The single feature that sets skeletal muscle apart from all other tissues is its phenomenal plasticity. No other tissue is capable of adapting to environmental demands or physical stimulation to the same extent. But skeletal muscle's main function is as the organ that drives locomotion. Hoppeler explains (p. 2143),`locomotion is essential for acquiring food and to escape predation' so it's not surprising that `skeletal muscle tissue functions in a species impact on the structural design of muscle tissue'. But are there any principles that unite skeletal muscle structures across the extreme scales of life? Hoppeler describes some trends that apply from woodmice to horses. For example, all the species that he analysed have the same relative quantities of contractile units in their muscle tissue. But Hoppeler is also intrigued by the molecular machinery that underpins muscle's versatility. He explains that the genes that control exercise-induced transcription include factors responsive to metabolic and mechanical events.
One of the world's tiniest mammals, the Etruscan shrew, is at the extreme of Hoppeler's scale. The tiny creature's large surface to volume ratio makes it difficult to maintain a constant body temperature, so it generates heat by shivering at incredibly high frequencies. When Klaus Jürgens began analysing the shrew's skeletal muscle, he realised that the animal had done away with slow twitch fibres in favour of fast twitch fibres(p. 2161), resulting in a unique form of muscle perfectly adapted for high speed performance and heat generation. By complete contrast, enormous birds have to generate a huge amount of muscular power as they lumber into the air. Graham Askew wondered how the blue breasted quail generates the power to lift it off the ground. Using a variety of techniques, he has found that the muscle has a high fibre density, rapid twitch contraction kinetics and a high maximum intrinsic velocity of shortening which allows the muscle to develop high stresses as it shortens very rapidly to get the bird airborne(p. 2153).
Some creature's diets have driven them to develop highly specialised muscles to hunt and consume their quarry. Chameleons have amazing ballistic tongues that they fire out of their mouths to catch passing insects. Once extended, the chameleon must drag its prey back, by retracting the tongue down to a fraction of its extended length. Skeletal muscle contracts when myosin filaments consume ATP, sliding the actin fibres along the length of the myosin filament until the actin fibres run into the Z disk structures. Tony Herrel wondered how the chameleon retractor muscle had got around this problem until he looked at electron micrographs of the muscle. Chameleon Z disks are perforated, which allows the actin fibres to pass through the Z disks so that the muscle continues contracting well beyond the limits of normal tissue(p. 2167).
The need to feed has also placed strong selective pressures on the muscles that clamp jaws shut. Different diets need different chewing techniques to rip, grind or cut food ready for digestion. Joe Hoh explains how masticatory myosin was one of the earliest isoforms of myosin to evolve and is found in today's fish and reptile jaw muscles(p. 2203). But as mastication became increasingly important for mammals, they adapted their jaw muscle structure to accommodate new diets, replacing the ancient masticatory myosin with other isoforms more commonly found in other forms of skeletal muscle.
During a contraction, skeletal muscle can store elastic energy for use during later contractions. Stan Lindstedt and his colleagues found that rat muscles became stiffer with eccentric training, in effect storing energy like a spring (p. 2211). Analysing the protein components of this stiffer muscle, Lindstedt believes that a rise in the cellular levels of the protein titin contributes to this increased stiffness.
Sound generating muscles having some of the fastest twitch velocities of any muscle must also conserve energy. Kevin Conley wondered how a single muscle that operates at almost 100 Hz generates the terrifying rattlesnake buzz. He was fascinated to find that the muscle has opted for the `short twitch/low force/short contraction length' approach to produce a structure that reduces its energy costs for a faster performance(p. 2175). Another species that makes a distinctive sound is the North American drummer fish, which produces its mating call with a single muscle(p. 2183). The muscle contracts and relaxes rapidly, forcing the swim bladder to vibrate and produce the fish's drumming call. The sound's pitch is set by the muscle's twitch duration. So the pitch is not an intrinsic property of the sound bladder, and the drumming muscle broadcasts the caller's fitness loud and clear.
Moving to the visual sense, Fred Schachat explains that even when we're staring at a fixed point, our eye muscles rotate the eye at speeds we don't even perceive, to protect the delicate retina from saturation. Using a phylogenetic approach, Schacht describes how the extraocular myosin heavy chain, which drives the muscle's superfast contraction, is probably the earliest example of a specialised group of superfast myosins(p. 2189). Surprisingly, the only other muscle where this unique form of myosin is found is laryngeal muscle, which also contracts very rapidly.
