What is the 'all-or-none' principle in nerve impulse conduction?

The 'all-or-none' principle states that if a stimulus applied to a neuron reaches a certain threshold potential, a full-strength action potential will be generated, always with the same amplitude and duration for that specific neuron. If the stimulus is below the threshold, no action potential will be generated at all. It's like flipping a light switch: it's either fully on or fully off; there's no 'half-on' state. This ensures reliable signal transmission without degradation over distance.

How does the sodium-potassium pump contribute to nerve impulse conduction?

While the sodium-potassium pump (Na+/K+ ATPase) is not directly involved in the rapid depolarization and repolarization phases of an action potential, it is absolutely crucial for maintaining the ion concentration gradients (high Na+ outside, high K+ inside) across the neuronal membrane. These gradients are the driving force for the passive diffusion of ions through voltage-gated channels during an action potential. Without the pump actively restoring these gradients after each impulse, the neuron would quickly lose its ability to fire subsequent action potentials.

What is the difference between absolute and relative refractory periods?

The absolute refractory period is a time during which a neuron cannot generate another action potential, regardless of the strength of the stimulus. This occurs when voltage-gated Na+ channels are either open or inactivated. The relative refractory period follows the absolute period, during which a new action potential can be generated, but only by a stronger-than-normal stimulus. This is because the membrane is still hyperpolarized, and some K+ channels are still open, making it harder to reach the threshold.

Why does a nerve impulse only travel in one direction?

A nerve impulse travels unidirectionally primarily due to the absolute refractory period. After a segment of the axon has just fired an action potential, its voltage-gated sodium channels become inactivated and cannot open again for a brief period. This prevents the local currents from re-exciting the previously depolarized region and ensures that the impulse propagates forward, away from the site of initiation (typically the axon hillock) towards the axon terminals.

How does myelin increase the speed of nerve impulse conduction?

Myelin, an insulating fatty sheath, significantly increases conduction speed through a process called saltatory conduction. Instead of regenerating the action potential at every point along the axon, the myelin prevents ion leakage, forcing the local currents to 'jump' rapidly between unmyelinated gaps called Nodes of Ranvier. Voltage-gated ion channels are concentrated at these nodes, allowing the action potential to be regenerated only at these specific points, thus speeding up transmission considerably compared to continuous conduction in unmyelinated axons.

What role do voltage-gated ion channels play in nerve impulse conduction?

Voltage-gated ion channels are the molecular machinery directly responsible for generating and propagating the action potential. Voltage-gated sodium channels open rapidly upon reaching threshold, causing depolarization. Voltage-gated potassium channels open more slowly and close slowly, causing repolarization and hyperpolarization. Their voltage-dependent opening and closing, along with their inactivation properties, dictate the precise timing and shape of the action potential and ensure its unidirectional propagation.

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Conduction of Nerve Impulse

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Version 1Updated 22 Mar 2026

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The conduction of a nerve impulse, also known as an action potential, is the fundamental mechanism by which information is transmitted throughout the nervous system. It involves a rapid, transient change in the electrical potential across the neuronal membrane, primarily driven by the sequential opening and closing of voltage-gated ion channels. This electrical signal originates at the axon hilloc…

Quick Summary

Nerve impulse conduction is the process by which electrical signals, called action potentials, travel along a neuron. It begins with a neuron at its resting membrane potential, maintained by the sodium-potassium pump and leak channels, making the inside negative.

A sufficient stimulus triggers depolarization, where voltage-gated sodium channels open, allowing Na+ ions to rush in and make the inside positive. This is followed by repolarization, as voltage-gated potassium channels open, allowing K+ ions to exit, restoring negativity.

A brief hyperpolarization may occur before returning to rest. This action potential propagates along the axon, either continuously in unmyelinated fibers or by 'jumping' from node to node (saltatory conduction) in myelinated fibers, which is much faster.

The absolute refractory period ensures unidirectional flow. Axon diameter and myelination are key factors determining conduction speed.

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Key Concepts

Ionic Basis of Resting Potential

The resting membrane potential is a crucial prerequisite for nerve impulse conduction. It's established by…

Threshold Potential and All-or-None Principle

For an action potential to be generated, the local depolarization caused by a stimulus must reach a critical…

Mechanism of Saltatory Conduction

Saltatory conduction is the specialized mode of impulse propagation in myelinated axons. The myelin sheath,…

Resting Potential: —

-70,\text{mV}

, high

K^+

inside, high

Na^+

outside. Maintained by

Na^+/K^+

pump (3

Na^+

out, 2

K^+

in) and

K^+

leak channels.

Threshold Potential: —

approx -55,\text{mV}

. Required to trigger action potential.

Depolarization: — Rapid

Na^+

influx via voltage-gated

Na^+

channels. Membrane becomes positive.

Repolarization: —

Na^+

channels inactivate, rapid

K^+

efflux via voltage-gated

K^+

channels. Membrane becomes negative.

Hyperpolarization: — Brief overshoot, membrane more negative than rest due to slow

K^+

channel closure.

Absolute Refractory Period: — No new AP possible;

Na^+

channels inactivated.

Relative Refractory Period: — Stronger stimulus can fire AP; membrane hyperpolarized.

Conduction Speed Factors: — Myelination (saltatory conduction), Axon Diameter (larger = faster).

Saltatory Conduction: — 'Jumping' of APs between Nodes of Ranvier in myelinated axons; faster, energy efficient.

Nerve Impulse: 'NaK-DRH-SC'

NaK: —

Na^+/K^+

pump maintains Resting Potential.

D: — Depolarization =

Na^+

Influx.

R: — Repolarization =

K^+

Efflux.

H: — Hyperpolarization = Extended

K^+

Efflux.

S: — Saltatory Conduction = Speed (Myelin).

C: — Continuous Conduction = Slower (Unmyelinated).

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Resting and Action Potential