Resting potential is the electrical potential difference across a neurone membrane when it is not transmitting an impulse, typically measured at approximately mV. This state is described as polarized, meaning there is a distinct difference in charge between the inside (negative) and outside (positive) of the axon.
The potential is established by the sodium-potassium pump, which uses ATP to actively transport three sodium ions () out of the axon for every two potassium ions () pumped in. This creates a concentration gradient for both ions across the membrane.
Membrane permeability plays a crucial role; the membrane is significantly more permeable to than to because more potassium leak channels are open. Consequently, diffuses out of the cell faster than can diffuse back in, resulting in a net loss of positive charge from the cytoplasm.
After the peak of the action potential, voltage-gated sodium channels close and voltage-gated potassium channels open. ions rapidly diffuse out of the axon down their electrochemical gradient, returning the membrane potential toward its resting state, a process called repolarisation.
The membrane often briefly becomes more negative than the resting potential (around to mV), known as hyperpolarisation, because potassium channels are slow to close. During this time, the neurone enters a refractory period where it is unresponsive to further stimulation.
The refractory period is essential for ensuring that nerve impulses are discrete (separate) and travel in only one direction. It prevents the impulse from spreading backward because the previous section of the membrane is temporarily incapable of generating a new action potential.
Impulse propagation occurs via local currents; the influx of at one point causes ions to diffuse sideways inside the axon. This sideways movement depolarises the adjacent section of the membrane to the threshold, triggering a new action potential further along the line.
In myelinated neurones, the axon is insulated by a fatty myelin sheath produced by Schwann cells, which prevents ion exchange across most of the membrane. Depolarisation can only occur at the uninsulated gaps known as nodes of Ranvier.
This results in saltatory conduction, where the action potential appears to 'jump' from one node to the next. This mechanism significantly increases the speed of transmission compared to unmyelinated neurones, where the wave of depolarisation must travel continuously along the entire membrane.
| Feature | Resting Potential | Action Potential |
|---|---|---|
| Membrane State | Polarised ( mV) | Depolarised ( mV) |
| Ion Movement | pump active; leak | Rapid influx; then efflux |
| Channels | Voltage-gated channels closed | Voltage-gated channels open/close |
Myelinated vs. Unmyelinated: Myelinated axons use saltatory conduction to achieve high speeds (up to 120 m/s), whereas unmyelinated axons rely on continuous propagation, which is much slower (approx. 0.5-2 m/s).
Frequency vs. Amplitude: The nervous system codes for stimulus strength by increasing the frequency of action potentials, never by increasing the voltage amplitude of a single impulse.
Direction of Ion Flow: Always double-check the direction of ion movement. During depolarisation, moves into the axon; during repolarisation, moves out of the axon. A common error is swapping these directions.
Active vs. Passive: Remember that the sodium-potassium pump is active (uses ATP) and maintains the gradient, while the movement of ions during an action potential is passive (facilitated diffusion) through voltage-gated channels.
The 'Jump' Misconception: In saltatory conduction, the impulse doesn't literally jump through the air; it is the electrical circuit (local currents) that completes between nodes, allowing the action potential to be regenerated only at the gaps.
