Slow and fast skeletal muscle fibres

Slow and fast skeletal muscle fibres

1. Slow-twitch muscle fibres. these are adapted to function over long periods. They respire aerobically to avoid the build-up of lactate which would quickly fatigue them. They have their own metabolic fuel but, because they are aerobic, they can almost use the almost limtiless supply of fat stores in the bdy. Their red colouration comes from having a high content of myoglobin and a good blood supply. this means they can obtain sufficient oxygen effieciently to generate large amounts of ATP. However, a disadvantage of aerobic respiration is that they cannot generate ATP at a very fast rate, thereofre slow-twitch fibres are not very powerful

2. Fast-twitch fibres, these are adapted for short bursts of explosive action. They generate ATP quickly and and anaerobically from a high-energy compounnd, phosphocreatine,a nd by lactate fermentation. When phosphocreatine breaks down it releases energy and phosphate groups which can be used to make ATP for up to 10 seconds of activity. Phosphocreatine is regenerated during aerobic respiration

3. Fast-twitch fibres appear white because they have a relatively low myoglobin content and a small number of mitrochondria, but htey are rich in the enzymes required for anaerobic respiration. However, during lactate fermentation, they can only use glycogen as a fuel, and the lactate and hydrogen ions they produce makes them fatigue quicker.

4. A single motor unit consists of only fast-twitch or slow-twitch fibres. A motor unit obeys the all-or-nothing principle; it either contracts completely or not at all. However, a whole muscle can produce graded responses by two mechanisms called temporal summation and muscle fibre recruitment

5. Temporal summation. This can be demonstrated by stimulation an isolated muscle electrically. If the isolating muscle is given a single electrical stimulus it will prodcue a simple twitch. If a second stimulus is given before the first twitch is over,the muscle tensions will add together to prodcue a greater response. If the rate of stimulation is fast enough, the twitches will fuse to produce a smoothed sustain contration caled tetanus

6. Muscle-fibre recruitment. The amount of tension produced in a muscle is altered by changing the number of motor units activated. When a situation requires more force, more muscle fibres are stimulated.