• Skeletal muscle is composed of muscle fibres connected together by tendons.
• Muscle cells contain myofibrils which are made of units called sarcomeres.
• Myofibrils are composed of thick and thin filaments.
• Filaments slide pas each other to cause a muscle contraction. The cross-bridge attaches to the actin and ‘walks’ along it, a process known as the cross-bridge cycle.
• Muscles contractions are controlled by the troponin and tropomyosin in the thin filament.
• The cross-bridge attaches to the actin and ‘walks’ along it, a process known as the cross-bridge cycle.
• The contraction of skeletal muscles relies on nerve impulses and the whole process, from impulse to contraction, is known as excitation-contraction coupling.
• Muscle contractions use more ATP than another other process.
• Muscle cells are able to produce ATP through three systems: the aerobic system, the anaerobic or glycogen-lactate system, and the creatine phosphate system.
• When the muscles are resting or only moderately active they can produce ATP via aerobic respiration.
• When not enough ATP can be produced through aerobic respiration the muscles use anaerobic respiration of glycogen.
• When muscle contractions reach their maximum speed ATP is produced from creatine phosphate.
• Slow-twitch muscles use aerobic respiration which means it’s able to contract for long periods.
• Fast-twitch muscles use anaerobic respiration which means it’s only used during short activity bursts.

——————————————————

TEST IT!

1. Muscles move due to contractions.
   1. Describe the sliding filament theory.
   2. What two molecules found in the thin filament control muscle contractions?
      i. _____________________________________
      ii. _____________________________________
   3. How is force maintained in a contraction?
2. The contraction of skeletal muscles relies on nerve impulses.
   1. An action potential in the cell membrane has to be initiated first. How is it initiated?
   2. What causes the thin filament to change shape so that the myosin cross-bridges can attach to the actin?
   3. Muscle contractions require ATP. Provide two ways in which muscle cells can produce ATP.
      i. ______________________________________
      ii. ______________________________________

3. Depending on the system of respiration they use and how quickly they contract, muscles are classified as either slow-twitch or fast-twitch.

1. An athlete is about to run a sprint. Which type of muscles will he use?
2. Why do slow-twitch muscles contract more slowly than fast-twitch muscles?
3. Fast-twitch and slow-twitch muscles also differ in terms of colour. Explain why.

ANSWERS

1. 1. When a muscle contracts, the sarcomere shortens which means the whole muscle shortens. However, there’s no change in length of the thick filament. This means that instead of contracting, filaments slide past each other instead.
   2. i. troponin
      ii. tropomyosin
   3. Each cross-bridge is out of sync with all the others so at any one time there are always numerous cross-bridges attached.
2. 1. When the neurotransmitter acetykcholine is released from the motor neurone.
   2. Calcium ions enter into the myofibrils and bind themselves to the troponin located on the thin filament.
   3. Answers include:
      i. the aerobic system
      ii. the anaerobic or glycogen-lactate system
      iii. the creatine phosphate system
3. 1. Fast-twitch
   2. Because they rely on oxygen from the blood supply.
   3. Slow-twitch muscles contain myoglobin which makes them red while fast-twitch muscles don’t and aren’t white