Sliding filament model

• A sarcomere is the functional unit of striated muscle.

• It contains actin (thin filaments) and myosin (thick filaments).

• During contraction, actin and myosin slide past each other — the filaments do not shorten.

• This causes the sarcomere to shorten and the whole muscle fibre to contract.

• Key exam point: contraction is due to sliding filaments, not “actin shrinking” or “myosin shrinking”.

• If describing sarcomere changes: Z lines move closer together, the I band and H zone become smaller, while the A band stays the same length.

This diagram shows the sliding filament theory, with actin, myosin heads, and cross-bridge formation during contraction. It is useful for explaining that filaments slide rather than shorten. Source

Muscle relaxation: titin and antagonistic muscles

• Titin is a very large protein in sarcomeres.

• It helps the sarcomere recoil after stretching and helps prevent overstretching.

• This stored elastic energy contributes to muscle relaxation after contraction.

• Because muscles only generate force by contracting, movement in opposite directions needs antagonistic muscles.

• In an antagonistic pair, one muscle contracts while the other relaxes/stretches.

• Example idea: flexor vs extensor muscles at a joint.

Motor units and skeletal muscle control

• A motor unit consists of a motor neuron and all the muscle fibres it innervates.

• Neuromuscular junctions connect the motor neuron to muscle fibres.

• When the motor neuron fires, the muscle fibres in that motor unit are stimulated to contract.

• Small motor units allow fine control; larger motor units allow powerful contractions.

• In exam responses, distinguish clearly between muscle fibre, motor neuron, motor unit, and neuromuscular junction.

This image shows a motor unit as a single motor neuron branching to multiple muscle fibres. It helps visualize how one neuron can coordinate contraction in several fibres at once. Source

Skeletons and levers

• Skeletons provide support, protection, and anchorage for muscles.

• They also act as levers, increasing the effectiveness of muscle force in producing movement.

• Arthropods have an exoskeleton.

• Vertebrates have an endoskeleton.

• When a muscle contracts, it pulls on a bone via a tendon, moving the bone around a joint.

• Always connect lever action to movement being produced at joints.

Movement at a synovial joint (human hip joint)

• A synovial joint allows a wide range of movement.

• The hip joint is a ball-and-socket joint between the head of the femur and the pelvis.

• Cartilage covers bone ends and reduces friction.

• Synovial fluid lubricates the joint and helps movement occur smoothly.

• Ligaments connect bone to bone and help stabilize the joint.

• Tendons connect muscle to bone.

• Muscles generate the force for movement by contracting.

• Know the named bones required here: femur and pelvis.

This diagram shows the basic anatomy of the hip joint, including the ball-and-socket arrangement and supporting structures. It is helpful for linking cartilage, ligaments, tendons, and synovial structures to movement and stability. Source

Range of motion

• The range of motion of a joint is how far it can move in one or more directions.

• Different joints allow movement in different planes and to different angles.

• Range of motion can be measured with a goniometer or by computer analysis of images.

• In practical work, compare movement in several dimensions where relevant.

• Exam skill: interpret or compare joint angle measurements.

Intercostal muscles as antagonistic muscles

• External intercostal muscles and internal intercostal muscles are an example of antagonistic muscle action.

• Their fibres run in different directions, so contraction moves the ribcage in opposite ways.

• External intercostals help move the ribs up and out during inhalation.

• Internal intercostals help move the ribs down and in during forced exhalation.

• When one set contracts, the other is stretched, storing potential energy in titin.

• This is a key example of antagonistic muscles producing internal body movement, not just limb movement.

This figure shows the diaphragm and intercostal muscles involved in ventilation. It supports the idea that different muscle fibre orientations produce opposite rib movements in breathing. Source

Reasons for locomotion

• Animals move to forage for food.

• They move to escape danger or avoid predators.

• They move to search for mates and increase reproductive success.

• They may migrate for breeding, feeding, or seasonal survival.

• Use at least one example if asked: e.g. predator avoidance, mate searching, or seasonal migration.

Adaptations for swimming in marine mammals

• Marine mammals show streamlining to reduce drag in water.

• Limbs are modified into flippers for steering and stability.

• The tail is modified into a fluke for propulsion.

• In cetaceans, the fluke moves up and down, unlike fish tails which usually move side to side.

• Airways are adapted for periodic breathing between dives.

• These adaptations are all linked to efficient movement in a dense, high-resistance medium.

This diagram labels the external anatomy of an orca, including structures relevant to swimming such as the flippers, dorsal fin, and fluke. It is useful for identifying how the body is adapted for streamlined aquatic locomotion. Source

Exam traps and high-yield distinctions

• Muscles contract by sliding filaments, not by filaments shortening.

• Muscles only pull, so opposite movement needs an antagonistic partner.

• Tendons connect muscle to bone; ligaments connect bone to bone.

• Motor unit = one motor neuron plus all the muscle fibres it controls.

• Exoskeleton = external support in arthropods; endoskeleton = internal support in vertebrates.

• Flippers are modified limbs; a fluke is the tail structure used for propulsion in marine mammals.