Which nerve fibre is most resistant to local anesthetic?

Discussion in 'Question Zone' started by Guest, Sep 11, 2010.

  1. Guest

    Guest Guest

    Which of the following nerve fibre is most resistant to local anesthetic?
    a. A alpha
    b. A gamma
    c. B
    d. C
  2. Guest

    Guest Guest

    Different fiber types are also differentially sensitive to local anesthetic blockade. In vivo experiments in which continuous superperfusion of peripheral nerve allows equilibration with drug and experiments in which a drug bolus is delivered by percutaneous injection, analogous to clinical peripheral nerve block, show unequivocally that small myelinated axons (Aγ motor and Aδ sensory fibers) are the most susceptible to impulse annihilation. Next in order of block are the large myelinated (Aα and Aβ) fibers, and the least susceptible are the small, nonmyelinated C fibers. In fact, in this last group, impulses in the slowest conducting population (conduction velocity of 0.5 to 0.8 msec) are the most resistant to local anesthetic. The generalized notion that local anesthetics block the smallest fibers first or most is clearly wrong.
  3. Guest

    Guest Guest

    Miller's text 6/e:

    "Traditional texts often state that small-diameter axons, such as C fibers, are more susceptible to local anesthetic block than are larger-diameter fibers. However, when careful measurements are made of single-impulse annihilation in individual nerve fibers, exactly the opposite differential susceptibility is seen. Repetitive stimulation, such as occurs during propagation of trains of impulses--the normal mode of operation for neuronal information coding produces a further, phasic inhibition of excitability, but it is unclear how this will effect a functionally selective failure of impulses. The length of drug-exposed nerve in the intrathecal space, imposed by anatomic restrictions,can explain clinically documented differential spinal or epidural blockade, because longer drug-exposed regions yield block by lower concentrations of local anesthetic. However, this reasoning does not explain the functionally differential loss from peripheral nerve block. Other factors may include the actual spread of the drug along the nerve, or its selective ability to inhibit Na + channels over K+ channels, which in itself can produce a differential block because these channels are present in very different proportions in different types of nerves. Because of these confounding factors, clinicians should be discouraged from making conclusions about fiber type involvement in chronic pain syndromes based on the dose or concentration requirement for pain relief in diagnostic nerve blockade."

    So in nutshell - Although Miller state's that Traditional texts suggest that "C" fibres have the maximum susceptibility to local anesthetics and careful examination shows that it is not so, in the end it also says:"Because of these confounding factors, clinicians should be discouraged from making conclusions about fiber type involvement in chronic pain syndromes based on the dose or concentration requirement for pain relief in diagnostic nerve blockade."

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