Nerve fiber most susceptible to local anesthetics.

a. A
b. Parasympathetic.
c.B.
d. C


Ans: D

Autonomic fibers, small unmyelinated C fibers (mediating pain) and small myelinated A-delta fibers (mediating pain and temperature sensation) are blocked before larger myelinated A-gamma, A-beta, or A-alpha fibers (mediating touch, pressure, muscle and postural inputs).


Local anesthetics block the generation and conduction of nerve impulses at the level of the cell membrane by preventing the transient increase in permeability of excitable membranes.

Local anesthetics bind directly within the intracellular portion of voltage-gated sodium channels. The degree of block produced by local anesthetics is dependent upon how the nerve has been stimulated and on its resting membrane potential.

Local anesthetics are only able to bind to sodium channels in their charged form and when the sodium channels are open. In this situation, the local anesthetic is able to bind more tightly to and stabilize the sodium channel.

Differences in pKa, lipid solubility, and molecular size influence the binding of local anesthetics to to the sodium channels.