Local anesthetics block nerve impulse propagation by occluding transmembrane sodium channels, so preventing depolarization. First, the uncharged lipid-soluble anesthetic base penetrates the membrane; then the positively charged cation binds to anionic components of the sodium channel's internal axoplasmic mouth. Though primarily a carrier, the base contributes to blockade by causing the membrane to swell, so pinching the sodium channels.
Dissolved in water, local anesthetic salt crystals dissociate into anesthetic cation and base—proportional to the drug's fixed pKa and the tissue's variable pH. The cation-base concentration ratio is critical to optimal neural blockade. If there is too little base, few anesthetic molecules will penetrate to the neural target; if too little cation, few sodium channels will be plugged.
(JAMA 238:1383-1385, 1977)
de Jong RH. Neural Blockade by Local Anesthetics. JAMA. 1977;238(13):1383–1385. doi:10.1001/jama.1977.03280140061020
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