Describe Local Anaesthetics — Mechanism & Classification in detail.

Definition & Mechanism

Local anaesthetics (LAs) are drugs that produce reversible blockade of nerve conduction in a circumscribed area, causing loss of sensation (and, at higher concentrations, motor block) without loss of consciousness. They act by blocking voltage-gated sodium channels in the nerve membrane from the inside, preventing the sodium influx needed to generate and propagate an action potential — so the nerve cannot depolarise and impulse conduction stops.

Local anaesthetics bind voltage-gated sodium channels from the inside, blocking sodium influx so the action potential cannot form or propagate.

Structure & Classification

A local anaesthetic molecule has an aromatic (lipophilic) ring and an amine (hydrophilic) group joined by an ester or amide link — and this link classifies them. Esters (procaine, amethocaine/tetracaine, cocaine) are metabolised by plasma cholinesterase and are more likely to cause allergy (a PABA metabolite). Amides (lignocaine, bupivacaine, ropivacaine, prilocaine) are metabolised in the liver and allergy is rare. (A memory aid: amides have an ‘i’ in the prefix — lignocaine, bupivacaine.)

Determinants of Action

Three physicochemical properties govern LA behaviour:

• pKa determines the speed of onset (agents with a pKa closer to physiological pH have more unionised drug to cross the membrane, so a faster onset)
• lipid solubility determines potency
• protein binding determines the duration of action. This is why bupivacaine (highly protein-bound, lipid-soluble) is potent and long-acting.

Differential Block

Nerve fibres are blocked in order of size and myelination: small, unmyelinated fibres first. So autonomic (sympathetic) and pain/temperature fibres are blocked before touch, and motor fibres last — producing a differential block in which pain sensation is lost while some motor power may be retained (exploited in labour epidurals).

Reversibility & Onset

An essential property of local anaesthetic block is that it is fully reversible: as the drug is absorbed away from the nerve and its concentration falls, the sodium channels recover and normal conduction returns, leaving no lasting damage when the drugs are used correctly. The onset of block depends on how much of the drug is in its unionised (lipid-soluble) form to cross the nerve sheath and membrane, which is why agents with a pKa nearer physiological pH act faster and why block is slow and unreliable in acidic, infected tissue where more of the drug is trapped in its ionised form. Once inside the cell the drug becomes ionised again to bind the channel, so both forms matter — the unionised form to get in, the ionised form to act.

• LAs reversibly block voltage-gated Na⁺ channels (from inside) → no action potential → conduction stops; consciousness preserved.
• Structure: aromatic ring + amine linked by ester or amide bond.
• Esters (procaine, amethocaine, cocaine): plasma cholinesterase, more allergy; amides (‘-i-caine’): hepatic, rare allergy.
• PKa → onset; lipid solubility → potency; protein binding → duration.
• Work poorly in acidic (infected) tissue; differential block — autonomic/pain before motor.