Muscles and movement (7.10, 7.2, 7.1)
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Muscles are able to flex, causing joints to bend, this is done by flexor muscles.
Muscles are also able to extend, causing the joints to straighten, this is caused by extensor muscles.
Muscles can only pull the skeleton, so they work in antagonistic pairs.
Ligaments are elastic tissues that join bone to bone; they stabilise joints, controlling movement.
Tendons are tough connective tissues joining muscle to bone; they transmit force from muscle contraction to bones.
Muscle fibres contain myofibrils, contractile threads with repeating units (sarcomeres). These are protein-based organelles specialised for contraction. Myofibrils are laid down in parallel and combined, are very strong.
Myofibrils contain two types of protein filaments:
- Actin is a thinner filament, made up of two twisted strands. Actin has binding sites for myosin, which are blocked by tropomyosin and held in place by troponin.
- Myosin filaments are thicker, long, rod-shaped fibres with hinged projections to the sides. Each head has one binding site for actin and one for ATP.
When viewed under a microscope, skeletal muscle has bands:
- A light band made of actin only.
- A dark band made up of actin and myosin.
- A medium band made up of myosin only.
Skeletal muscle contraction occurs when actin and myosin filaments slide past one another, shortening the sarcomere. This is known as the sliding filament theory:
- A nerve impulse triggers release from the sarcoplasmic reticulum into the sarcoplasm.
- binds to troponin, causing tropomyosin to move off actin’s myosin binding sites.
- Myosin heads attach to exposed actin, forming cross-bridges.
- ADP and Pi on myosin is released.
- Myosin heads bend forward and actin slides over the myosin
- ATP binds to myosin heads and actin is released.
- ATPase on myosin head hydrolyses ATP to ADP + Pi.
- The myosin head returns to its original position.
When there are no more impulses, a muscle relaxes.
ions are actively pumped out of the sarcoplasm; this process uses ATP.
Troponin and tropomyosin go back to their original positions.
A skeletal muscle fibre is a single, elongated multinucleated cell. Each fibre is surrounded by a cell surface membrane called the sarcolemma.
The sarcoplasm (cytoplasm) contains many mitochondria (to supply ATP for contraction), an extensive sarcoplasmic reticulum (a specialised endoplasmic reticulum that stores and releases calcium ions), and bundles of myofibrils (the contractile units of the muscle).
Each myofibril is made up of repeating units called sarcomeres, containing the protein filaments actin (thin) and myosin (thick) that interact to produce contraction via the sliding filament mechanism.
Fast twitch (Type II) muscle fibres are adapted for short bursts of intense, powerful activity such as sprinting and weightlifting.
They have a large diameter, few mitochondria, a limited blood supply, low myoglobin content (appearing pale / white) and large glycogen stores.
They contract rapidly and with great force but fatigue quickly because they rely primarily on anaerobic respiration, which produces lactate.
Slow twitch (Type I) muscle fibres are adapted for sustained, endurance activity such as long–distance running and maintaining posture.
They have a small diameter, many mitochondria, a rich blood supply (from a dense capillary network) and high myoglobin content (appearing red).
They contract slowly but are highly resistant to fatigue because they rely primarily on aerobic respiration, which requires a steady oxygen supply.