The perineurium serves as the primary barrier to local anesthetic diffusion because it is a dense, multilayered connective tissue sheath that tightly surrounds each nerve fascicle. It has low permeability which restricts the passage of hydrophilic molecules, including many local anesthetics. This structural characteristic ensures that anesthetic agents must cross this barrier to reach the endoneurium where nerve fibers lie. The epineurium and endoneurium are more permeable, while the myelin sheath affects conduction but does not significantly impede anesthetic penetration. Thus, the perineurium is the main determinant in the onset and efficacy of nerve blocks.

Reference: Miller’s Anesthesia, Ronald D. Miller, 9th Edition.