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16 | NewScientist | 29 June 2013 THE ability to urinate normally is one thing that paralysed people often say would improve their lives. New work offers some hope. When the spinal cord is severed, the brain can no longer control muscles below the break, which affects bladder control. Grafting nerve tissue onto the injured spinal cord so new neurons from the brainstem can grow over the gap could help to reverse this, but scar tissue gets in the way. Jerry Silver of Case Western Reserve University’s School of Medicine in Cleveland, Ohio, and colleagues performed such a graft on rats whose spinal cords had been completely severed, and also added a bacterial enzyme that breaks down scar tissue. Six months later, the rats still could not walk but they were able to pee unaided. The pressure in Hello, tetraquark: you’re real at last THE myriad forms matter can take just grew by one with the discovery of the fabled tetraquark. Quarks clump in threes to form protons or neutrons, which in turn clump to form atomic nuclei. Whether particles with more than three quarks exist was unclear, with glimpses of penta and tetraquarks proving inconclusive. Now the Belle accelerator in Tsukuba, Japan, and the BESIII accelerator in Beijing, China, have separately detected a particle – the unromantically named Z c (3900) – that appears to be the most convincing sighting of a tetraquark yet (Physical Review Letters, doi.org/m2m and doi.org/ m2k). It should give new insights into quantum chromodynamics, which describes how quarks interact. That could help predict the behaviour of matter under extreme conditions, such as inside neutron stars or in the early universe. Erosion helps keep mountains standing tall WHAT goes up must come down. So why are many mountain ranges, such as the Appalachians, still standing tall long after rivers should have eaten them down? Paradoxically, the erosive forces that should wear them away might instead carve them into a stable shape. Recent research has it that rivers only chew through bedrock if their waters carry lots of abrasive sand and grit. When mountains are being forced upwards by tectonic processes, their slopes are generally steep and liable to collapse in landslides (see picture). These provide rivers with abrasive particles, further destabilising the IGOR SHPILENOK/NATUREPL.COM IN BRIEF Nerve grafts let paralysed rats pee again their bladders indicated that they had regained two-thirds of their bladder function. The team found that nerves from the brainstem had regrown across the break and were transmitting signals to the muscles that control the bladder (Journal of Neuroscience, DOI: 10.1523/jneurosci.1116-12.2013). It seems some brainstem neurons can grow down the spinal cord if helped through scar tissue, says Silver, who thinks the same may happen in humans. mountainside and in turn causing more landslides. Now David Egholm at Aarhus University in Denmark and colleagues have modelled the fate of mountains after the tectonic uplift is over. They found that mountain slopes soon erode to become more gentle and less likely to collapse. There are fewer landslides on mountains of that shape, so rivers carry less abrasive sediment and lose their ability to cut through the rock. Erosion no longer wears down the mountain, instead it largely grinds to a halt (Nature, DOI: 10.1038/nature12218). Existing models suggest that a 4-kilometre-tall mountain range would lose half of its height within 20 million years. Under Egholm’s team’s scenario, it would take more than 200 million years, which is closer to the age of many mountain ranges. A PARTICLE proposed 50 years ago may be masquerading as a fraction of a neutrino. The Goldstone boson is supposed to be part of the Higgs mechanism, which gives mass to other particles. But no hints of it had been seen in nature. High- resolution maps of the early universe make the most sense if there are between three and four types of neutrino. We know of only three. The Goldstone could fill the gap, says Steven Weinberg of the University of Texas at Austin. It is its own antiparticle, so would have half the effect of a particle pair on the maps. And interacting less with matter as the universe expands means it could look like 0.39 of a neutrino (Physical Review Letters, doi.org/m2s). Is partial neutrino a boson in disguise?
