Delay argument. The assumption is that, since perception of pain is delayed, a different set of neurons, with slower conduction times, is required. There is good evidence for a progressively increasing latency of action potentials in blocked nerves. Clark, Hughes, and Gasser (9) found slight slowing of conduction rates apparent within 15 minutes. Records of action potentials in the human ulnar nerve show progressive increase in latency after inflation of the pressure cuff (10). There was no discontinuity in the curve as would be expected if the composition of the group of active neurons had changed. Similar delays in perception occur in the other cutaneous senses also (6, 7). I would assume the perceptual delay to be due to the increasing latency of action potentials in neurons subjected to pathological conditions, and possibly also to synaptic delays occasioned by reduction in number of afferent impulses reaching the central nervous system.Reliability of ischemia-nerve block data. The assumption is that such data are reliable and give reliable indices of conduction times of fibers. It is clear that the results of such experiments on human subjects are variable. If a large number of subjects is used and if the results are treated statistically (6, 7), the order of loss is seldom significant. Landau and Bishop themselves (4) found procaine blocks to be "inconclusive" because prick and deep pain disappeared together-that is, because the sensory results did not bear out the results of action-potential studies. Whereas touch may usually fail before pain in compression of a limb, the difference is not sufficiently dramatic to enable one to distinguish between small delta fibers and C fibers. There is evidence that the survival time of fibers under compression block is influenced by factors other than conduction rates. Frankenhauser (11), who dealt with touch fibers of different types, found that slowly adapting touch receptors in the rabbit were blocked later than hair touch fibers in spite of the fact that their conduction rates completely overlap those of the latter. He concluded that the fibers themselves have properties which are not predictable from observation of the impulses. In man, skin touch and hair touch also have different survival times (12), and in some areas hair touch survives pain (6).There are some interesting results which suggest that the somatic sensory apparatus is much more complex than the current popular notions would have it. Between giving up all specificity, as Sinclair (13) does, and being bound to one or even two specific pain modali-Delay argument. The assumption is
Since 1996, commercial spinach cultivars with resistance to four previously described races of Peronospora farinosa f. sp. spinaciae (races 1, 2, 3, and 4) were observed with high incidences of downy mildew both in California and Europe. Isolates of P. farinosa f. sp. spinaciae collected in California between 1997 and 2001, Arizona in 1999, and a single isolate collected in the Netherlands in 1996 were examined for their disease reaction on differential spinach cultivars and a set of commercial spinach cultivars. Disease reactions on the differential cultivars indicated the occurrence of three new races of P. farinosa f. sp. spinaciae. Two newly identified races, designated race 5 (isolate CA1) and race 6 (isolate SP1), were detected in the United States. The isolate from the Netherlands also was distinct and designated race 7 (isolate JVN7). Some cultivars with resistance to races 1, 2, 3, and 4 were susceptible to race 5, whereas others were resistant, indicating that resistance to a given race may be governed by different genes (or alleles) depending on the source of resistance. A survey of races in California indicated that races 5 and 6 predominated. Although the majority of the cultivars examined were susceptible to race 6 based on the traditional qualitative cotyledon inoculation assay, significant quantitative differences in resistance to race 6 were observed using a true-leaf greenhouse screening procedure. Although more work is needed to confirm the results of the true-leaf assays, the quantitative resistance observed using this procedure appears to be race specific.
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