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Skeletal muscle's made up of overlapping striped muscle fibres, held together by connective
tissue with a tendon at each end. Tendons attach muscle to bone. When the fibres contract, the
muscle shortens, pulls the tendons and moves the bones.
Striped muscle fibre has cytoplasm, called sarcoplasm, surrounded by a plasma membrane
called sarcolemma. Striped muscle fibres have many nuclei in the cytoplasm, which lie near the
The central part of striped muscle fibre consists of mainly thin fibrelike structures called
myofibrils, which are composed mainly of protein molecules. These transfer chemical energy from
food into movement.
Each myofibril is made up of two protein filament types thick filaments composed of the
myosin protein and thin filaments composed of the actin protein.
When a myofibril contracts the sarcomeres (lengths of muscle fibres between Z discs) become
shorter, the light bands become shorter and the dark bands stay the same length.
Electron micrographs of the dark bands show crossbridges between the actin and myosin
filaments. These are part of myosin molecules and push on actin filaments to make the myofibril
shorten. This is known as the ratchet mechanism.
Each myosin crossbridge has a wider `head' that engages in a `binding site' on the actin
filament. The myosin crossbridge then performs its `power stroke', remaining stiff so the actin
filament moves. The myosin head then disengages and moves through a `recovery stroke' to
engage with the next binding site. The cycle repeats. think of it as an oar moving, then
returning it to its starting position. Muscle contraction is smooth due to crossbridges' spiral
arrangement, they are present at different positions in the cycle.
Tropomyosin, calcium ions & ATP:
Muscle contraction is turned on/off by a calcium switch. When a nerve impulse reaches a muscle
fibre, calcium channels open and calcium ions diffuse in. This allows actin and myosin to bind.
Crossbridges don't form in resting muscle because tropomyosin, another protein molecule,
blocks actin/myosin binding sites.
When calcium binds to tropomyosin, it alters the shape of the protein and it can no longer block
the binding site. Calcium ions also activate myosin molecules to break down ATP, releasing
energy needed for contraction.
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Calcium ions are stored in the ER of the cytoplasm that surrounds each myofibril. When the
muscle's resting, calcium ions are pumped into the ER by active transport.
Action potential travels along motor neurone.
Acetylcholine released at neuromuscular junction (synaptic cleft).
Action potential travels along muscle fibre membrane (sarcolemma).
Action potential passed into fibre via Ttubule. Ttubule leads to centre of muscle fibre.
Action potential makes ER permeable to calcium ions which rapidly diffuse out.…read more