Muscle and motility
🎯What you need to be able to do
- Explain why adaptations for movement are universal, comparing a motile and a sessile species.
- Explain the sliding filament model of contraction of a sarcomere.
- Explain the roles of titin and of antagonistic muscles in relaxation.
- Describe the structure and function of a motor unit.
- Compare exoskeletons and endoskeletons as anchorage for muscles and as levers.
- Describe movement at the hip joint, including bones, cartilage, synovial fluid, ligaments, muscles and tendons.
- Measure and compare the range of motion of a joint.
- Explain the antagonistic action of the internal and external intercostal muscles.
- Outline reasons for locomotion, and adaptations for swimming in marine mammals.
📚The biology
Movement is universal
All living organisms move in some way, although not all of them move from place to place.
Move from place to place (locomotion). A cheetah runs using skeletal muscles pulling on an endoskeleton; a jellyfish swims by contracting its bell; a bacterium swims with a flagellum.
Fixed in one place, yet they still move parts of themselves. A barnacle beats feathery legs to sweep food into its shell; a sea anemone moves its tentacles to catch prey; a plant turns its leaves towards light and a Venus flytrap snaps shut.
Even inside cells there is constant movement: cytoplasm streams, vesicles are carried along microtubules, and chromosomes are pulled apart in division.
The sliding filament model
Skeletal muscle is made of long muscle fibres, each packed with myofibrils. A myofibril is a chain of repeating contractile units called sarcomeres, joined end to end at Z lines. Each sarcomere contains two kinds of protein filament:
Attached to the Z lines at each end of the sarcomere and extend towards the centre.
In the centre of the sarcomere, overlapping the actin. Each has many protruding heads that can bind to actin.
The overlapping pattern gives light and dark bands. The dark A band is the length of the myosin filaments. The light I band contains actin only, on either side of a Z line. The H zone, in the centre of the A band, contains myosin only.
Contraction happens when the actin filaments slide over the myosin towards the centre of the sarcomere. Neither filament changes length:
- A nerve impulse causes calcium ions to be released inside the muscle fibre. Calcium causes the binding sites on actin, which are normally covered, to be exposed.
- Myosin heads bind to the exposed sites on actin, forming cross-bridges.
- Each myosin head swivels, pulling the actin filament towards the centre of the sarcomere (the power stroke).
- ATP binds to the myosin head, causing it to detach from actin. Hydrolysis of the ATP re-cocks the head, ready to bind further along the actin.
- The cycle repeats many times per second while calcium and ATP are available, with different heads attached at any moment, so the actin is pulled steadily inwards.
As a result the Z lines move closer together and the sarcomere shortens. Since thousands of sarcomeres lie end to end, the whole muscle shortens. During contraction the I band and H zone get shorter, but the A band stays the same length — the key evidence that filaments slide rather than shrink.
Relaxation: titin and antagonistic muscles
Muscle can only exert force when it contracts. It cannot push. Two things return a muscle to its original length after contraction:
- Titin is an immense, springy protein — the largest known — that runs from the Z line to the centre of the sarcomere, along the myosin filaments. When a sarcomere is stretched, titin acts like a spring, storing energy and helping the sarcomere recoil. It also prevents overstretching, which would pull the actin and myosin apart until they no longer overlapped.
- Antagonistic muscles work in pairs with opposite effects. When one contracts, it pulls the bone in one direction and stretches its relaxed partner. To reverse the movement, the partner contracts and the first is stretched. At the elbow, the biceps bends (flexes) the arm and the triceps straightens (extends) it.
Motor units
A motor unit consists of:
- a single motor neuron;
- all the muscle fibres it stimulates;
- the neuromuscular junctions connecting them — synapses at which the neuron releases acetylcholine, which triggers an action potential in the muscle fibre (C2.2).
When the motor neuron fires, all the fibres in its unit contract together. The strength of a whole muscle’s contraction is controlled by how many motor units are activated. Muscles used for fine control, such as those moving the eye or fingers, have small motor units with few fibres each; large postural muscles have units with thousands of fibres.
Skeletons: anchorage and levers
Skeletons give muscles something firm to pull against. Muscles are attached to the skeleton at both ends, across a joint, so that contraction moves one part relative to another. Bones then act as levers, with the joint as the pivot, changing the size or direction of the force or the distance moved.
A hard external skeleton, made of chitin. Muscles are attached to its inside surface. It also protects and prevents water loss, but must be moulted for the animal to grow.
An internal skeleton of bone and cartilage. Muscles are attached to its outside by tendons. It grows with the animal.
Movement at a synovial joint: the hip
The hip is a ball-and-socket joint between the femur (thigh bone) and the pelvis. The rounded head of the femur fits into a cup-shaped socket in the pelvis.
Femur and pelvis act as levers and give attachment for muscles.
Smooth, tough tissue covering the ends of the bones, reducing friction and absorbing shock.
Fills the joint cavity, lubricating the joint so the surfaces move smoothly, and nourishing the cartilage.
Encloses the joint and contains the synovial fluid.
Tough bands connecting bone to bone, holding the joint together and restricting movement to safe directions.
Muscles pull on bones to cause movement; tendons attach muscle to bone. Antagonistic muscle groups move the leg forwards, backwards, outwards, inwards and rotate it.
Range of motion
The range of motion of a joint is the extent of movement possible, measured as an angle. A ball-and-socket joint such as the hip allows movement in several dimensions — forwards and backwards (flexion and extension), sideways (abduction and adduction) and rotation — whereas a hinge joint such as the knee moves mainly in one plane.
Joint angles can be measured with a goniometer, a protractor with two arms aligned with the bones on either side of the joint, or by computer analysis of images, marking points on a photograph or video and using software to calculate the angle. Repeating measurements and comparing people, or the left and right sides, gives reliable comparisons.
The intercostal muscles: antagonism inside the body
The muscles between the ribs work as an antagonistic pair for ventilation (B3.1). The external and internal intercostal muscles have their fibres running in different directions, so they move the ribcage in opposite directions:
Ribs move up and out: inspiration. The internal intercostals are stretched.
Ribs move down and in: forced expiration. The external intercostals are stretched.
When one layer contracts and stretches the other, potential energy is stored in the stretched titin of the relaxed muscle’s sarcomeres, which helps it recoil when it next contracts.
Why animals move from place to place
e.g. honeybees flying from flower to flower to collect nectar and pollen.
e.g. a gazelle sprinting from a cheetah; a squid jetting away from a predator.
e.g. male moths flying long distances following the pheromone trail of a female.
e.g. arctic terns flying between Arctic and Antarctic each year; wildebeest following seasonal rains across the Serengeti.
Adaptations for swimming in marine mammals
Whales, dolphins and seals evolved from land mammals and have adapted to life in water:
- Streamlining — a smooth, torpedo-shaped body with no external ears, reducing drag in viscous water (A1.1).
- Flippers — forelimbs adapted into flat paddles for steering; the pentadactyl bone structure is still present inside (A4.1).
- A tail fluke — in whales and dolphins, the tail is broadened into a horizontal fluke that is moved up and down for propulsion, unlike the side-to-side tail of a fish.
- Changes to the airways — the nostrils have moved to the top of the head as a blowhole, which closes when diving, so the animal can breathe at the surface with little of its body exposed and make periodic breaths between long dives.
✏️Worked example
sarcomere length 2.5 µm
A band 1.6 µm
H zone 0.6 µm
sarcomere length 2.0 µm
A band 1.6 µm
(b) Predict the length of the H zone in the contracted sarcomere, and explain your prediction.
(c) Explain how these measurements support the sliding filament model.
(d) A muscle contains 20 000 sarcomeres end to end along its length. Calculate how far it shortens.
(a) The I band is the part of the sarcomere not occupied by the A band (half on each side of the Z line, so the total is simply the difference):
2.5 − 1.6 = 0.9 µm
2.0 − 1.6 = 0.4 µm
(b) The sarcomere shortened by 0.5 µm. The myosin filaments stay the same length and stay centred, so the actin filaments from both ends move 0.5 µm further into the A band in total, reducing the region with myosin only by the same amount: H zone = 0.6 − 0.5 = 0.1 µm.
(c) The A band, which is the length of the myosin filaments, did not change, so myosin did not shorten. The I band and H zone, which are regions where actin and myosin do not overlap, both got shorter, meaning the overlap increased. This is what would happen if the filaments slid past each other without changing length, as the sliding filament model proposes.
(d) Each sarcomere shortens by 0.5 µm:
📝Practise
Work through these on paper, then reveal the answer. All are HL only.
1. Outline the roles of calcium ions and ATP in muscle contraction.
2. Explain why muscles are arranged in antagonistic pairs.
3. State the components of a motor unit, and explain how the force of a muscle contraction can be varied.
4. Describe the roles of cartilage, synovial fluid, ligaments and tendons at the hip joint.
5. Explain how the internal and external intercostal muscles act antagonistically, including the role of titin.
6. Outline three adaptations of a dolphin for swimming.
🔗Go deeper — other people’s work
These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.
- HHMI BioInteractive — animation of the sliding filament model and the cross-bridge cycle.
- RCSB Protein Data Bank, Molecule of the Month — actin, myosin and titin.
- Kinovea — free video analysis software that measures joint angles from video, for the range-of-motion skill.