[Pain perception, mechanisms of action of local anesthetics and possible causes of failure].

First, the fundamentals of impulse transmission and pain perception are revised. The role of the primary afferent nociceptors is explained. Dental pain is described as a form of acute pain and the mechanism of nociception is fundamental. Peripheral and central sensitization can evolve. The second part covers the pharmacological aspects. Local anesthetics reduce impulse transmission by interfering with the mechanism of normal depolarisation. Binding to specific receptors located at the nerve membrane, more specifically on the sodium channel, results in decreased or eliminated permeability to sodium ions and leads to interruption of nerve conduction. The different types of local anesthetics used in dentistry are discussed in more detail with respect to their physico-chemical characteristics and analgetic properties. The importance of factors such as lipophilicity, degree of protein binding and dissociation constant pKa are explained together with the clinical implications of pH and possible toxic effects. Failure of local anesthesia can be the result of problems with the administration of the product or can have a pharmacological basis. Injection of the anesthetic should take place in amounts large enough, with suitable volume and as close as possible to the nerve. When infection and inflammation are present, the intravascular resorption of the anesthetic will accelerate and the lowered pH influences diffusion negatively. Repetitive administration can induce the phenomenon of tachyfylaxis (decreased anesthetic effect).