1The idea in one paragraph

Every organism moves something, but animals do it fast and on a large scale because they have muscle. Muscle shortens because, inside each fibre, thin actin filaments are pulled past thick myosin filaments by myosin heads that bind, pull, let go and bind again, using ATP each time. A muscle can only pull, so muscles work in antagonistic pairs, each stretching the other, with the giant spring protein titin helping the stretched one recoil. Muscles are switched on in motor units by motor neurons. They pull on skeletons, which act as levers turning about joints, and the synovial joint at the hip shows every part that makes that smooth. Animals use the result to find food, escape, find mates and migrate, and a dolphin shows how a whole body can be reshaped around one way of moving.

2Movement is universal

All living things move in some way. What differs is what moves and how far. The guide asks you to compare a motile and a sessile species.

A motile organism can move its whole body from place to place. The common octopus (Octopus vulgaris) crawls across the sea floor on its arms and, to escape, shoots backwards by squeezing water out of its body cavity through a funnel: jet propulsion, driven by muscle.

A sessile organism is fixed in one place for its adult life, but it still moves. The acorn barnacle (Semibalanus balanoides) is cemented head-down to a rock. At high tide it opens the plates of its shell and sweeps feathery legs through the water to comb out plankton, then pulls them in and shuts the plates as the tide falls. Its larvae, before settling, swim freely. Plants are sessile too, but the sensitive plant Mimosa pudica folds its leaflets within seconds of being touched, and many plants turn their leaves towards light.

Movement also happens inside cells in every organism: chromosomes are pulled apart in mitosis, vesicles are carried along the cytoskeleton, and cilia and flagella beat. Muscle is the animal kingdom's way of doing the same thing at the scale of a whole body.

3The sliding filament model

A skeletal muscle is a bundle of muscle fibres, long cells with many nuclei. Each fibre is packed with myofibrils, and each myofibril is a chain of repeating units called sarcomeres. The sarcomere is the unit that contracts. Figure 1 shows one relaxed and one contracted.

A sarcomere runs from one Z line to the next.

• Actin filaments (thin) are anchored to the Z lines and point inward.
• Myosin filaments (thick) sit in the middle, overlapping the actin, held at the centre by the M line. Myosin has heads that stick out towards the actin.
• The A band is the length of the myosin filaments. The I band is the region with only actin, either side of a Z line. The H zone is the central region with only myosin.

What happens in contraction. The myosin heads pull the actin filaments towards the M line. The actin filaments from both ends slide inwards, the Z lines are drawn closer, and the sarcomere shortens. Thousands of sarcomeres shortening at once shorten the whole fibre.

Read Figure 1 to see what changes and what does not.

The filaments do not get shorter. They slide past each other, so the overlap increases and the sarcomere shortens.

The percentage shortening here is (2.4 − 2.0) ÷ 2.4 × 100 = 16.7%. The A band staying constant is the evidence for the model: if the filaments themselves shrank, the A band would shrink too.

How the pulling happens. Each myosin head works through a cycle, shown in Figure 2.

1. Attach. An energised myosin head binds to a binding site on actin, forming a cross-bridge.
2. Power stroke. The head swivels, pulling the actin filament a few nanometres towards the M line. ADP and phosphate are released.
3. Detach. A new ATP molecule binds to the head, which makes it let go of the actin.
4. Re-cock. The head hydrolyses the ATP to ADP and phosphate, and the energy released swings it back to its energised position, ready to bind further along.

Many heads cycle at different moments, like a team pulling a rope hand over hand, so the actin is never let go all at once. Two consequences are worth knowing. ATP is needed both to detach the head and to re-cock it; without ATP the heads stay locked on, which is why muscles stiffen after death. And the cycle only runs when calcium ions are present: a nerve impulse causes calcium to be released inside the fibre, and calcium moves the proteins that normally cover the binding sites on actin. When the calcium is pumped away, the sites are covered and the muscle stops contracting.

4Relaxation: titin and antagonistic muscles

Contraction is active. Relaxation, as far as the sarcomere is concerned, is just the myosin heads letting go. Nothing in the cross-bridge cycle pushes the filaments apart again. So how does a contracted muscle get back to its original length? Two ways.

Titin. Titin is an immense protein, the largest known, running from each Z line to the M line alongside the myosin, as in Figure 3.

Part of titin is coiled and behaves like a spring. When a sarcomere is stretched, titin extends and stores potential energy; when the stretching force stops, titin recoils and helps the sarcomere return to its resting length. Titin also resists stretching more and more strongly as it extends, which prevents overstretching: the sarcomere cannot be pulled so far that actin and myosin no longer overlap.

Antagonistic muscles. Muscle tissue can only exert force when it contracts. It can pull; it cannot push. So a muscle that has contracted must be stretched out again by something else, and that is usually a second muscle pulling the other way. Two muscles that move a joint in opposite directions are an antagonistic pair. Figure 4 shows the biceps and triceps at the elbow.

When the biceps contracts, the forearm is raised (flexion) and the triceps is stretched. When the triceps contracts, the forearm is lowered (extension) and the biceps is stretched. At any moment one muscle is contracting and the other is relaxed and being lengthened, its titin storing energy that helps it recoil. Antagonistic pairs also let you hold a joint still and control a movement precisely, by contracting both a little.

5Motor units

A muscle does not contract all at once or not at all. It is controlled in motor units, shown in Figure 5.

A motor unit is one motor neuron together with all the muscle fibres it stimulates. The motor neuron's cell body is in the spinal cord (or brain stem). Its long axon runs to the muscle and branches, and each branch ends on one muscle fibre at a neuromuscular junction, a synapse between neuron and muscle. When an impulse arrives, the neuron releases the neurotransmitter acetylcholine, which triggers an impulse in the muscle fibre and so contraction.

Three consequences follow.

• All the fibres in a unit contract together, because one neuron fires them all.
• Force is graded by recruitment. A light task activates a few motor units; a heavy one activates many more.
• Unit size matches the job. Muscles that make fine, precise movements, such as those that move the eye or the fingers, have small motor units with few fibres each. Big postural and leg muscles have large motor units with many fibres each, giving power but less precision.

6Skeletons: anchorage and levers

A contracting muscle needs something to pull on. The skeleton gives it two things.

Anchorage. Muscles are attached to the skeleton, so when they shorten they pull on something rigid rather than just bunching up. In vertebrates, muscles attach to bones through tendons.

Levers. A lever is a rigid bar that turns about a fixed point, the fulcrum. In the body, a bone is the bar, a joint is the fulcrum, the muscle supplies the effort, and the weight being moved is the load. Figure 6(a) shows the forearm.

The biceps attaches close to the elbow, so it moves only a short distance, but the hand at the end of the forearm moves a long way and fast. The price is force: the biceps must pull much harder than the weight in the hand. Many limb levers in the body trade force for speed and range of movement in this way.

Exoskeleton and endoskeleton. Arthropods (insects, crustaceans, spiders) have an exoskeleton, a hard outer case containing chitin, made of tube-like sections joined by flexible joints. Their muscles are inside the tubes and attach to the inner surface, as in Figure 6(b). Vertebrates have an endoskeleton of bone and cartilage inside the body, with the muscles on the outside of the bones. Both provide anchorage and levers; an exoskeleton also protects the soft tissue and reduces water loss, but it must be moulted for the animal to grow.

7The synovial joint at the hip

A synovial joint is a freely movable joint where the ends of two bones are separated by a fluid-filled cavity. The hip is the example the guide uses: the rounded head of the femur (thigh bone) fits into a cup-shaped socket in the pelvis. It is a ball-and-socket joint. Figure 7 shows its parts. You need not name any muscle or ligament, but you must name the femur and pelvis.

Learn the pair that is most often confused: ligaments join bone to bone; tendons join muscle to bone.

8Range of motion

The range of motion of a joint is how far it can move, measured as an angle, in each direction it can move. A ball-and-socket joint such as the hip moves in three planes: forwards and back (flexion and extension), out to the side and back in (abduction and adduction), and twisting (rotation). A hinge joint such as the knee moves mainly in one plane, flexion and extension.

Joint angles are measured with a goniometer, a protractor with two long arms: the pivot is placed over the centre of the joint, one arm is lined up with the fixed bone and the other with the moving bone. They can also be measured by photographing or filming the movement and measuring the angles with image-analysis software, which lets you measure at the exact moment of greatest movement. Figure 8 shows one student's results. The data are invented for practice.

Each bar is the mean of three readings. For hip flexion, for example:

The comparison is the point: the hip has a range in every one of the six directions, while the straight knee has almost none outside flexion and extension. That difference is the difference between a ball-and-socket and a hinge. When you evaluate such data, look for the variability of the repeats, whether the same person measured each time, whether the start position was controlled, and whether warming up changed the result.

9The intercostal muscles: an antagonistic pair inside the body

Antagonistic muscles do not only move limbs. The guide's example of internal body movement is the ribcage, which you met in B3.1. Between each pair of ribs there are two layers of intercostal muscle, and their fibres run at roughly right angles to each other, as Figure 9 shows.

• When the external intercostals contract, they pull the ribcage up and out: this is inspiration.
• When the internal intercostals contract, they pull the ribcage down and in: this is forced expiration.

Because the fibres are oriented differently, the two layers move the ribs in opposite directions. And because they are attached to the same ribs, when one layer contracts it stretches the other. The stretched muscle's sarcomeres lengthen and the titin in them extends, storing potential energy that helps the muscle recoil when the other layer relaxes. The intercostals are a neat case of every idea in section 4 working together.

10Why animals move from place to place

Locomotion is movement of the whole organism from one place to another. The guide wants four reasons, with an example of each.

Migration differs from the other three because it is a regular, seasonal movement between two regions, usually to exploit food or breeding conditions that are only available at certain times of year.

11Built for swimming: marine mammals

Whales and dolphins evolved from land mammals, and their bodies have been reshaped for life in water. The guide names four adaptations; Figure 10 shows them on a bottlenose dolphin (Tursiops truncatus).

• Streamlining. The body is a smooth, torpedo shape, widest near the front and tapering to the tail, with no hind limbs, no external ears and almost no hair. Water flows over it with little drag, so less energy is needed to swim.
• Limbs adapted to form flippers. The forelimbs are stiff, paddle-shaped flippers. Inside, the bones are the same as in a human arm, which is evidence of their land ancestry, but they are used for steering and balance, not for propulsion.
• A tail adapted to form a fluke, moved up and down. The fluke is horizontal, and powerful back muscles beat it up and down. This is the key contrast with fish, whose tail fin is vertical and moves side to side. It reflects the mammal's backbone, which flexes up and down as a running land mammal's does.
• Airways changed for periodic breathing. Dolphins breathe air but live under water, so they breathe in bursts between dives. The nostril has moved to the top of the head as a blowhole, so the animal can breathe with most of its body under water. A muscular flap closes the blowhole tightly during a dive. The airway is also separated from the mouth, so the dolphin can open its mouth to catch fish under water without water entering its lungs. Each breath exchanges a large proportion of the air in the lungs, so the few breaths between dives count.