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781.
782.
When disease or injury causes partial loss of innervation from a muscle, the remaining axons sprout and form new connections to the denervated muscle fibres. Sprouting can occur in two ways: from axon terminals (terminal sprouting) or from the intramuscular axons themselves, probably from the nodes of Ranvier (collateral sprouting). Terminal sprouting has been induced experimentally using various methods, including partial denervation, nerve conduction block and nerve transmission block. A common factor in the induction of terminal sprouting seems to be changes in the surface membrane of muscle fibres; these changes and terminal sprouting are prevented by direct stimulation of the muscle. Collateral sprouting has been induced only by partial denervation and is not prevented by direct stimulation. This has been taken as evidence for an earlier suggestion that products of nerve or axon degeneration may be a direct stimulus for collateral sprouting. We report here that axon degeneration products alone are probably not the stimulus for collateral sprouting. 相似文献
783.
784.
A basic difficulty for the nervous system is integrating locally ambiguous sensory information to form accurate perceptions about the outside world. This local-to-global problem is also fundamental to motor control of the arm, because complex mechanical interactions between shoulder and elbow allow a particular amount of motion at one joint to arise from an infinite combination of shoulder and elbow torques. Here we show, in humans and rhesus monkeys, that a transcortical pathway through primary motor cortex (M1) resolves this ambiguity during fast feedback control. We demonstrate that single M1 neurons of behaving monkeys can integrate shoulder and elbow motion information into motor commands that appropriately counter the underlying torque within about 50 milliseconds of a mechanical perturbation. Moreover, we reveal a causal link between M1 processing and multi-joint integration in humans by showing that shoulder muscle responses occurring ~50 milliseconds after pure elbow displacement can be potentiated by transcranial magnetic stimulation. Taken together, our results show that transcortical processing through M1 permits feedback responses to express a level of sophistication that rivals voluntary control; this provides neurophysiological support for influential theories positing that voluntary movement is generated by the intelligent manipulation of sensory feedback. 相似文献
785.
The conserved Polycomb repressive complex 2 (PRC2) generates trimethylation of histone 3 lysine 27 (H3K27me3), a modification associated with stable epigenetic silencing. Much is known about PRC2-induced silencing but key questions remain concerning its nucleation and stability. Vernalization, the perception and memory of winter in plants, is a classic epigenetic process that, in Arabidopsis, involves PRC2-based silencing of the floral repressor FLC. The slow dynamics of vernalization, taking place over weeks in the cold, generate a level of stable silencing of FLC in the subsequent warm that depends quantitatively on the length of the prior cold. These features make vernalization an ideal experimental system to investigate both the maintenance of epigenetic states and the switching between them. Here, using mathematical modelling, chromatin immunoprecipitation and an FLC:GUS reporter assay, we show that the quantitative nature of vernalization is generated by H3K27me3-mediated FLC silencing in the warm in a subpopulation of cells whose number depends on the length of the prior cold. During the cold, H3K27me3 levels progressively increase at a tightly localized nucleation region within FLC. At the end of the cold, numerical simulations predict that such a nucleation region is capable of switching the bistable epigenetic state of an individual locus, with the probability of overall FLC coverage by silencing H3K27me3 marks depending on the length of cold exposure. Thus, the model predicts a bistable pattern of FLC gene expression in individual cells, a prediction we verify using the FLC:GUS reporter system. Our proposed switching mechanism, involving the local nucleation of an opposing histone modification, is likely to be widely relevant in epigenetic reprogramming. 相似文献
786.
Keane TM Goodstadt L Danecek P White MA Wong K Yalcin B Heger A Agam A Slater G Goodson M Furlotte NA Eskin E Nellåker C Whitley H Cleak J Janowitz D Hernandez-Pliego P Edwards A Belgard TG Oliver PL McIntyre RE Bhomra A Nicod J Gan X Yuan W van der Weyden L Steward CA Bala S Stalker J Mott R Durbin R Jackson IJ Czechanski A Guerra-Assunção JA Donahue LR Reinholdt LG Payseur BA Ponting CP Birney E Flint J Adams DJ 《Nature》2011,477(7364):289-294
787.
788.
Abbot P Abe J Alcock J Alizon S Alpedrinha JA Andersson M Andre JB van Baalen M Balloux F Balshine S Barton N Beukeboom LW Biernaskie JM Bilde T Borgia G Breed M Brown S Bshary R Buckling A Burley NT Burton-Chellew MN Cant MA Chapuisat M Charnov EL Clutton-Brock T Cockburn A Cole BJ Colegrave N Cosmides L Couzin ID Coyne JA Creel S Crespi B Curry RL Dall SR Day T Dickinson JL Dugatkin LA El Mouden C Emlen ST Evans J Ferriere R Field J Foitzik S Foster K Foster WA Fox CW Gadau J Gandon S 《Nature》2011,471(7339):E1-4; author reply E9-10
Arising from M. A. Nowak, C. E. Tarnita & E. O. Wilson 466, 1057-1062 (2010); Nowak et al. reply. Nowak et al. argue that inclusive fitness theory has been of little value in explaining the natural world, and that it has led to negligible progress in explaining the evolution of eusociality. However, we believe that their arguments are based upon a misunderstanding of evolutionary theory and a misrepresentation of the empirical literature. We will focus our comments on three general issues. 相似文献
789.
790.
Nugent PE Sullivan M Cenko SB Thomas RC Kasen D Howell DA Bersier D Bloom JS Kulkarni SR Kandrashoff MT Filippenko AV Silverman JM Marcy GW Howard AW Isaacson HT Maguire K Suzuki N Tarlton JE Pan YC Bildsten L Fulton BJ Parrent JT Sand D Podsiadlowski P Bianco FB Dilday B Graham ML Lyman J James P Kasliwal MM Law NM Quimby RM Hook IM Walker ES Mazzali P Pian E Ofek EO Gal-Yam A Poznanski D 《Nature》2011,480(7377):344-347
Type Ia supernovae have been used empirically as 'standard candles' to demonstrate the acceleration of the expansion of the Universe even though fundamental details, such as the nature of their progenitor systems and how the stars explode, remain a mystery. There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary body could be anything from a main-sequence star to a red giant, or even another white dwarf. This uncertainty stems from the fact that no recent type Ia supernova has been discovered close enough to Earth to detect the stars before explosion. Here we report early observations of supernova SN 2011fe in the galaxy M101 at a distance from Earth of 6.4 megaparsecs. We find that the exploding star was probably a carbon-oxygen white dwarf, and from the lack of an early shock we conclude that the companion was probably a main-sequence star. Early spectroscopy shows high-velocity oxygen that slows rapidly, on a timescale of hours, and extensive mixing of newly synthesized intermediate-mass elements in the outermost layers of the supernova. A companion paper uses pre-explosion images to rule out luminous red giants and most helium stars as companions to the progenitor. 相似文献