8.4:
Local Anesthetics: Differential Sensitivity of Nerve Fibers
A local anesthetic, or LA, can block conduction in all nerves, not just the ones around the target site. The variable response of nerve fibers to the LA, or their differential sensitivity, restricts the anesthesia to a limited region.
Nerve fibers proximal to the injection site are blocked first, followed by the distal ones. Additionally, structural variations in neurons also contribute to their differential sensitivity.
Typically, neurons can be myelinated or unmyelinated. In myelinated neurons, the axons are surrounded by multilayered myelin sheaths interspersed with unmyelinated gaps or nodes of Ranvier.
Differential nerve blockades depend on node spacing, myelination, or nerve type differences.
For example, narrow myelinated fibers with short internodal distances are more susceptible to LAs and are blocked faster than thicker fibers.
LA susceptibility also differs between sensory and motor neurons.
At low LA concentrations, sensory neurons are blocked first, while motor neurons remain unaffected. Such a differential blockade helps relieve pain during labor and delivery.
8.4:
Local Anesthetics: Differential Sensitivity of Nerve Fibers
Local anesthetics (LAs) block the sodium channels of nerve trunks, sensory nerve endings, and neuromuscular junctions. Although LAs can block all kinds of nerves, the sensitivity of nerve fibers differs according to nerve types and structures. LAs are known to block myelinated fibers faster than unmyelinated ones. Also, they block pain or sensory neurons at low concentrations without affecting the motor neurons involved in muscle contractions. This helps relieve labor pain without affecting the muscular contraction required to deliver the baby.
However, LAs fail to control pain effectively in inflamed tissues. During inflammation, the tissue pH becomes low. At such an acidic pH, LAs ionize, and their movement across the cell membrane gets disturbed. As a result, LAs stay outside the cell in their ionized form. Furthermore, inflamed tissues have higher vascularization and permeability, which allows the rapid removal of LAs into systemic circulation. Vasoconstrictors like adrenaline are usually given with LAs to reduce the blood flow and prolong the effect of LA. But its effectiveness is diminished in inflamed tissues.