describe and explain changes to the membrane potential of neurones, including: how the resting potential is maintained, the events that occur during an action potential, how the resting potential is restored during the refractory period

Control and Coordination in Mammals

Objective

Describe and explain changes to the membrane potential of neurones, including:

• How the resting potential is maintained
• The events that occur during an action potential
• How the resting potential is restored during the refractory period

1. Resting Membrane Potential

The resting membrane potential (RMP) of a typical neurone is about \$-70\ \text{mV}\$. It is established and maintained by three main factors:

1. Ion concentration gradients across the plasma membrane.
2. Selective permeability of the membrane to different ions.
3. Sodium‑potassium pump (Na⁺/K⁺‑ATPase) which actively transports ions against their gradients.

Ion Concentrations

Key Processes Maintaining RMP

• Leak channels – mainly K⁺ leak channels allow K⁺ to diffuse out of the cell, making the interior negative.
• Na⁺/K⁺‑ATPase – pumps 3 Na⁺ out and 2 K⁺ in per ATP hydrolysed, contributing to the negative charge inside.
• Electrochemical equilibrium – the Nernst equation predicts the equilibrium potential for each ion; the RMP is closest to the K⁺ equilibrium potential because of its high permeability.

2. Action Potential

An action potential is a rapid, self‑propagating change in membrane potential that travels along the axon. It consists of several phases:

2.1 Depolarisation

1. A stimulus raises the membrane potential to the threshold (≈ \$-55\ \text{mV}\$).
2. Voltage‑gated Na⁺ channels open rapidly.
3. Na⁺ rushes into the cell (driven by both concentration and electrical gradients), causing the membrane potential to become positive (peak ≈ \$+30\ \text{mV}\$).

2.2 Repolarisation

1. Voltage‑gated Na⁺ channels close (inactivation).
2. Voltage‑gated K⁺ channels open.
3. K⁺ exits the cell, driving the membrane potential back toward the negative resting value.

2.3 Hyperpolarisation (After‑potential)

Because K⁺ channels close slowly, the membrane potential often becomes slightly more negative than the resting potential (≈ \$-80\ \text{mV}\$) before returning to \$-70\ \text{mV}\$.

2.4 Summary of Ion Movements

\$\$\begin{aligned}

\text{Depolarisation:}&\quad \text{Na}^{+}{\text{out}} \rightarrow \text{Na}^{+}{\text{in}} \\

\text{Repolarisation:}&\quad \text{K}^{+}{\text{in}} \rightarrow \text{K}^{+}{\text{out}} \\

\text{Hyperpolarisation:}&\quad \text{K}^{+} \text{ continues to leave until channels close}

\end{aligned}\$\$

3. Refractory Period and Restoration of Resting Potential

After an action potential, the neurone experiences a refractory period during which a second impulse cannot be generated (or requires a stronger stimulus). It consists of two phases:

3.1 Absolute Refractory Period

• All voltage‑gated Na⁺ channels are inactivated.
• Another action potential cannot be initiated, regardless of stimulus strength.

3.2 Relative Refractory Period

• Some Na⁺ channels have returned to the closed (but activatable) state, while K⁺ channels remain open.
• A stronger than normal stimulus can elicit an action potential.

Restoration Mechanisms

1. Na⁺/K⁺‑ATPase – restores the original ion gradients by pumping Na⁺ out and K⁺ in.
2. Closing of voltage‑gated K⁺ channels – stops K⁺ efflux, allowing the membrane potential to settle at the resting value.
3. Leak channels – maintain the baseline permeability that defines the RMP.

Key Points to Remember

• The resting potential is primarily determined by K⁺ permeability and the Na⁺/K⁺ pump.
• An action potential is an all‑or‑none event that depends on the rapid opening and closing of voltage‑gated Na⁺ and K⁺ channels.
• The refractory period ensures unidirectional propagation of the nerve impulse and limits the frequency of firing.
• Energy from ATP hydrolysis (via Na⁺/K⁺‑ATPase) is essential for resetting ion distributions after each impulse.