. Observe how the Nernst potential $E_{\text{K}}$ becomes less negative (from $-92.8\text{ mV}$ to $-75.0\text{ mV}$), shifting the resting membrane potential closer to threshold.
Frequently Asked Questions (FAQ)
What is the "all-or-none" law in neuronal action potentials?
If a depolarizing stimulus fails to reach the critical threshold (around $-55\text{ mV}$), the neuron only produces a local graded potential that decays. Once the membrane crosses $-55\text{ mV}$, voltage-gated $\text{Na}^+$ channels open in a positive feedback loop, firing a full-amplitude action potential.
What phases make up a neuronal action potential?
It begins at resting potential ($-70\text{ mV}$), followed by rapid depolarization via $\text{Na}^+$ influx up to $+35\text{ mV}$, repolarization via $\text{Na}^+$ inactivation and $\text{K}^+$ efflux, and a brief hyperpolarization near $-80\text{ mV}$ before returning to rest.
How does the myelin sheath speed up nerve impulse conduction?
Myelin acts as an electrical insulator that increases membrane resistance and decreases capacitance. The action potential leaps from one Node of Ranvier to the next (saltatory conduction), boosting speed from $\sim 2\text{ m/s}$ to over $90\text{ m/s}$.
Related Interactive Labs & Tools
🎓 Interactive Science Education Hub
Explore all virtual laboratories across Physics, Mathematics, Chemistry, and Anatomy.
🫀 Cardiovascular Anatomy & Physiology
Simulate cardiac output, blood pressure, and Poiseuille laminar blood flow.
🔌 RC Circuits & Time Constant (τ)
Study the electrical RC circuit model equivalent to the cell lipid bilayer.