How Can Part of You Stop Feeling? Interrupting the Signal on the Way
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Injecting a local anaesthetic beside a nerve stops signals passing along it, so everything that nerve serves loses sensation while the person stays awake. The chemistry and the anatomy both have to be right.
How the drugs work
A nerve signal propagates because sodium ions rush across the membrane through channels that open when the voltage changes, regenerating the impulse at each point along the fibre. Local anaesthetics bind to those channels from the inside and block them, so the impulse cannot regenerate and stops where the drug is. The drug must cross the membrane to reach its binding site, which it does in an uncharged form, and then acts in a charged form, which is why the acidity of the surrounding tissue affects how well it works and why anaesthetics are less effective in inflamed tissue. Fibres differ in how readily they are blocked, with small unmyelinated pain fibres affected before large motor fibres, which is why sensation can be removed while movement is partly retained.
How the drug is placed
Where it is injected determines how much of the body is affected:
- •Infiltration, injecting directly into the tissue to be worked on, which suits small procedures
- •Peripheral nerve block, placing the drug beside a named nerve to numb everything it supplies
- •Plexus block, targeting a bundle of nerves to anaesthetise a whole limb
- •Spinal anaesthesia, injecting into the fluid around the spinal cord, which affects everything below
- •Epidural, placing the drug outside that membrane, which allows continuous infusion
- •Topical application, which works where the surface is thin enough for the drug to penetrate
Finding the nerve
Placing the drug accurately used to depend on anatomical landmarks and on asking the patient, and the change in technique has been substantial. Traditional methods located a nerve by feel and by anatomical relationship, sometimes confirmed by provoking a sensation when the needle touched it, which is unpleasant and risks damage. Nerve stimulation passes a small current through the needle and watches for the muscle the nerve supplies to twitch, which confirms proximity without contact. Ultrasound now allows the nerve, the needle and the spreading drug to be seen directly in real time, which has improved success rates, reduced the volume of drug required and reduced complications, and it has become standard practice within about two decades.
Where the drugs came from
The whole class descends from one plant compound and the development is worth tracing. Cocaine, isolated from coca leaves in the mid nineteenth century, was found to numb tissue it contacted and was introduced for eye surgery in 1884, which was the first local anaesthetic and which transformed several kinds of operation immediately. Its toxicity and its potential for dependence drove a search for substitutes, and chemists produced synthetic compounds sharing the essential structure while removing the rest, beginning with procaine in 1905 and continuing through the compounds in use today, which differ in how fast they act, how long they last and how toxic they are. Adding a compound that constricts blood vessels keeps the drug where it was placed and extends its effect, which is why many preparations contain one.
Why it matters beyond avoiding a general
The advantages go well past staying awake. Avoiding general anaesthesia removes its risks, which matter more for elderly and frail patients and for those with lung and heart disease. Pain control continuing after the procedure reduces the need for opioid medication substantially, which matters for side effects and for dependence. Recovery is faster and discharge earlier. Blocks are also used to treat pain rather than to permit surgery, including for chronic conditions, and diagnostically to establish which nerve is carrying a pain. The limitations are real, since a block can fail or be incomplete, the drugs are toxic if injected into a blood vessel or given in excess, and some procedures require the patient to be unconscious regardless.
The takeaway
Local anaesthetics block the sodium channels that regenerate a nerve impulse, so signals stop where the drug is, and small pain fibres are blocked before large motor ones. Placement determines coverage, from a single nerve to everything below the waist. Ultrasound showing nerve, needle and spreading drug in real time replaced landmarks and stimulation within about two decades.