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Peripheral nerve and local anaesthetic drugs.

Local anaesthetics are drugs that block reversibly the conduction of impulses in the peripheral nervous system. By definition, the latter comprises the roots, rami and distal branches of both cranial and spinal nerves, and includes components of the autonomic nervous system. The peripheral and central nervous systems may be distinguished on both gross anatomical and histological grounds, with both factors having some relevance to local anaesthesia. The gross anatomical arrangement of the nerves governs the distribution of blockade once an agent has been applied at a particular site. It has the same significance whatever factor (drug, temperature, surgery, etc.) is used to interrupt function and bears no relationship to the mode of action of that factor. In contrast histological, in addition to neurophysiological, factors affect the action of drugs, since they must be able to penetrate the often considerable covering of a nerve fibre before its function can be interrupted.