Sarcomere Banding (A, I, H Zones), Tropomyosin, Ca²⁺ Ions & Sliding Filament Theory
Skeletal muscle fibres are multinucleate cells (syncytia) packed with parallel myofibrils composed of repeating contractile units called sarcomeres. Under an electron microscope, each sarcomere displays alternating dark anisotropic A-bands (overlapping thick myosin and thin actin filaments) and light isotropic I-bands (thin actin filaments only), bounded by Z-lines. According to the sliding filament theory, neither actin nor myosin filaments shorten during contraction; instead, globular myosin heads form cross-bridges with actin and execute ATP-dependent power strokes that slide actin filaments inward toward the M-line. Examiners frequently test how changes in I-band, H-zone, and A-band widths prove the sliding filament mechanism, alongside the precise roles of Ca²⁺ ions, tropomyosin, ATP, and phosphocreatine.
- Ultrastructure of a Sarcomere (A-Band, I-Band, H-Zone, Z-Line)
- A sarcomere is the functional unit between two adjacent Z-lines: 1. I-band (light band): contains thin actin filaments only. 2. A-band (dark band): equals the entire length of the thick myosin filaments (including regions where actin overlaps myosin). 3. H-zone: the paler central region of the A-band containing thick myosin filaments only (no overlapping actin). 4. M-line: the attachment line down the centre of the H-zone/myosin filaments. During muscle contraction: the I-band becomes narrower, the H-zone becomes narrower, the Z-lines move closer together (sarcomere shortens), and the A-band stays the exact same width.
- Neuromuscular Junction & T-Tubule Depolarisation
- When an action potential arrives at a cholinergic neuromuscular junction, acetylcholine (ACh) is released and binds to receptors on the sarcolemma (muscle fibre membrane), triggering depolarisation. The action potential travels deep into the muscle fibre down invaginations called transverse tubules (T-tubules) that contact the sarcoplasmic reticulum (SR), causing voltage-gated Ca²⁺ channels in the SR membrane to open and Ca²⁺ ions to diffuse rapidly into the sarcoplasm.
- Role of Calcium Ions (Ca²⁺), Troponin & Tropomyosin
- In a relaxed muscle, the fibrous protein tropomyosin is wound around the actin filament, physically blocking the myosin-binding sites (actin-myosin binding sites) on the actin monomers. When Ca²⁺ ions diffuse from the sarcoplasmic reticulum into the sarcoplasm, Ca²⁺ binds to troponin (a globular protein attached to tropomyosin), causing troponin to change its 3D tertiary shape and pull tropomyosin aside, exposing the myosin-binding sites on the actin filament.
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Key Concepts & Exam Definitions (6)
- Ultrastructure of a Sarcomere (A-Band, I-Band, H-Zone, Z-Line)
- A sarcomere is the functional unit between two adjacent Z-lines: 1. I-band (light band): contains thin actin filaments only. 2. A-band (dark band): equals the entire length of the thick myosin filaments (including regions where actin overlaps myosin). 3. H-zone: the paler central region of the A-band containing thick myosin filaments only (no overlapping actin). 4. M-line: the attachment line down the centre of the H-zone/myosin filaments. During muscle contraction: the I-band becomes narrower, the H-zone becomes narrower, the Z-lines move closer together (sarcomere shortens), and the A-band stays the exact same width.
- Neuromuscular Junction & T-Tubule Depolarisation
- When an action potential arrives at a cholinergic neuromuscular junction, acetylcholine (ACh) is released and binds to receptors on the sarcolemma (muscle fibre membrane), triggering depolarisation. The action potential travels deep into the muscle fibre down invaginations called transverse tubules (T-tubules) that contact the sarcoplasmic reticulum (SR), causing voltage-gated Ca²⁺ channels in the SR membrane to open and Ca²⁺ ions to diffuse rapidly into the sarcoplasm.
- Role of Calcium Ions (Ca²⁺), Troponin & Tropomyosin
- In a relaxed muscle, the fibrous protein tropomyosin is wound around the actin filament, physically blocking the myosin-binding sites (actin-myosin binding sites) on the actin monomers. When Ca²⁺ ions diffuse from the sarcoplasmic reticulum into the sarcoplasm, Ca²⁺ binds to troponin (a globular protein attached to tropomyosin), causing troponin to change its 3D tertiary shape and pull tropomyosin aside, exposing the myosin-binding sites on the actin filament.
- Sliding filament power stroke & cross-bridge cycling
- 1. Once binding sites on actin are exposed, the myosin head (carrying ADP + Pᵢ) binds to the actin filament to form an actin-myosin cross-bridge. 2. Release of ADP and Pᵢ causes the myosin head to change angle (nod/bend by ~45° in the power stroke), pulling the actin filament along the myosin filament toward the centre of the sarcomere. 3. A new ATP molecule binds to the myosin head, causing it to detach from the actin binding site. 4. Ca²⁺ ions activate ATP hydrolase (ATPase) on the myosin head, which hydrolyses ATP → ADP + Pᵢ; the energy released returns (recocks) the myosin head to its original upright position ready to bind further along the actin filament.
- Muscle Relaxation & Rigor Mortis
- When nervous stimulation stops: 1. Ca²⁺ ions are actively transported back into the sarcoplasmic reticulum using energy from ATP hydrolysis. 2. Without bound Ca²⁺, troponin returns to its original shape and tropomyosin moves back to block the myosin-binding sites on actin. 3. No actin-myosin cross-bridges can form, so the filaments slide back passively (relaxation). After death (rigor mortis), Ca²⁺ leaks out of the SR (exposing binding sites so cross-bridges form), but respiration stops so no ATP is available to detach myosin heads from actin, leaving muscles locked stiff.
- Phosphocreatine & Slow-Twitch vs Fast-Twitch Muscle Fibres
- Phosphocreatine stored in muscle sarcoplasm rapidly regenerates ATP anaerobically in a single step by donating a phosphate group directly to ADP (ADP + Phosphocreatine → ATP + Creatine), powering intense bursts of contraction for ~5-10 seconds before being replenished aerobically at rest. Slow-twitch fibres (posture/endurance): contract slowly, fatigue-resistant, rich in myoglobin, dense capillaries, many mitochondria for aerobic respiration. Fast-twitch fibres (sprinting/weightlifting): contract rapidly and powerfully, fatigue quickly (lactate build-up), thick myosin filaments, high glycogen and phosphocreatine stores, high glycolytic enzyme activity for anaerobic respiration.
Quick-Fire Flashcards (6)
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What happens to the width of the A-band, the I-band, and the H-zone when a sarcomere contracts?
Explain the exact role of calcium ions (Ca²⁺) in initiating skeletal muscle contraction.
State the three specific roles of ATP in skeletal muscle contraction and relaxation.
Why does rigor mortis occur a few hours after death?
How does phosphocreatine provide energy for muscle contraction, and when is it replenished?
Give three structural differences between slow-twitch and fast-twitch muscle fibres.
Exam-Style Questions & Mark Schemes (4)
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- Q1
Describe the roles of calcium ions, tropomyosin, and ATP in the contraction of a myofibril after an action potential reaches the sarcoplasmic reticulum. (6 marks)
Click “Check Mark Scheme” to compare your answer, 1. Depolarisation of the T-tubules opens calcium ion channels in the sarcoplasmic reticulum, allowing Ca²⁺ ions to diffuse down their concentration gradient into the sarcoplasm [1 mark]. 2. Ca²⁺ ions bind to troponin molecules, causing them to change shape and pull attached tropomyosin molecules away from the actin-myosin binding sites on the actin filament [1 mark]. 3. Globular myosin heads (with ADP + Pᵢ attached) bind to the exposed binding sites on actin, forming actin-myosin cross-bridges [1 mark]. 4. The myosin heads release ADP and Pᵢ and bend (change angle in a power stroke), pulling the actin filaments along the myosin filaments toward the centre of the sarcomere [1 mark]. 5. A new ATP molecule binds to each myosin head, causing the myosin head to detach from the actin binding site [1 mark]. 6. Ca²⁺ ions activate ATP hydrolase on the myosin head to hydrolyse ATP into ADP + Pᵢ, releasing energy that returns the myosin head to its original upright position to repeat the cycle [1 mark].
- Q2
A scientist measured the lengths of different regions of a sarcomere before and during contraction. In the relaxed state, the sarcomere was 2.4 µm long, the A-band was 1.6 µm, and the H-zone was 0.6 µm. During contraction, the sarcomere shortened to 1.8 µm. Calculate the width of the I-band and the H-zone in the contracted sarcomere, and explain how these measurements support the sliding filament theory. (4 marks)
Click “Check Mark Scheme” to compare your answer, 1. Because the A-band length equals the fixed length of the myosin filaments, the A-band remains 1.6 µm in the contracted sarcomere [1 mark]. 2. Total I-band width in the contracted sarcomere = Sarcomere length - A-band length = 1.8 µm - 1.6 µm = 0.2 µm (or 0.1 µm on each side of the A-band, down from 0.8 µm total when relaxed) [1 mark]. 3. Since the sarcomere shortened by 2.4 - 1.8 = 0.6 µm, the actin filaments slid inward from both ends by 0.6 µm total, reducing the H-zone from 0.6 µm to 0.0 µm [1 mark]. 4. The constant width of the A-band combined with the narrowing of the I-band and H-zone proves that actin and myosin filaments do not shorten themselves, but instead overlap more by sliding past one another [1 mark].
- Q3
Compare and contrast the structure and function of a cholinergic neuromuscular junction and a cholinergic synapse between two neurones. (4 marks)
Click “Check Mark Scheme” to compare your answer, 1. Similarity: Both release the neurotransmitter acetylcholine (ACh) from presynaptic vesicles by exocytosis when Ca²⁺ ions diffuse into the knob, and both use acetylcholinesterase to hydrolyse ACh after binding [1 mark]. 2. Difference in target: A neuromuscular junction connects a motor neurone to a skeletal muscle fibre (sarcolemma), whereas a cholinergic synapse connects a presynaptic neurone to a postsynaptic neurone [1 mark]. 3. Difference in summation/potentials: Neuromuscular junctions are always excitatory and a single motor neurone impulse releases enough ACh to trigger a muscle action potential without needing summation, whereas neurone-neurone synapses can be excitatory or inhibitory and usually require spatial or temporal summation [1 mark]. 4. Difference in postsynaptic folds: The postsynaptic sarcolemma at a neuromuscular junction has deep clefts/folds containing high densities of receptors and acetylcholinesterase that lead into T-tubules [1 mark].
- Q4
Elite marathon runners have up to 80% slow-twitch muscle fibres in their calf muscles, whereas elite 100 m sprinters have up to 80% fast-twitch muscle fibres. Explain how the biochemical and structural properties of slow-twitch fibres adapt a marathon runner for endurance running. (4 marks)
Click “Check Mark Scheme” to compare your answer, 1. Slow-twitch fibres contain a very high density of mitochondria to carry out aerobic respiration (Krebs cycle and oxidative phosphorylation), producing a continuous, high yield of ATP over long periods [1 mark]. 2. They are surrounded by a dense network of blood capillaries with a short diffusion distance to supply high rates of oxygen and glucose and remove CO₂ [1 mark]. 3. They contain high concentrations of myoglobin, which has a very high affinity for oxygen and acts as an intracellular oxygen store in the muscle [1 mark]. 4. Because they respire aerobically rather than relying on anaerobic glycolysis, they produce very little lactate, preventing drop in pH and delaying muscle fatigue [1 mark].
Common Exam Questions (4)
If you cut a transverse cross-section through different zones of a myofibril, what pattern of filaments do you see?
A cross-section through the I-band shows thin dots only (actin filaments). A cross-section through the H-zone shows thick dots only (myosin filaments). A cross-section through the outer overlap region of the A-band shows both thick myosin dots surrounded by a hexagonal arrangement of thin actin dots.
Why do muscles work in antagonistic pairs across a synovial joint?
Sliding filaments can only actively pull actin toward the centre of the sarcomere (generating tension during contraction); myosin heads cannot push actin filaments apart. Therefore, a second antagonistic muscle (e.g. triceps vs biceps) must contract to pull the bone in the opposite direction and stretch the relaxed muscle back to its resting length.
How does lactate build-up in fast-twitch muscle fibres cause muscle fatigue?
Lactate dissociates to release H⁺ ions, lowering the pH of the sarcoplasm. The acidic pH alters ionic bonds in the tertiary structure of glycolytic enzymes and myosin ATPase (slowing ATP hydrolysis) and interferes with Ca²⁺ binding to troponin.
What is a motor unit and how does the nervous system control the force of contraction of a whole muscle?
A motor unit consists of a single motor neurone and all the individual muscle fibres it innervates. Because individual muscle fibres contract in an all-or-nothing manner, the nervous system increases the force of contraction by recruiting a greater number of motor units simultaneously.
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