 neurotoxic substances affect the nervous system in a selective manner . one possible basis for this selectivity is blood vessel permeability . in general , the central nervous system and 
 the peripheral nerve trunks have impermeable blood vessels , but in certain parts the capillaries are " leaky , " allowing the passage of a plasma filtrate . 
 intravenously injected protein tracers rapidly reach nerve cells in these regions , with the implication that these nerve cells are also readily accessible to circulating neurotoxic substances . 
 some examples of neurotoxicity in the central nervous system show a selectivity that could be due to capillary permeability . in experimental methylmercury poisoning , 
 cranial nerve v and sensory dorsal root ganglia , which lie in regions of vascular permeability , are particularly susceptible . 
 a number of drug and chemically induced neuropathies are predominantly sensory , and may be due , directly or indirectly , to the accessibility of neurotoxic substances to sensory neurons . 
 examination of areas of potential vulnerability to circulating toxic substances may be of value in the experimental testing of substances for neurotoxicity , where pharmacological tests may be negative and clinical symptoms difficult to assess.imagesfigure 1 . 
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