5.1Unit 5: Respiration, Internal Environment, Coordination and Gene Technology
Muscles & Movement — IAL Edexcel Modular Revision Notes & Topical Questions
5 min read 11 parts Lesson 2 of 7 1 quiz
Lesson summary
5.1 Muscles and Movement — IAL EDEXCEL MODULAR notes: Describe how muscles and the skeleton produce movement, the structure of muscle fibres and sarcomeres, fast and slow twitch fibres, and the sliding filament mechanism.Skeleton and muscle — effective movement needs muscles pull…
- Skeletal muscles
- Synovial joint
- Antagonistic pair
- Sarcoplasm contents
- Sarcoplasmic reticulum
- Thick filaments
Section 1 of 11The musculoskeletal system
The musculoskeletal system
Skeleton and muscle
- Skeleton and muscle effective movement needs muscles pulling on an incompressible skeleton,
- because muscles produce movement only by pulling on a structure that does not shorten or bend.
Skeletal muscles
- Skeletal muscles over 600 muscles are effectors stimulated by impulses from motor neurones.
Tendons
- Tendons strong, flexible connective tissue joining muscle to bone that does not stretch when the muscle contracts.
Ligaments
- Ligaments strong connective tissue joining bone to bone and keeping the skeleton intact.
Synovial joint
- Synovial joint cartilage reduces friction and absorbs shock, synovial fluid lubricates and nourishes cartilage, the synovial membrane secretes the fluid,
- and the capsule and ligaments give stability.
Section 2 of 11Antagonistic muscle action
- Antagonistic pair
- muscles can only pull, so they work in pairs pulling in opposite directions at a joint.
| Movement | Contracting muscle | Relaxing muscle |
|---|---|---|
| Raise the lower arm (flexion) | Biceps (flexor) | Triceps |
| Lower the lower arm (extension) | Triceps (extensor) | Biceps |
- Reset
- the contracting muscle stretches the relaxed muscle back to full length so it can contract again.
Section 3 of 11Structure of a muscle fibre
- Muscle fibre
- a specialised multinucleate cell-like unit surrounded by a cell surface membrane, containing contractile proteins in the cytoplasm.
| Muscle fibre term | Equivalent in a normal cell |
|---|---|
| Sarcolemma | Cell surface membrane |
| Sarcoplasm | Cytoplasm |
| Sarcoplasmic reticulum (SR) | Endoplasmic reticulum |
Structure of a muscle fibre
T-tubules
- T-tubules deep tube-like folds of the sarcolemma that run close to the SR and spread electrical impulses through the fibre.
Sarcoplasm contents
- Sarcoplasm contents mitochondria make ATP by aerobic respiration, and myofibrils are bundles of actin and myosin filaments.
Sarcoplasmic reticulum
- Sarcoplasmic reticulum membrane pumps transport calcium ions into its lumen, and calcium is needed for contraction.
Section 4 of 11Myofibrils and the sarcomere
Myofibril
- Myofibril it has thick filaments of myosin and thin filaments of actin in a regular arrangement that creates bands.
Thick filaments
- Thick filaments many myosin molecules lie side by side with globular heads pointing away from the M line.
Thin filaments
- Thin filaments two chains of globular actin twist together, with tropomyosin twisted round them and troponin attached at intervals.
Sarcomere
Sarcomere the unit between two Z discs, with sarcomeres end to end giving a striated appearance.
- The A band is dark and contains myosin.
- The I band is light and contains only actin.
- The H zone is the centre, with myosin only.
- The Z line anchors actin and the M line anchors myosin.
Contraction
- Contraction the Z discs move closer, so the sarcomere, I band and H zone shorten while the A band stays the same length.
- This shows the filaments slide and do not shorten.
Section 5 of 11Fast and slow twitch fibres
| Feature | Fast twitch | Slow twitch |
|---|---|---|
| Contraction | Rapid, myosin heads cycle about five times faster | Slower, sustained |
| ATP supply | Anaerobic respiration | Aerobic respiration |
| Fatigue | Quickly, because of lactate | Slowly, suited to endurance |
| Capillaries | Fewer | Denser network with short diffusion distance |
| Myoglobin and mitochondria | Low amounts, so paler | High amounts, so dark red |
| Calcium | Large amounts needed | Less |
| Examples | Cheetah legs, robin wings, human eyelids | Wolf legs, goose wings, human back muscles |
Myoglobin
Myoglobin a red pigment like haemoglobin that stores oxygen in muscle and speeds oxygen absorption from capillaries.
Fibre proportions
- Fibre proportions they are partly genetic and altered by training, with sprinters having more fast twitch and endurance athletes more slow twitch fibres.
Section 6 of 11Sliding filament mechanism
1
An action potential reaches the neuromuscular junction, where acetylcholine depolarises the sarcolemma.
2
The depolarisation spreads along the sarcolemma and down the T-tubules, and calcium ions are released from the sarcoplasmic reticulum.
3
Calcium ions bind to troponin, which changes shape and moves tropomyosin, exposing myosin-binding sites on actin.
4
Myosin heads bind to actin, forming cross-bridges.
5
The heads bend in the power stroke, pulling the actin filaments towards the centre of the sarcomere and releasing ADP and phosphate.
6
ATP binds to the myosin heads, which detach from actin.
7
ATPase hydrolyses ATP to ADP and phosphate, returning the heads to their original position (recovery stroke).
8
The heads bind to new sites closer to the Z disc and the cycle repeats while calcium and ATP are available.
Relaxation
- Relaxation when stimulation stops, calcium leaves troponin and is actively transported back to the sarcoplasmic reticulum using ATP.
- Troponin returns to its shape, tropomyosin blocks the binding sites again and actin slides back as the antagonist pulls.
Rigor mortis
- Rigor mortis after death there is no ATP to detach the myosin heads, so muscles stay contracted.
Section 7 of 11ATP supply in muscle
ATP supply in muscle
ATP hydrolysis
- ATP + H2O → ADP + Pi releases about 30.5 kJ mol−1 and is needed for detaching and re-cocking myosin heads and for calcium pumping.
Creatine phosphate
- Creatine phosphate it regenerates ATP quickly: creatine phosphate + ADP → creatine + ATP.
Aerobic and anaerobic respiration
- Aerobic and anaerobic respiration slow twitch fibres use aerobic respiration,
- and fast twitch fibres rely on anaerobic respiration that produces lactate.
Section 8 of 11More from the specification
1Electromyographic muscle tension assessment
- Electromyographic muscle tension assessment measuring motor unit action potentials provides an electrophysiological means to quantify musculoskeletal force production during contraction.
2Cardiac Output Equation
- Cardiac output is the total volume of blood pumped by a ventricle per minute, calculated using the equation Cardiac Output = Heart Rate * Stroke Volume.
3Cardiovascular Response to Exercise
- During physical exercise, sympathetic nerve stimulation triggers an increase in heart rate and ventricular contractility (stroke volume), elevating cardiac output to deliver oxygen to working muscles.
4Ventilation Changes in the Lungs
- Increased muscular activity elevates blood carbon dioxide levels, stimulating the medulla oblongata to increase breathing rate and tidal volume in the lungs.
5Venous Return and Stroke Volume
- Increased muscle contractions compress veins, augmenting venous return to the heart;
- according to Starling's law, greater ventricular stretching results in a higher stroke volume per contraction.
6Cardiac Output Equation
- Cardiac output is the total volume of blood pumped by a ventricle per minute, calculated using the equation Cardiac Output = Heart Rate * Stroke Volume.
7Cardiovascular Response to Exercise
- During physical exercise, sympathetic nerve stimulation triggers an increase in heart rate and ventricular contractility (stroke volume), elevating cardiac output to deliver oxygen to working muscles.
8Ventilation Changes in the Lungs
- Increased muscular activity elevates blood carbon dioxide levels, stimulating the medulla oblongata to increase breathing rate and tidal volume in the lungs.
9Venous Return and Stroke Volume
- Increased muscle contractions compress veins, augmenting venous return to the heart;
- according to Starling's law, greater ventricular stretching results in a higher stroke volume per contraction.
10Cardiac output
- Heart rate multiplied by stroke volume.
11Tidal volume
- The volume of air breathed in or out in one normal breath.
12Medulla oblongata
- The brain region controlling heart rate and breathing.
13Minute ventilation
- Tidal volume multiplied by breathing rate.
14Volume
- The amount of space something takes up.
15Breathing
- Moving air into and out of the lungs.
16Ventricle
- Lower chamber pumping blood out.
17Ventilation
- Movement of air into and out of the lungs.
18Spirometer
- Instrument that measures the volume of air breathed.
Section 9 of 11More on this topic
- Muscle contraction
- a nerve impulse releases calcium ions in the muscle fibre, and calcium exposes binding sites on actin.
- Myosin heads attach to actin and pull the filaments past each other.
- ATP supplies energy for myosin to detach and reset.
- Repeated cross-bridge cycles shorten sarcomeres while the filaments themselves stay the same length.
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Sarcomere | The unit of a myofibril between two Z discs. |
| Sarcolemma | The cell surface membrane of a muscle fibre. |
| Sarcoplasmic reticulum | The endoplasmic reticulum of a muscle fibre that stores calcium ions. |
| Troponin | A protein on actin that binds calcium and moves tropomyosin. |
| Tropomyosin | A protein on actin that blocks myosin binding sites at rest. |
| Antagonistic pair | Two muscles that pull in opposite directions at a joint. |
| Myoglobin | A red oxygen-storing pigment in muscle. |
| Power stroke | The bending of myosin heads that pulls actin filaments. |
Section 11 of 11Exam technique
1
Saying the filaments shorten in contraction — they slide, and only the sarcomere, I band and H zone shorten.
2
Saying muscles push as well as pull — they only pull, so antagonistic pairs are needed.
3
Forgetting that ATP is needed for relaxation — it detaches myosin and pumps calcium back into the sarcoplasmic reticulum.
4
Confusing tendons and ligaments — tendons join muscle to bone and ligaments join bone to bone.
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