The study of the ultrastructure of the spinal arachnoid and its relationship with the spinal nerve roots and dural sac may help explain the different patterns of local anesthetic distribution in the subarachnoid space after spinal anesthesia, as well as some neurological conditions associated with it. Under the electron microscope, the spinal arachnoid matter in humans is composed of two distinct layers: one compact and one trabecular. The trabecular arachnoid, which has a variable thickness, forms tubular structures that enclose each spinal nerve root as well as the spinal cord. The mesh-like structure of the spinal trabecular arachnoid could affect the even distribution of local anesthetics in the dural sac, leading to higher concentrations at certain sites. This anatomical feature would explain, at least in part, some of the neurological manifestations that have been described following spinal anesthesia. In this article, we discuss some of the available literature and present our own experience relevant to the ultrastructure of the spinal arachnoid in humans and its impact on distribution of local anesthetics within the spinal dural sac.