Muscles, skeleton and movementEdexcel A-Level Biology A: Revision notes
Section 1
Muscles, tendons, ligaments and the skeleton
Movement needs the skeleton, muscles and connective tissue to work together:
- Bones are rigid and act as levers.
- Skeletal muscles contract to pull on bones.
- Tendons are inelastic cords that join muscle to bone and transmit the force of contraction.
- Ligaments are slightly elastic and join bone to bone, stabilising the joint and limiting its movement.
- At a joint, cartilage covers the bone ends and synovial fluid reduces friction.
Do not mix up tendons and ligaments: tendons join muscle to bone, ligaments join bone to bone.
Section 2
Antagonistic muscle pairs
Muscles can only pull by contracting; they cannot push. Movement in two directions therefore needs a pair of antagonistic muscles. One contracts (the agonist) while the other relaxes.
At the elbow, the biceps is the flexor: it contracts to bend the arm. The triceps is the extensor: it contracts to straighten the arm. At the knee, the hamstring is the flexor and the quadriceps is the extensor.
Name which muscle contracts and which relaxes for each movement, and say why a pair is needed: muscles only pull.
Section 3
Structure of a skeletal muscle fibre
A muscle fibre is a long multinucleate cell, formed by fusion of many cells. Its cell surface membrane is the sarcolemma, which folds inwards as T-tubules so that action potentials spread quickly into the fibre. The cytoplasm (sarcoplasm) contains many mitochondria and the sarcoplasmic reticulum, which stores calcium ions.
The fibre is packed with myofibrils, made of repeating units called sarcomeres between Z lines. Thin actin filaments attach to the Z lines; thick myosin filaments lie in the centre. The I band has only actin, the A band is the length of the myosin, and the H zone has only myosin.
Link each feature to its function: many mitochondria for ATP, T-tubules for fast spread of the action potential, SR for calcium release.
Section 4
Slow twitch and fast twitch fibres
Skeletal muscle contains two main fibre types.
Slow twitch fibres: contract slowly and weakly, but resist fatigue. They have many mitochondria, much myoglobin (red colour), and a rich capillary supply, so they respire aerobically. They are used for posture and endurance.
Fast twitch fibres: contract quickly and powerfully but fatigue rapidly. They have thicker myosin filaments, high glycolytic enzyme activity, large stores of phosphocreatine and glycogen, but fewer mitochondria and little myoglobin (pale). They respire anaerobically and are used in sprinting and lifting.
Fast twitch fibres do not have more mitochondria. They have fewer, and rely on glycolysis and phosphocreatine for fast ATP supply.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Muscles, skeleton and movement
- A physiotherapist is assessing a patient who is recovering from an elbow injury. She asks the patient to bend the arm at the elbow and then straighten it again, and observes which muscles of the upper arm are working. She is also checking the tissues that hold the joint stable.Explain why skeletal muscles are arranged in antagonistic pairs.2 marks
- A sports scientist takes small samples of thigh muscle from a marathon runner and a sprinter. Sample X has many mitochondria, a high concentration of myoglobin and a dense network of capillaries. Sample Y has fewer mitochondria, thicker myosin filaments, large stores of phosphocreatine and glycogen, and a high activity of glycolytic enzymes.Explain why the fibres in sample Y can contract rapidly but fatigue quickly.2 marks
- Skeletal muscle fibres are long cells formed by the fusion of many cells, so each fibre contains many nuclei. The fibre is surrounded by a cell surface membrane called the sarcolemma and is packed with myofibrils. The sarcolemma folds inwards at intervals to form T-tubules, and a network of membrane called the sarcoplasmic reticulum lies around the myofibrils.Describe how a myofibril is organised into sarcomeres.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).