Although some things look similar superficially, scratch beneath the surface and there can be significant differences. Travis Tune and Simon Sponberg, from the Georgia Institute of Technology, USA, explain that two muscles in the cockroach leg – the dorsal and ventral femoral extensors – appear identical at first glance; their contraction characteristics are indistinguishable. Yet, the ventral muscle functions as a brake, absorbing energy as the insects scamper about, while the dorsal muscle works like a motor powering the insect's manoeuvres. ‘It is difficult to explain why these muscles’ contraction characteristics are so similar but have very different functions during running’, says Tune. Yet the duo realised that minute differences in the structure of the two muscles on the nanometer scale (0.000000001 m), could account for their radically different performances. To figure out what was going on, Tune and Sponberg needed to visit one of the world's largest X-ray microscopes, the Advanced Photon Source (APS), USA, to peek inside the muscles as they contracted.
But why did the duo have to use such a powerful microscope? It turns out that muscles have a lot in common with a grain of sugar. The sliding filaments packed together in a muscle are arranged a little like tightly packed candy sticks in a box, or sugar molecules in a grain of sugar. Shining X-rays on a grain of sugar tells you how the molecules are arranged within and how tightly they are packed, so the X-ray microscope could reveal whether there are any differences in how the filaments in the two muscles are arranged on this minute scale.
‘The APS facility is a kilometre-long apparatus that feels enormous’, says Sponberg, and Tune adds, ‘Working there can be stressful though, since beam time is limited and you need to work round the clock to collect enough data’. The duo X-rayed each muscle before re-creating the electrical nerve signals that stimulate the tissues to contract as if the cockroach was running. ‘We had to take X-ray images several hundred times a second’, says Sponberg. Then, the duo with Weikang Ma and Thomas Irving (Illinois Institute of Technology) analysed how the muscle filaments in the static muscles pack together, only to discover that the arrangements were essentially identical. The differences in the braking and driving muscles were not down to the way in which the fibres are arranged in the muscle. However, when they measured how tightly the filaments are packed, the fibres in the brake muscle were arranged more loosely (52 nm apart) compared with the motor muscle fibres, which were packed 51 nm apart. ‘A single nanometer in the myofilament lattice is the first structural difference detected in these otherwise identical muscles’, says Tune.
But, when Tune and Sponberg triggered the muscles to contract, the spacing between the motor muscle fibres increased slightly (1 nm), until it matched that of the more loosely packed brake muscles. Even this tiny (2%) change in the packing distance between muscle filaments could have a dramatic effect on how the actin and myosin filaments slide past each other to generate force. ‘The brake-like muscle is like a morning person’, says Sponberg; ‘as soon as it's activated, it's ready to go. However, the motor-like muscle has to stretch to get to the same lattice spacing. This expansion can shift both when and how much force it produces during a stride and, potentially, account for its difference from the braking muscle’, he concludes.