Generation and Conduction of Nerve Impulse — Core Principles

Core Principles

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.

Important Differences

vs Continuous Conduction

Aspect	This Topic	Continuous Conduction
Occurs in	Unmyelinated axons	Myelinated axons
Mechanism	Sequential depolarization of adjacent membrane segments	Action potential 'jumps' from one Node of Ranvier to the next
Speed	Slower (e.g., $0.5 - 10, ext{m/s}$)	Much faster (e.g., $10 - 120, ext{m/s}$)
Energy Efficiency	Less efficient; $ ext{Na}^+/ ext{K}^+$ pump works along entire axon	More efficient; $ ext{Na}^+/ ext{K}^+$ pump only active at Nodes of Ranvier
Ion Channel Distribution	Voltage-gated channels distributed uniformly along the axon	Voltage-gated channels concentrated at Nodes of Ranvier
Myelin Sheath	Absent	Present

Continuous conduction, found in unmyelinated axons, involves the sequential depolarization of every segment of the axonal membrane, making it a relatively slow and energy-intensive process. In contrast, saltatory conduction, characteristic of myelinated axons, allows the nerve impulse to 'jump' between the Nodes of Ranvier. This 'jumping' mechanism significantly boosts the speed of impulse transmission and enhances energy efficiency by limiting the active regeneration of the action potential to only the unmyelinated nodes.