The subarachnoid angle: An area of transition in peripheral nerve

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Abstract
The lateral limit of the subarachnoid space, where nerve roots enter and leave, forms the subarachnoid angle. This is an important site of transition for nerve sheaths. Here the perineurium of peripheral nerve leaves the surface of the nerve and extends between the dura mater and the arachnoid. The perineurium is therefore open‐ended with respect to the subarachnoid space. The central perineurial extension is histologically the same as perineurium in some areas but in others forms a layer of hydrated cells without basement membranes. These lie in close apposition with the outermost cells of the arachnoid membrane. At the subarachnoid angle the arachnoid membrane may either reflect onto the root sheath or be attached to it by punctate junctions. The root sheath covers the nerve roots as they pass through the subarachnoid space. It is composed of loosely arranged cells bound by punctate junctions. Its intercellular spaces may contain connective tissue fibrils. A single basement membrane separates it from the endoneurium. The histological structure in the region of the subarachnoid angle is consistent with clinical evidence implicating the endoneurium of nerve trunks as a pathway for the transmission of infection from the periphery to the central nervous system.