Skeletal muscle and movementEdexcel International A Level Biology: Revision notes
Section 1
Skeleton, tendons and ligaments
The skeleton provides rigid levers and a framework to which muscles attach. Bones meet at joints, which act as pivots.
- Tendons connect muscle to bone. They are strong, inelastic collagen cords, so the force of a contraction is transmitted to the bone without being lost in stretching.
- Ligaments connect bone to bone. They are tough but slightly elastic, hold the joint together and limit excess movement.
When a muscle contracts it pulls on the bone through its tendon and the bone pivots about the joint.
Tendons join muscle to bone; ligaments join bone to bone. Examiners penalise swapping them.
Section 2
Antagonistic muscle pairs
Muscles can only pull when they contract; they cannot push. Movement in two directions therefore needs two muscles that work in opposite directions, called an antagonistic pair. One contracts while the other relaxes.
- Flexor: contracts to bend (flex) a joint, decreasing the angle, e.g. the biceps at the elbow, and the hamstrings at the knee.
- Extensor: contracts to straighten (extend) a joint, increasing the angle, e.g. the triceps at the elbow, and the quadriceps at the knee.
Example (elbow): to flex the elbow the biceps contracts and pulls on the radius while the triceps relaxes. To extend it the triceps contracts and pulls on the ulna while the biceps relaxes.
Say that one muscle contracts while the other relaxes, and name the bone being pulled.
Section 3
Structure of a skeletal muscle fibre
A skeletal muscle fibre is a long, cylindrical, multinucleate cell, formed by the fusion of many cells. Its parts are:
- Sarcolemma: the cell surface membrane. It folds inwards to form T-tubules, which carry the action potential deep into the fibre.
- Sarcoplasm: the cytoplasm, containing many mitochondria (ATP for contraction), glycogen and the oxygen-storing pigment myoglobin.
- Sarcoplasmic reticulum: a specialised endoplasmic reticulum that stores and releases Ca²⁺.
- Myofibrils: long parallel bundles of contractile protein filaments filling most of the fibre.
Section 4
Myofibrils and the sarcomere
Each myofibril is made of repeating units called sarcomeres, the contractile unit of muscle, running from one Z line to the next.
- Thin filaments: actin.
- Thick filaments: myosin.
The arrangement gives a banded pattern:
- A band (dark): the whole length of the myosin filaments, including where actin overlaps.
- I band (light): actin only.
- H zone: myosin only, in the centre of the A band.
- M line: the middle of the sarcomere, holding the myosin filaments together.
Section 5
Fast and slow twitch fibres
Skeletal muscle contains two main fibre types.
Slow twitch (type I), suited to endurance and posture, e.g. marathon running:
- many mitochondria, much myoglobin (so red) and a dense capillary supply
- aerobic respiration; contract slowly and resist fatigue
- relatively low glycogen and thinner myosin
Fast twitch (type II), suited to short, powerful bursts, e.g. sprinting:
- thicker myosin filaments and high ATPase activity, so contract rapidly and forcefully
- large glycogen and phosphocreatine stores; few mitochondria, little myoglobin (so pale)
- rely on anaerobic glycolysis, producing lactate, so they fatigue quickly
Link each structural feature to its function: more mitochondria and myoglobin mean aerobic ATP supply and resistance to fatigue.
Must know
- Tendons: muscle to bone; ligaments: bone to bone
- Muscles only pull, so they work in antagonistic pairs (flexor and extensor)
- Fibre: sarcolemma, T-tubules, sarcoplasm, sarcoplasmic reticulum, myofibrils, many nuclei and mitochondria
- Sarcomere: Z line to Z line, with actin and myosin; A, I and H bands
- Slow twitch: aerobic, fatigue-resistant; fast twitch: anaerobic, powerful, fatigue quickly
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Skeletal muscle and movement
- A physiotherapist is demonstrating how the human arm moves at the elbow joint. The biceps muscle on the front of the upper arm and the triceps muscle on the back of the upper arm both attach, by tendons, to the bones of the lower arm. The bones of the upper and lower arm are held together at the elbow by ligaments.Explain how the biceps and triceps interact with the bones of the arm to extend (straighten) the elbow.2 marks
- A student prepares a thin longitudinal section of mammalian skeletal muscle and examines a single muscle fibre with an electron microscope. The fibre is long and cylindrical, contains many nuclei, and is packed with parallel bundles of protein filaments that show a regular pattern of light and dark bands.The muscle fibre contains many mitochondria lying close to the myofibrils. Explain the advantage of this.2 marks
- A sports scientist takes small muscle samples (biopsies) from the thigh of two athletes, a marathon runner and a 100 m sprinter, and examines the fibres. Sample X: dark red fibres with a very high density of mitochondria, a high concentration of myoglobin and a dense network of capillaries; low stores of glycogen. Sample Y: pale fibres with few mitochondria, little myoglobin, a sparse capillary network, high stores of glycogen and phosphocreatine, thicker myosin filaments and a high ATPase activity.Deduce which sample came from the sprinter. Explain your answer using the data.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).