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61.
Often we must balance being prepared to act quickly with being prepared to suddenly stop. The stop signal task (SST) is widely used to study inhibitory control, and provides a measure of the speed of the stop process that is robust to changes in subjects’ response strategy. Previous studies have shown that preparation affects inhibition. We used fMRI to separate activity that occurs after a brief (500 ms) warning stimulus (warning-phase) from activity that occurs during responses that follow (response-phase). Both of these phases could contribute to the preparedness to stop because they both precede stop signals. Warning stimuli activated posterior networks that signal the need for top-down control, whereas response phases engaged prefrontal and subcortical networks that implement top-down control. Regression analyses revealed that both of these phases affect inhibitory control in different ways. Warning-phase activity in the cerebellum and posterior cingulate predicted stop latency and accuracy, respectively. By contrast, response-phase activity in fronto-temporal areas and left striatum predicted go speed and stop accuracy, in pre-supplementary motor area affected stop accuracy, and in right striatum predicted stop latency and accuracy. The ability to separate hidden contributions to inhibitory control during warning-phases from those during response-phases can aid in the study of models of preparation and inhibitory control, and of disorders marked by poor top-down control. 相似文献
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In extant brachiopods, parental brooding of the larvae occurs exclusively within Rhynchonelliformea. Methods of larval protection range from simple retention of the larvae within the mantle cavity, to sophisticated brood care within highly specialized brood pouches found in Argyrotheca and Joania (Terebratulida, Megathyridoidea), Gwynia (Terebratulida, Gwynioidea), and all Thecideoidea (Thecideida). Previous studies on the reproductive biology of Argyrotheca yielded contrasting results on the epithelial origin of the brood pouches in this genus. Here, representatives of different species of Argyrotheca from the Belize Barrier Reef were examined using histological section series. Brood pouches of four species, A. cf. schrammi and Argyrotheca sp. 1–3, are of the same basic structure, formed by invaginations of the anterior body wall and connected to the visceral cavity via the metanephridia. The same four species are simultaneously hermaphroditic, suggesting that fertilization is achieved, at least partly, through selfing. One species, Argyrotheca rubrocostata, differs significantly from all others as it has no brood pouch and gonochoric gonads. Thus, the presence of brood pouches and simultaneous hermaphroditism are concluded to be correlated within Megathyridoidea and proposed to be homologous traits of Joania and several but not all species of Argyrotheca, questioning the monophyletic status of both genera. In contrast to the brood pouches of Thecideoidea, lophophoral epithelium is not involved in the formation of the pouches of Argyrotheca and Joania. Therefore, megathyridoid and thecideoid brood pouches are not homologous but evolved independently within rhynchonelliform brachiopods. All brachiopods with brood pouches share a micromorphic form and a short life span, limiting the space and time available for gamete and larval development. We suggest that the brood pouches and the hermaphroditic gonads of Argyrotheca spp. and Joania compensate these limitations by minimizing the loss of gametes and larvae, and by maximizing the chances of successful fertilization. J. Morphol., 2013. © 2013 Wiley Periodicals, Inc. 相似文献
65.
Myosin associated with the male germ cells of angiosperms interacts with actin, promoting transport of the non-motile generative
and later sperm cells in the pollen tube. Myosin localizing on the sperm cell plasma membrane seems negligible in Plumbago, as reflected by the absence of: (i) anti-myosin labeling using immunoelectron microscopy, (ii) sperm motility on actin matrices,
and (iii) electrophoretic movement changes after addition of antibody. Sperm cells injected directly into actively streaming
Nitella internodal cells, however, follow actin bundles and their movement is sensitive to ATP and Mg2+. This may be based on simple charge binding since negatively charged latex beads also migrate on actin, whereas neutral or
positively-charged latex beads do not. Sperm cells are negatively charged according to capillary microelectrophoresis, whereas
killed sperm cells, which are positively charged do not migrate. The sperm cell that normally fertilizes the egg has a higher
calculated charge (8.277 × 103 esu/cm2) compared with the sperm cell that fuses with the central cell (6.120 × 103 esu/cm2).
Received: 15 December 1998 / Accepted: 21 January 1999 相似文献
66.
J. L. V. Broers Barbie M. Machiels Helma J. H. Kuijpers Frank Smedts Ronald van den Kieboom Yves Raymond Frans C. S. Ramaekers 《Histochemistry and cell biology》1997,107(6):505-517
A selection of normal human tissues was investigated for the presence of lamins B1, B2, and A-type lamins, using a panel
of antibodies specific for the individual lamin subtypes. By use of immunoprecipitation and two-dimensional immunoblotting
techniques we demonstrated that these antibodies do not cross-react with other lamin subtypes and that a range of different
phosphorylation isoforms is recognized by each antibody. The lamin B2 antibodies appeared to decorate the nuclear lamina in
all tissues examined, except hepatocytes, in which very little lamin B2 expression was observed. In contrast to previous studies,
which suggested the ubiquitous expression of lamin B1 in mammalian tissues, we show that lamin B1 is not as universally distributed
throughout normal human tissues as was to be expected from previous studies. Muscle and connective tissues are negative, while
in epithelial cells lamin B1 seemed to be preferentially detected in proliferating cells. These results correspond well with
those obtained for lamin B1 in chicken tissues. The expression of A-type lamins is most prominent in well-differentiated epithelial
cells. Relatively undifferentiated and proliferating cells in epithelia showed a clearly reduced expression of A-type lamins.
Furthermore, most cells of neuroendocrine origin as well as most hematopoietic cells were negative for A-type lamin antibodies.
Accepted: 4 February 1997 相似文献
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68.
Josiah?P. Zayner Chloe Antoniou Alexander?R. French Ronald?J. Hause Jr. Tobin?R. Sosnick 《Biophysical journal》2013,105(4):1027-1036
To investigate the relationship between a protein’s sequence and its biophysical properties, we studied the effects of more than 100 mutations in Avena sativa light-oxygen-voltage domain 2, a model protein of the Per-Arnt-Sim family. The A. sativa light–oxygen–voltage domain 2 undergoes a photocycle with a conformational change involving the unfolding of the terminal helices. Whereas selection studies typically search for winners in a large population and fail to characterize many sites, we characterized the biophysical consequences of mutations throughout the protein using NMR, circular dichroism, and ultraviolet/visible spectroscopy. Despite our intention to introduce highly disruptive substitutions, most had modest or no effect on function, and many could even be considered to be more photoactive. Substitutions at evolutionarily conserved sites can have minimal effect, whereas those at nonconserved positions can have large effects, contrary to the view that the effects of mutations, especially at conserved positions, are predictable. Using predictive models, we found that the effects of mutations on biophysical function and allostery reflect a complex mixture of multiple characteristics including location, character, electrostatics, and chemistry. 相似文献
69.
70.
Mouse specific-locus test for the induction of heritable gene mutations by dibromochloropropane (DBCP) 总被引:1,自引:0,他引:1
The nematocide DBCP (1,2-dibromo-3-chloropropane) produced negative results in a specific-locus test for gene-mutation induction in the germline of male (101 X C3H)F1 mice, most of which were treated with 5 daily intraperitoneal injections of 80 mg/kg (total exposure, 400 mg/kg); a few received lower exposures. For treated spermatogonial stem cells, the finding of 2 mutations among 39519 offspring--a rate almost identical to the control rate--rules out (at the 5% significance level) an induced mutation frequency greater than 2.0 times the historical control rate. From treated poststem-cell stages, no mutants were found among 6240 offspring, ruling out (at the 5% significance level) a multiple of 8.0 times the control for these cell types. A multiple rearrangement (7 chromosomes involved in 3 translocations) found in one of the mutants probably arose as a postmeiotic event not associated with the DBCP treatment. The fertility of DBCP-treated males was not disturbed, in keeping with the absence of germ-cell toxicity and dominant lethals found by other investigators in these mice, and in contrast to results in certain other species. While the treated (101 X C3H)F1 mice are Ah-responsive, other findings make it questionable whether biotransformation of DBCP to reactive intermediates is accomplished via the Ah-receptor system. 相似文献