Kiernan J A
Department of Anatomy, University of Western Ontario, London, Canada.
Microsc Res Tech. 1996 Oct 1;35(2):122-36. doi: 10.1002/(SICI)1097-0029(19961001)35:2<122::AID-JEMT3>3.0.CO;2-S.
Endothelium, choroidal epithelium, and arachnoid exclude plasma proteins from most parts of the mammalian central nervous system (CNS). Nerve roots, in contrast, have permeable capillaries and permeable pia-arachnoid sheaths. Diffusion of plasma proteins into the cerebrospinal fluid is probably prevented by slow bulk flow along a pressure gradient from the subarachnoid space into the veins of the roots. In nerves, the perineurium prevents diffusion of proteins from the epineurium into the endoneurium. Capillaries within fascicles are permeable to macromolecules, though less so than the microvessels of roots and ganglia. Endoneurial vascular permeability is lowest in rats and mice, but even in these species albumin is normally present in the extracellular spaces around the nerve fibers. The so-called blood-nerve barrier is not equivalent to the blood-brain barrier. Capillaries in sensory and sympathetic ganglia are fully permeable to macromolecules, and extravasated protein is in contact with neuronal cell bodies and neurites. An impenetrable perineurium surrounds each ganglion, but serves no obvious purpose when the vessels inside are as permeable as those outside. The enteric nervous system lacks a perineurium, and the neurons in its avascular ganglia and tracts are exposed to extracellular fluid formed by permeable vessels in adjacent tissues of the gut. The reasons for excluding macromolecules from some parts of the nervous system are obscure. Carrier-mediated transport, which maintains a constant supply of ions, glucose, and other metabolites to cells in the CNS, would be impossible if larger molecules could diffuse freely. Presumably the metabolic needs of ganglia are adequately met by exchange vessels similar to those of nonnervous tissues. Most of the CNS is protected from exogenous toxic substances that bind to plasma proteins. Peripheral neurons and glial cells are damaged by some such substances because of the lack of blood-tissue barriers.
内皮、脉络膜上皮和蛛网膜可阻止血浆蛋白进入哺乳动物中枢神经系统（CNS）的大部分区域。相比之下，神经根具有可渗透的毛细血管和可渗透的软膜 - 蛛网膜鞘。血浆蛋白向脑脊液的扩散可能是由于沿着从蛛网膜下腔到神经根静脉的压力梯度的缓慢总体流动而受到阻止。在神经中，神经束膜可防止蛋白质从神经外膜扩散到神经内膜。束内的毛细血管对大分子具有通透性，尽管其通透性低于神经根和神经节的微血管。神经内膜血管通透性在大鼠和小鼠中最低，但即使在这些物种中，白蛋白通常也存在于神经纤维周围的细胞外间隙中。所谓的血 - 神经屏障与血 - 脑屏障并不等同。感觉神经节和交感神经节中的毛细血管对大分子完全通透，外渗的蛋白质与神经元细胞体和神经突接触。每个神经节周围都有一层不可渗透的神经束膜，但当内部血管与外部血管一样具有通透性时，它并没有明显的作用。肠神经系统没有神经束膜，其无血管神经节和神经束中的神经元暴露于由肠道相邻组织中可渗透血管形成的细胞外液中。从神经系统某些部分排除大分子的原因尚不清楚。如果较大分子能够自由扩散，那么维持向CNS细胞持续供应离子、葡萄糖和其他代谢物的载体介导转运将是不可能的。据推测，神经节的代谢需求可通过与非神经组织类似的交换血管得到充分满足。CNS的大部分区域受到与血浆蛋白结合的外源性有毒物质的保护。由于缺乏血 - 组织屏障，外周神经元和神经胶质细胞会受到某些此类物质的损害。
