B3.3 Muscle and motilityIB Biology HL: Revision notes

Movement is a universal feature of living organisms, even if not every organism moves from place to place. Motile species (e.g. the humpback whale, Megaptera novaeangliae) move their whole body; sessile species (e.g. adult acorn barnacles, or plants) are fixed but still move parts: barnacles sweep their legs to feed, plants turn leaves towards light.

A sarcomere runs from one Z-line to the next. Thin actin filaments are attached to the Z-lines; thick myosin filaments sit in the middle and make up the A-band. The I-band is actin only; the H-zone is myosin only.

During contraction, myosin heads bind actin (forming cross-bridges), pivot (the power stroke) and pull actin towards the centre. ATP binds to detach each head, and its hydrolysis re-cocks the head. The filaments slide; they do not shorten. So the A-band stays the same length while the I-band and H-zone shorten and the Z-lines move closer.

Titin is an immense elastic protein linking myosin to the Z-lines. It stores potential energy when a sarcomere is stretched, helps it recoil, and prevents overstretching.

Muscle can exert force only when it contracts; it cannot push. So muscles work in antagonistic pairs: when one contracts, it stretches the other, which later contracts to reverse the movement.

A motor unit is one motor neuron, its branches, the neuromuscular junctions and all the muscle fibres it supplies. An impulse in the neuron makes all fibres of that unit contract together. Force is graded by recruiting more units. Small units (a few fibres) give fine control, as in eye muscles; large units (over a thousand fibres) give powerful but coarse control, as in the calf.

Skeletons provide anchorage for muscles and act as levers. Arthropods have an exoskeleton with muscles attached inside; vertebrates have an endoskeleton with muscles attached outside the bones by tendons.

The hip is a synovial ball-and-socket joint between the femur and the pelvis. Cartilage covers bone ends and reduces friction; synovial fluid lubricates; ligaments join bone to bone and stabilise; muscles provide force and tendons attach them to bone. Range of motion is measured in several directions (flexion, extension, abduction, adduction, rotation) with a goniometer or by computer analysis of images.

The external and internal intercostal muscles have fibres at different orientations, so they move the ribcage in opposite directions: external intercostals move ribs up and out (inhalation); internal intercostals move them down and in (forced exhalation). When one layer contracts it stretches the other, storing potential energy in titin, which helps the next movement.

Animals move to forage (e.g. whales swimming to krill), escape danger (e.g. a gazelle fleeing a cheetah), search for a mate (e.g. male moths following female pheromones) and migrate (e.g. humpback whales between polar and tropical waters).

Marine mammals are adapted for swimming by: streamlining; forelimbs forming flippers; the tail forming a fluke moved up and down; and airways changed for periodic breathing between dives, with the nostrils as a blowhole on top of the head.