Resting and Action Potential — Revision Notes

⚡ 30-Second Revision

Resting Potential: — $approx -70,\text{mV}$. Maintained by Na$^+$/K$^+$ pump ($3,\text{Na}^+_{\text{out}} / 2,\text{K}^+_{\text{in}}$) and K$^+$ leak channels ($P_{\text{K}^+} gg P_{\text{Na}^+}$).
Threshold Potential: — $approx -55,\text{mV}$. Required to trigger AP.
Action Potential Phases:

- Depolarization (Rising): Voltage-gated Na$^+$ channels open $ightarrow$ Rapid Na$^+$ influx $ightarrow$ Inside becomes positive ($approx +30,\text{mV}$). - Repolarization (Falling): Voltage-gated Na$^+$ channels inactivate + Voltage-gated K$^+$ channels open $ightarrow$ Rapid K$^+$ efflux $ightarrow$ Inside becomes negative. - Hyperpolarization (Undershoot): Prolonged K$^+$ efflux $ightarrow$ Membrane more negative than rest ($approx -80,\text{mV}$).

Refractory Periods:

- Absolute: No AP possible (Na$^+$ channels inactivated). - Relative: AP possible with stronger stimulus (some Na$^+$ channels reset, K$^+$ channels still open).

All-or-None Principle: — AP fires fully or not at all, fixed amplitude.

2-Minute Revision

The neuron's electrical state is defined by its membrane potential. At rest, it maintains a resting potential of about $-70,\text{mV}$, negative inside. This is a dynamic equilibrium, primarily established by the **Na$^+$/K$^+$ pump** (actively moving $3,\text{Na}^+$ out, $2,\text{K}^+$ in) and the membrane's higher permeability to **K$^+$ ions** through leak channels, allowing K$^+$ to diffuse out.

When a stimulus reaches the threshold potential (around $-55,\text{mV}$), an action potential is triggered.

1
Depolarization: — Voltage-gated Na$^+$ channels rapidly open, causing a massive influx of Na$^+$ ions. The inside of the cell quickly becomes positive (e.g., $+30,\text{mV}$).
2
Repolarization: — Voltage-gated Na$^+$ channels inactivate, and slower voltage-gated K$^+$ channels open, leading to a rapid efflux of K$^+$ ions. The membrane potential returns to negative.
3
Hyperpolarization: — K$^+$ channels close slowly, causing a brief 'undershoot' where the membrane becomes more negative than the resting potential.

Action potentials follow the 'all-or-none' principle and are followed by refractory periods (absolute and relative), which prevent immediate re-firing and ensure unidirectional signal propagation. Understanding the specific ion movements and channel types for each phase is key.

5-Minute Revision

The foundation of nerve impulse transmission lies in the dynamic changes of the neuronal membrane potential, specifically the resting and action potentials. The resting membrane potential is the steady electrical state of an inactive neuron, typically around $-70,\text{mV}$ (negative inside).

This state is maintained by two primary factors: the **sodium-potassium pump (Na$^+$/K$^+$ ATPase)**, which actively transports $3,\text{Na}^+$ ions out and $2,\text{K}^+$ ions into the cell, consuming ATP and creating ion concentration gradients; and the differential permeability of the membrane, which is much more permeable to K$^+$ ions (via K$^+$ leak channels) than to Na$^+$ ions at rest, allowing K$^+$ to slowly diffuse out and contribute to the internal negativity.

When a neuron receives a sufficient stimulus, it depolarizes to a critical level called the threshold potential (around $-55,\text{mV}$). This triggers an action potential, an 'all-or-none' electrical signal with a fixed amplitude, characterized by a rapid sequence of events:

1
Depolarization (Rising Phase): — Upon reaching threshold, voltage-gated Na$^+$ channels rapidly open. Driven by both concentration and electrical gradients, Na$^+$ ions flood into the cell, causing the membrane potential to swiftly reverse and become positive (e.g., $+30,\text{mV}$). This is the peak of the action potential.
2
Repolarization (Falling Phase): — This rapid positive phase is short-lived. Voltage-gated Na$^+$ channels quickly inactivate (close and become unresponsive), stopping Na$^+$ influx. Simultaneously, voltage-gated K$^+$ channels, which open more slowly, become fully active. K$^+$ ions rush out of the cell, driven by their concentration gradient and the now positive internal charge. This efflux of positive charge rapidly restores the negative membrane potential.
3
Hyperpolarization (Undershoot): — The voltage-gated K$^+$ channels are slow to close, leading to a brief period where the membrane potential becomes even more negative than the resting potential (e.g., $-80,\text{mV}$). Eventually, these channels close, and the membrane returns to its resting state, aided by the Na$^+$/K$^+$ pump.

Following an action potential, the neuron enters a refractory period. The absolute refractory period occurs during depolarization and early repolarization, when Na$^+$ channels are inactivated, making it impossible to fire another action potential.

The relative refractory period follows, during late repolarization and hyperpolarization, when a stronger-than-normal stimulus is required to trigger a new action potential because some K$^+$ channels are still open and the membrane is hyperpolarized.

These periods ensure unidirectional signal propagation and regulate firing frequency.

Worked Example: Consider a neuron at $-70,\text{mV}$. A stimulus causes it to depolarize to $-50,\text{mV}$ (threshold). Voltage-gated Na$^+$ channels open, Na$^+$ rushes in, potential goes to $+30,\text{mV}$. Na$^+$ channels inactivate, voltage-gated K$^+$ channels open, K$^+$ rushes out, potential drops to $-75,\text{mV}$ (hyperpolarization), then returns to $-70,\text{mV}$.

Prelims Revision Notes

Resting Membrane Potential (RMP)

Definition: — Electrical potential difference across the neuronal membrane at rest, typically $-70,\text{mV}$ (inside negative).
Maintenance:

1. **Na$^+$/K$^+$ Pump:** Active transport ($3,\text{Na}^+_{\text{out}} / 2,\text{K}^+_{\text{in}}$), electrogenic, maintains concentration gradients. 2. Differential Permeability: Membrane much more permeable to K$^+$ (via K$^+$ leak channels) than Na$^+$ (fewer Na$^+$ leak channels) at rest. K$^+$ efflux contributes significantly to negativity. 3. Impermeant Anions: Large, negatively charged proteins/phosphates trapped inside.

Action Potential (AP)

Definition: — Rapid, transient, self-propagating reversal of membrane potential (nerve impulse).
All-or-None Principle: — Fires fully or not at all, fixed amplitude once threshold is reached.
Threshold Potential: — Critical depolarization level (approx. $-55,\text{mV}$) required to trigger AP.
Phases:

1. Depolarization (Rising Phase): * Stimulus reaches threshold. * Rapid opening of **voltage-gated Na$^+$ channels**. * Massive **Na$^+$ influx** (down electrochemical gradient). * Membrane potential rapidly becomes positive (e.

g., $+30,\text{mV}$). 2. Repolarization (Falling Phase): * **Voltage-gated Na$^+$ channels inactivate** (close and become unresponsive). * Slower opening of **voltage-gated K$^+$ channels**. * Rapid **K$^+$ efflux** (down electrochemical gradient).

* Membrane potential rapidly returns to negative. 3. Hyperpolarization (Undershoot): * Voltage-gated K$^+$ channels close slowly. * Excessive **K$^+$ efflux** causes membrane to become more negative than RMP (e.

g., $-80,\text{mV}$). * Na$^+$/K$^+$ pump and K$^+$ leak channels restore RMP.

Refractory Periods

Absolute Refractory Period:

* During depolarization and early repolarization. * **Voltage-gated Na$^+$ channels are inactivated.** * No new AP can be generated, regardless of stimulus strength. * Ensures unidirectional propagation and discrete APs.

Relative Refractory Period:

* During late repolarization and hyperpolarization. * Some Na$^+$ channels have reset, but K$^+$ channels are still open, and membrane is hyperpolarized. * A stronger-than-normal stimulus is required to generate a new AP. * Allows for modulation of firing frequency.

Vyyuha Quick Recall

Na$^+$ In, K$^+$ Out, Ready Again!

Na$^+$ In: Depolarization is due to Sodium (Na$^+$) rushing In.
K$^+$ Out: Repolarization is due to Potassium (K$^+$) rushing Out.
Ready Again: The Na$^+$/K$^+$ pump gets the neuron Ready Again for the next Action potential by restoring ion gradients.