Skeletal muscles are stimulated to contract by nerves and act as
AQA
A-Level
Muscles can only pull. A muscle shortens when it contracts and generates force, but it cannot push itself back to its original length, so something else must stretch it, and that something is another muscle pulling the same bone the other way. That is an antagonistic pair: as one muscle, the agonist or prime mover, contracts, its partner, the antagonist, relaxes and is stretched. At the elbow the biceps contracts to flex the joint while the triceps relaxes; to extend it, the triceps contracts and the biceps relaxes. The bones between them are incompressible, so the pull is transmitted rather than absorbed, and the joint acts as a pivot so the bone works as a lever. Tendons, made of collagen and almost inelastic, attach muscle to bone so that contraction moves the bone rather than stretching the attachment. Without a rigid skeleton the force would simply deform the body.
18
Objectives
16
Exam Tips
27
Pitfalls
31
Key Terms
30
Mark Points
Subtopics in this area
Muscles act in antagonistic pairs against an incompressible skeleton.
Gross and microscopic structure of skeletal muscle. The ultrastructure of a myofibril.
The roles of actin, myosin, calcium ions and ATP in myofibril contraction.
The roles of calcium ions and tropomyosin in the cycle of actinomyosin bridge formation.
) The roles of ATP and phosphocreatine in muscle contraction.
The structure, location and general properties of slow and fast skeletal muscle fibres.
Skeletal muscles are stimulated to contract by nerves and act as Revision Guide
Learning Objectives
What you need to know and understand
- Explain why a muscle cannot return itself to its resting length and therefore needs an antagonist.
- Name the agonist and the antagonist for flexion and for extension at a named joint.
- Explain how an incompressible skeleton allows the force of contraction to produce movement.
- Label a sarcomere with the Z line, A band, I band and H zone and state which filaments lie in each region.
- Describe the structure of a muscle fibre, explaining why it is multinucleate and why it contains many mitochondria.
- Calculate the length of a sarcomere from a labelled electron micrograph and its magnification.
- Sequence the sliding filament mechanism from calcium ion release through to sarcomere shortening.
- State three distinct roles for ATP in muscle contraction and relaxation.
- Predict which regions of the sarcomere narrow during contraction and explain why the A band does not.
- Explain how tropomyosin prevents contraction in a resting muscle.
- Describe the effect of calcium ions on troponin and tropomyosin, and hence on actinomyosin bridge formation.
- Explain why a muscle relaxes when calcium ions are actively transported back into the sarcoplasmic reticulum.
- State the three roles of ATP in a contracting and relaxing muscle fibre.
- Explain how phosphocreatine regenerates ATP without oxygen.
- Distinguish between the immediate ATP store, the phosphocreatine reserve and aerobic respiration as energy sources for muscle contraction.
- Compare slow and fast skeletal muscle fibres in terms of structure, type of respiration and fatigue.
- Predict which fibre type predominates in a named muscle or athlete and justify the prediction using structural evidence.
- Explain how high myoglobin and mitochondria content allows a slow fibre to keep contracting for long periods.
Marking Points
Key points examiners look for in your answers
- one mark for stating that a muscle can only shorten and pull, and cannot push
- one mark for describing an antagonistic pair, with one muscle contracting while the other relaxes
- one mark for the contracting muscle stretching its relaxed partner back to its original length
- one mark for naming a correct pair acting at the same joint, such as the biceps and triceps at the elbow
- one mark for the skeleton being incompressible, so the force of contraction is transmitted and the bone acts as a lever
- one mark for describing a muscle fibre as multinucleate, containing sarcoplasm, many mitochondria and a sarcolemma
- one mark for the sarcoplasmic reticulum storing calcium ions
- one mark for identifying a sarcomere as the region between two Z lines
- one mark for actin as the thin filament and myosin as the thick filament
- one mark for correctly identifying the A band, I band and H zone on a diagram or electron micrograph
- Calcium ions diffuse out of the sarcoplasmic reticulum into the sarcoplasm, where they bind to troponin, causing tropomyosin to move and expose the myosin binding sites on actin.
- Myosin heads attach to the binding sites on the actin, forming actinomyosin bridges.
- The myosin head bends, pulling the actin filament towards the centre of the sarcomere (the power stroke), which is driven by the release of ADP and Pi.
- A new ATP molecule attaches to each myosin head, causing it to detach from the actin; hydrolysis of this ATP provides energy to recock the head.
- ATP is also used to actively transport calcium ions back into the sarcoplasmic reticulum, allowing the muscle to relax.
- Calcium ions diffuse from the sarcoplasmic reticulum into the sarcoplasm following an action potential.
- Calcium ions bind to troponin, causing it to change its tertiary structure.
- This structural change pulls tropomyosin away, exposing myosin binding sites on the actin filament.
- Myosin heads bind to the exposed sites on actin, forming actinomyosin cross-bridges.
- Muscle relaxation requires ATP to actively transport calcium ions back into the sarcoplasmic reticulum, allowing tropomyosin to block the sites again.
- ATP attaches to the myosin head to break the actinomyosin bridge, allowing detachment.
- ATP hydrolysis to ADP and Pi provides energy to recock the myosin head before the power stroke.
- ATP is used in the active transport of calcium ions back into the sarcoplasmic reticulum during relaxation.
- Phosphocreatine donates a phosphate group to ADP to form ATP rapidly and anaerobically.
- Phosphocreatine is a phosphate donor, not an energy source in its own right, and its use does not produce lactate.
- one mark for fast fibres being used during short-term or intense exercise, such as a sprint
- one mark for slow fibres being used during longer-term or lower-intensity exercise, such as a marathon
- one mark for slow fibres respiring aerobically, with many mitochondria, many capillaries and much myoglobin
- one mark for fast fibres respiring anaerobically, with stores of glycogen and phosphocreatine and few mitochondria
- one mark for linking lactate production in fast fibres to their rapid fatigue
Examiner Tips
Expert advice for maximising your marks
- 💡Answer in pairs: for every muscle you say contracts, name the one that relaxes.
- 💡Use the word antagonistic explicitly, since it is often the marking word.
- 💡If given an unfamiliar animal or joint, apply the same reasoning; the mark is for the logic, not the species.
- 💡Learn the bands by what they contain, and the changes during contraction then follow automatically.
- 💡On an electron micrograph find the Z lines first, then measure a sarcomere between them.
- 💡Many marks depend on the calcium-storing role of the sarcoplasmic reticulum, so never name it without that function.
- 💡Name all four players in every answer: actin, myosin, calcium ions and ATP. Which are required depends on the wording of the question.
- 💡Clearly distinguish the roles of ATP: binding causes detachment, while hydrolysis provides energy to recock the myosin head.
- 💡When asked for the roles of ATP, remember the third use, active transport of calcium ions back into the sarcoplasmic reticulum.
- 💡If a question asks about factors affecting the force of muscle contraction, link this to the number of actinomyosin cross-bridges formed, which depends on calcium ion availability.
- 💡Always specify the exact source and destination of calcium ions; state they are released from and actively pumped back into the sarcoplasmic reticulum, not just 'the cell'.
- 💡Answer roles of ATP questions with a clear list of the three distinct jobs.
- 💡When comparing energy sources, distinguish the immediate ATP store, the phosphocreatine reserve and aerobic respiration.
- 💡Pair every structural feature with the property it explains: many mitochondria, aerobic respiration, resistance to fatigue.
- 💡Fast twitch and slow twitch are both accepted, so use whichever you prefer, but stay consistent.
- 💡Where data on an athlete are given, quote the percentages when you justify which fibre type dominates.
Common Mistakes
Pitfalls to avoid in your exam answers
- saying the antagonist contracts in the opposite direction, when it relaxes
- stating that a relaxed muscle pushes the bone back into position
- claiming tendons are elastic and stretch during contraction, which would waste the force generated
- naming a pair of muscles that do not act on the same joint
- describing the skeleton only as support, without linking incompressibility to transmission of force
- calling a muscle fibre a muscle, or a myofibril a fibre, when the hierarchy is muscle, fibre, myofibril, sarcomere, filament
- placing actin in the middle of the sarcomere and myosin at the Z lines
- assuming the A band contains actin because both begin with the same letter, when the A band is the dark band containing myosin
- forgetting that the fibre is multinucleate, which follows from the fusion of many cells
- labelling the sarcoplasmic reticulum without mentioning that it stores calcium ions
- Calling the actin binding site an active site; use binding site.
- Saying the filaments themselves contract or shorten, when they slide past each other.
- Stating that ATP hydrolysis directly causes the power stroke. Correction: ATP hydrolysis recocks the myosin head, while the release of ADP and Pi drives the power stroke.
- Saying calcium ions are released into the myofibril. Correction: they are released into the sarcoplasm, the cytoplasm of the muscle fibre.
- Stating that calcium ions bind directly to tropomyosin. Correction: Calcium ions bind to troponin, which then undergoes a conformational change to move tropomyosin.
- Confusing the roles of calcium ions and ATP in the bridge cycle. Correction: Calcium ions expose the binding sites, whereas ATP binding is required to break the actinomyosin cross-bridges.
- Referring to the binding sites on actin as 'active sites'. Correction: Use the term 'binding sites', as 'active sites' strictly refers to enzymes.
- Forgetting that calcium ion reuptake is an active process. Correction: Always specify that calcium ions are actively transported back into the sarcoplasmic reticulum using ATP.
- Treating phosphocreatine as an energy source in its own right. Correction: it acts as a phosphate donor to regenerate ATP.
- Claiming phosphocreatine use produces lactate. Correction: it does not produce lactate.
- Stating ATP hydrolysis bends the head during the stroke. Correction: hydrolysis cocks the myosin head before the power stroke.
- Confusing the roles of ATP: binding causes detachment, hydrolysis recocks the head, and active transport returns calcium ions.
- saying slow fibres never fatigue rather than that they fatigue slowly
- claiming fast fibres contain more mitochondria because they release energy quickly
- describing myoglobin as transporting oxygen in the blood, when it stores oxygen in the muscle
- giving a location such as in the legs without naming the activity that justifies the fibre type
- treating a whole muscle as being made of only one fibre type