B3.3 Muscle and motility [HL only]

Muscle and motility connect sarcomere contraction, antagonistic muscles, skeletons, synovial joints and locomotor adaptations to movement across biological scales efficiently.

Movement changes position or shape; locomotion moves the whole organism from one place to another. A sessile organism remains attached yet may move body parts or grow directionally.

A side-by-side visual showing a motile animal moving its whole body and a sessile organism still showing local movement or growth toward a stimulus.

Locomotion can improve access to food, escape from predators, mate finding and migration. The benefit must outweigh its metabolic cost and exposure to risk.

A sarcomere runs from Z line to Z line. During contraction, myosin pulls thin actin filaments toward the centre, increasing overlap and bringing Z lines closer; neither filament shortens.

An electron micrograph is paired with an interpretive drawing labeling thick myosin filaments and thin actin filaments in a single sarcomere.

I band and H zone narrow → overlap increases
A band stays constant → myosin length is unchanged
Z lines approach → sarcomere shortens

Ca²⁺ permits cross-bridge formation, while ATP controls detachment and re-cocking. Repeated myosin power strokes slide actin toward the sarcomere centre.

Show ordered contraction states so the learner can sequence Contraction.

1 Ca²⁺ binds troponin; tropomyosin moves
2 energized myosin–ADP–Pi binds actin
3 Pi/ADP release drives the power stroke
4 ATP binding detaches myosin
5 ATP hydrolysis re-cocks the head

Skeletal muscles generate active force by shortening; they cannot actively push themselves longer. Titin and antagonistic muscle pairs provide two different routes back toward the starting state.

titin spans from thick filament toward the Z line, centres myosin and resists overstretching
stretched titin stores elastic energy and recoils as the sarcomere relaxes
an agonist contracts while its antagonist relaxes; reversing the movement swaps their roles

Titin restores sarcomere alignment and passive length; it does not replace the opposing muscle force needed to rotate a joint in the reverse direction.

A motor unit is one motor neuron and all muscle fibres it controls. At each neuromuscular junction, neuronal excitation is converted into sarcolemma excitation and then Ca²⁺ release from sarcoplasmic reticulum.

motor-neuron action potential → terminal Ca²⁺ entry → ACh exocytosis → receptor activation → muscle action potential → T-tubules → SR Ca²⁺ release → cross-bridge cycling

Muscles pull; they do not push. A tendon transmits tension to a rigid skeletal element, a joint acts as fulcrum, and an antagonistic muscle produces the reverse movement.

A diagram shows the biceps flexing the forearm as a lever, with the hand carrying the load, the muscle providing effort, and the elbow joint acting as the fulcrum, alongside a labeled lever schematic.

At the elbow: joint = fulcrum, biceps force = effort, forearm/hand load = resistance. Attachment close to the joint sacrifices force advantage for speed and range.

A synovial joint combines smooth cartilage, lubricating synovial fluid, stabilizing ligaments and force-transmitting tendons. Articulating surface shape constrains its directions of movement.

ball-and-socket hip/shoulder → movement in three planes, including circumduction
hinge knee/elbow → mainly flexion and extension
goniometer → angular change between starting and safe maximum positions

External and internal intercostals have opposing fibre directions. Their contraction moves ribs differently, changing thoracic volume and therefore lung pressure.

A front rib-cage diagram showing external intercostals raising the ribs up and out and internal intercostals pulling the ribs down and in.

external intercostals contract → ribs up/out → volume rises → pressure falls → inspiration
internal intercostals contract in forced expiration → ribs down/in → volume falls → pressure rises → air exits

Water is dense, so marine mammals combine a streamlined body with controlled thrust and rapid surface ventilation. They remain air-breathing mammals.

An annotated dolphin or whale showing a streamlined outline, flippers, horizontal tail flukes, reduced pelvic region, and blowhole at the top of the head.

Feature	Mechanical or respiratory effect
tapered body and reduced projections	reduces drag in dense water
forelimb flippers	steer and stabilize the body
horizontal tail flukes driven by axial muscles	generate up-down thrust
dorsal blowhole	permits rapid ventilation with little emergence

motor signal → ACh → muscle action potential → SR Ca²⁺ → exposed actin sites → cross-bridge cycling → sarcomere shortening → tendon tension → joint rotation

ATP detaches and re-cocks myosin; Ca²⁺ reuptake stops new bridges; titin restores passive sarcomere length, while antagonistic muscle action reverses joint movement. Whole-body form then determines whether force becomes efficient locomotion.

[ 2 ]

Microscopic eukaryotes include Euglena and Paramecium. Outline the range of cellular structures used for locomotion in these organisms.

[ 8 ]

Actin and myosin are two proteins found in muscles. Explain how skeletal muscle contracts, including the interaction of these proteins.

[ 2 ]

Explain the role of the protein titin in muscle relaxation.

[ 1 ]

What happens when an action potential reaches motor end plates?

[ 1 ]

Outline how the muscle attachment of insects differs from humans.

[ 2 ]

State the function of structures I and II.

I:

II:

[ 2 ]

Compare the movements of the hip joint and the knee joint.

Next Topic

—B4.1 Adaptation to environmentRead

B3.3 Muscle and motility [HL only]

Movement Does Not Always Mean Locomotion

Sliding Filaments Shorten the Sarcomere

Calcium Opens the Site; ATP Resets the Myosin Head

Titin Recoils; Antagonistic Muscles Reverse Movement

A Motor-Neuron Signal Releases Calcium around Myofibrils

A Skeleton Converts Muscle Tension into Rotation

Joint Shape Sets the Available Range of Motion

Antagonistic Intercostals Change Thoracic Volume

Marine Mammals Reduce Drag and Surface Efficiently

Summary: From Molecular Pull to Locomotion

Next Topic

Movement adaptations

Sliding filament model

Titin and antagonistic muscles

Motor units

Skeletons as anchorage and levers

Synovial joint movement

Range of motion