What is the primary difference between a graded potential and an action potential?

A graded potential is a localized change in membrane potential that varies in amplitude depending on the strength of the stimulus. It can be depolarizing or hyperpolarizing and typically decays over distance. It's like a ripple in a pond. An action potential, on the other hand, is an 'all-or-none' event; once a threshold is reached, it fires with a fixed amplitude and propagates without decrement over long distances. It's a self-regenerating signal, unlike a graded potential.

Why is the resting membrane potential negative inside the neuron?

The resting membrane potential is negative primarily due to three factors: the differential permeability of the membrane to ions (more $ ext{K}^+$ leak channels than $ ext{Na}^+$ leak channels, allowing $ ext{K}^+$ to diffuse out), the presence of large, negatively charged proteins trapped inside the cell, and the electrogenic action of the $ ext{Na}^+/ ext{K}^+$ pump, which expels three $ ext{Na}^+$ ions for every two $ ext{K}^+$ ions it brings in, resulting in a net loss of positive charge from the cell.

What is the significance of the absolute refractory period?

The absolute refractory period is crucial for ensuring the unidirectional propagation of the nerve impulse and for limiting the frequency at which a neuron can fire. During this period, voltage-gated $ ext{Na}^+$ channels are either open or inactivated, making it impossible to generate another action potential. This prevents the impulse from traveling backward along the axon and ensures that each action potential is a distinct, separate event.

How does myelination increase the speed of nerve impulse conduction?

Myelination increases conduction speed through saltatory conduction. The myelin sheath acts as an electrical insulator, preventing ion leakage across the membrane. This forces the local currents to 'jump' from one Node of Ranvier (unmyelinated gaps) to the next. Action potentials are only regenerated at these nodes, reducing the number of times the membrane needs to depolarize and repolarize, thus significantly speeding up the transmission and conserving energy.

Can a nerve impulse travel backward?

Under normal physiological conditions, a nerve impulse cannot travel backward. This is due to the absolute refractory period. Immediately after a segment of the axon has generated an action potential, it enters an absolute refractory period during which it cannot be re-excited. This ensures that the impulse propagates unidirectionally, moving away from the point of origin (e.g., axon hillock) towards the axon terminal.

What role does the sodium-potassium pump play in nerve impulse generation and conduction?

The sodium-potassium pump is vital for maintaining the ion concentration gradients (high $ ext{Na}^+$ outside, high $ ext{K}^+$ inside) across the neuronal membrane. These gradients are essential for the passive diffusion of ions that underlies the resting potential and the rapid ion movements during an action potential. While the pump doesn't directly generate the action potential, its continuous activity restores the ion balance after each impulse, ensuring the neuron is ready to fire again and maintaining the resting membrane potential.

Generation and Conduction of Nerve Impulse

Biology

NEET UG

Version 1Updated 22 Mar 2026

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The generation and conduction of a nerve impulse, also known as an action potential, is the fundamental mechanism by which neurons transmit information throughout the nervous system. It involves a rapid, transient change in the electrical potential across the neuronal membrane, moving from a resting state to a depolarized state and then repolarizing. This electrochemical signal is initiated when a…

Quick Summary

The generation and conduction of a nerve impulse, or action potential, is the fundamental electrical signal of neurons. It begins with a neuron at its resting membrane potential (RMP), typically $-70,\text{mV}$ , maintained by the $ext{Na}^+/\text{K}^+$ pump and differential ion permeability.

A stimulus, if strong enough to reach the threshold potential (around $-55,\text{mV}$ ), triggers rapid depolarization. This involves the opening of voltage-gated $ext{Na}^+$ channels, causing a massive influx of $ext{Na}^+$ ions and making the inside of the cell positive (up to $+30,\text{mV}$ ).

Immediately, $ext{Na}^+$ channels inactivate, and voltage-gated $ext{K}^+$ channels open, leading to $ext{K}^+$ efflux and repolarization, restoring the negative charge. A brief hyperpolarization (undershoot) may occur before the RMP is re-established.

This entire process follows the 'all-or-none' principle. Once generated, the impulse propagates along the axon. In unmyelinated axons, it's continuous. In myelinated axons, it 'jumps' between Nodes of Ranvier (saltatory conduction), significantly increasing speed and efficiency.

Refractory periods ensure unidirectional propagation and limit firing frequency.

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Key Concepts

Resting Membrane Potential (RMP) Maintenance

The RMP is a dynamic equilibrium, not a static state. It's primarily established by a higher permeability of…

Action Potential Phases: Depolarization and Repolarization

The action potential is a rapid sequence of events. Depolarization begins when a stimulus causes the membrane…

Saltatory Conduction Mechanism

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

Resting Potential: —

\approx -70,\text{mV}

. Inside negative. Maintained by

\text{Na}^+/\text{K}^+

pump (

3\text{Na}^+_{\text{out}} / 2\text{K}^+_{\text{in}}

) and

\text{K}^+

leak channels.

Threshold Potential: —

\approx -55,\text{mV}

. Minimum depolarization to trigger AP.

Depolarization (Rising Phase): — Stimulus reaches threshold

\rightarrow

Rapid opening of voltage-gated

\text{Na}^+

channels

\rightarrow

\text{Na}^+

influx

\rightarrow

Membrane potential becomes positive (up to

+30,\text{mV}

). All-or-none principle.

Repolarization (Falling Phase): —

\text{Na}^+

channels inactivate

\rightarrow

Slow opening of voltage-gated

\text{K}^+

channels

\rightarrow

\text{K}^+

efflux

\rightarrow

Membrane potential returns to negative.

Hyperpolarization (Undershoot): — Slow closing of

\text{K}^+

channels

\rightarrow

Excessive

\text{K}^+

efflux

\rightarrow

Membrane potential more negative than RMP.

Absolute Refractory Period: — During depolarization/early repolarization.

\text{Na}^+

channels open/inactivated. No new AP possible. Ensures unidirectional flow.

Relative Refractory Period: — During late repolarization/hyperpolarization. Stronger stimulus can trigger AP.

Conduction:

- Continuous: Unmyelinated axons. Slower. Sequential depolarization. - Saltatory: Myelinated axons. Faster, energy-efficient. AP 'jumps' between Nodes of Ranvier.

NaK+ pump sets the Rest, then Depolarization is Na+ in a Rush. Repolarization is K+ out, then Hyperpolarization's a K+ Slow-down. Saltatory Jumps are Fast!