The nerve fibers which are most sensitive to local anesthetic belong to ----- group
Classification of peripheral nerves is impoant in determining the sequence of local anesthetic blockade. B fibers are the most sensitive. Dilation of cutaneous blood vessels is often the first sign of local anesthetic onset. C fibers and A-d are next-in sensitivity. They result in the inability to feel cool sensations such as an alcohol wipe. Next-in sensitivity are the A-g, A-b, and A-a fibers, which result in the loss of sensation, pressure, proprioception, and finally motor paralysis. Local anesthetic concentration must be adequate to block nerve fibers. It takes approximately twice the concentration of local anesthetic to block motor fibers as it does to block sensory fibers, and sympathetic fibers require the least. The order of blockade onset is B fibers > C fibers = A-d fibers > A-g fibers > A-b fibers > A-a fibers. Fiber type myelin function A-a +++ Motor (efferent: to skeletal muscle) A-b ++ Touch, pressure, proprioception (afferent: from skin) A-g ++ Motor (efferent: to muscle spindles) A-d ++ Pain (sharp, fast) and temperature (efferent: from skin) B + Preganglionic sympathetic (efferent: to vascular smooth muscle) C none Pain (dull, slow) and temperature (afferent from skin); postganglionic sympathetic (efferent: to vascular smooth muscle)
A
B
C
D
Fundamental concepts