Mechanism of action of local anesthesia

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Here is a thorough breakdown of the mechanism of action of local anesthetics, drawn from authoritative textbooks.

Mechanism of Action of Local Anesthetics

Core Mechanism: Sodium Channel Blockade

Local anesthetics produce reversible nerve blockade by inhibiting voltage-gated sodium (Na+) channels in the axonal membrane. This prevents sodium influx, which is required for the upstroke of the action potential. Without a rising action potential, the firing threshold is never reached and nerve impulse conduction is blocked entirely.

Local anesthetic impulse inhibition diagram

Impulse propagation (left) is halted when local anesthetics block sodium channels (right). - Rosen's Emergency Medicine

Step-by-Step Mechanism

1. The drug exists in two forms

Local anesthetic solutions contain both:

An uncharged (lipid-soluble, free base) form - able to cross tissue barriers and the nerve membrane
A charged (cationic) form - the pharmacologically active form at the channel

The ratio of these two forms depends on the tissue pH and the drug's pKa. In an alkaline environment (higher pH), more uncharged form is present, which explains why local anesthetics work faster in normal (non-inflamed) tissue. Inflamed, acidic tissue reduces efficacy because less uncharged drug is available for membrane penetration.

2. Tissue penetration

The uncharged lipid-soluble form crosses the perineurium, nerve sheath, and axonal membrane to reach the axoplasm (interior of the nerve).

3. Re-equilibration inside the axon

Once inside, the drug re-equilibrates back into both charged and uncharged forms based on the axoplasmic pH.

4. Channel binding and blockade

The charged (cationic) form enters the sodium channel from the axoplasmic (inner) side and binds to a specific receptor site within the channel pore. This stabilizes the channel in its inactivated state, blocking Na+ influx. With the channel inactivated:

The rate of rise and amplitude of depolarization decrease
The firing threshold is not reached
No action potential is formed
Conduction is blocked

This is called the "specific receptor" theory and is the dominant accepted mechanism. (Note: it cannot fully explain the action of neutral compounds like benzocaine, which act via a separate membrane-expansion or direct pore-blocking mechanism without needing the charged form.)

Sodium channel states with local anesthetic

The channel transitions from closed → open → inactivated (blocked by the local anesthetic). - Rosen's Emergency Medicine

Use-Dependent (Phasic) Blockade

Local anesthetics bind preferentially to sodium channels in their open or inactivated states rather than the resting (closed) state. This means nerves that fire frequently (high frequency) are blocked more readily - a property called use-dependent or phasic blockade. Rapidly firing pain fibers are therefore more susceptible than resting motor fibers.

Physicochemical Properties Governing Activity

Property	Determines	Example
pKa	Onset of action	Lower pKa → more uncharged form → faster onset (e.g., lidocaine is faster than bupivacaine)
Lipid solubility	Potency	Higher solubility → better membrane penetration → greater potency (e.g., bupivacaine > lidocaine)
Protein binding	Duration	Tighter binding → longer duration (e.g., bupivacaine > procaine)

Differential Nerve Blockade

Local anesthetics block nerve fibers in a predictable order based on fiber size and myelination:

Small myelinated fibers (Aδ) - pain, temperature - blocked first
Small unmyelinated fibers (C) - pain, autonomic
Large myelinated fibers (Aα, Aβ) - touch, pressure, motor - blocked last (require higher concentrations)

This produces the characteristic clinical sequence: loss of pain → temperature → touch → proprioception → motor function.

Role of Epinephrine

Adding epinephrine to a local anesthetic causes local vasoconstriction, which:

Slows systemic absorption
Prolongs duration of anesthesia
Reduces the peak plasma concentration (less systemic toxicity)

Summary

Step	Event
Injection	Drug equilibrates into uncharged and charged forms based on tissue pH
Tissue penetration	Uncharged form crosses barriers
Inside axon	Re-equilibration; charged form predominates
Channel binding	Charged form binds inside Na+ channel from axoplasmic side
Blockade	Channel locked in inactivated state; Na+ influx blocked
Result	No action potential; no impulse conduction; reversible anesthesia

Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 633
Rosen's Emergency Medicine: Concepts and Clinical Practice, p. 3097
Lippincott Illustrated Reviews: Pharmacology, p. 692

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