Lidocaine is a local anaesthetic. It is injected by dentists before fillings, and used in creams and for minor operations, to stop pain in one area of the body while the patient stays conscious.Lidocaine acts on the axons of neurones rather than at synapses. It blocks voltage-gated sodium ion channels in the neurone membrane.Normally, when a stimulus reaches threshold, voltage-gated Na+ channels open and Na+ floods in, causing depolarisation. When lidocaine is present:• the Na+ channels cannot open, so Na+ cannot diffuse into the axon
• the membrane cannot be depolarised, so no action potentials are generated
• impulses cannot travel along the sensory neurones from the pain receptors to the brain
• the brain does not receive the information, so no pain is feltThe resting potential is not affected, because it depends on the sodium-potassium pump and K+ channels, not on voltage-gated Na+ channels.Lidocaine acts only near where it is applied, which is why it is a local anaesthetic, unlike general anaesthetics, which act on the brain and cause unconsciousness. Thin pain neurones are blocked most easily, so pain is lost before touch and pressure. Higher doses also block motor neurones in the area, which is why a lip can feel numb and move less easily after a dental injection.The effect wears off as lidocaine is carried away in the blood and broken down in the liver. Because it also blocks Na+ channels in heart muscle, lidocaine is sometimes used to treat abnormal heart rhythms.
• the membrane cannot be depolarised, so no action potentials are generated
• impulses cannot travel along the sensory neurones from the pain receptors to the brain
• the brain does not receive the information, so no pain is feltThe resting potential is not affected, because it depends on the sodium-potassium pump and K+ channels, not on voltage-gated Na+ channels.Lidocaine acts only near where it is applied, which is why it is a local anaesthetic, unlike general anaesthetics, which act on the brain and cause unconsciousness. Thin pain neurones are blocked most easily, so pain is lost before touch and pressure. Higher doses also block motor neurones in the area, which is why a lip can feel numb and move less easily after a dental injection.The effect wears off as lidocaine is carried away in the blood and broken down in the liver. Because it also blocks Na+ channels in heart muscle, lidocaine is sometimes used to treat abnormal heart rhythms.