Mechanism of action of Local anesthetics:
Correct Answer: Inhibition of Na+ channels
Description: Ref: Miller Textbook of Anaesthesia. 6th editionExplanation:Mechanism of action of Local anestheticsThe primary mechanism of action of local anesthetics is blockade of voltage-gated sodium channels.The excitable membrane of nerve axons, like the membrane ot cardiac muscle and neuronal cell bodies, maintains a resting transmembrane potential of -90 to -60 mV.During excitation, the sodium channels open, and a fast inward sodium current quickly depolarizes the membrane toward the sodium equilibrium potential t+40 mV).As a result of depolarization, the sodium channels close (inactivate) and potassium channels open. The outward flow of potassium repolarizes the membrane toward the potassium equilibrium potential (about -95 mV): repolarization returns the sodium channels to the rested state.Biologic toxins such as batrachotoxin. aconitine, veratridine. and some scorpion venoms bind to receptors within the channel and prevent inactivation. This results in prolonged influx of sodium through the channel and depolarization of the resting potential.The marine toxins tetrodotoxin (TTX) and saxitoxin block sodium channels by binding to channel receptors near the extracellular surface. Their clinical effects superficially resemble those of local anesthetics (ie, block of conduction without a change in the resting potential) even though their receptor site is quite different.The blockade of sodium channels by most local anesthetics is both voltage- and time-dependent: Channels in the rested state, which predominate at more negative membrane potentials, have a much lower affinity for local anesthetics than activated (open state) and inactivated channels, w-hich predominate at more positive membrane potentials.Elevated extracellular calcium partially antagonizes the action of local anesthetics owing to the calcium-induced increase in the surface potential on the membrane (which favors the low-affinity rested state).Increases in extracellular potassium depolarize the membrane potential and favor the inactivated state, enhancing the effect of local anesthetics.
Category:
Anaesthesia
Get More
Subject Mock Tests
Practice with over 200,000 questions from various medical subjects and improve your knowledge.
Attempt a mock test nowMock Exam
Take an exam with 100 random questions selected from all subjects to test your knowledge.
Coming SoonGet More
Subject Mock Tests
Try practicing mock tests with over 200,000 questions from various medical subjects.
Attempt a mock test now