Skeletal Muscles as Effectors

AQA spec ref: 3.6.5 - Skeletal muscles as effectors

Skeletal (voluntary) muscles are the effectors of the somatic nervous system, converting the chemical energy of ATP into mechanical movement. Their mechanism of contraction - the sliding filament model - is one of the most detail-heavy topics in AQA Biology and requires precise knowledge of protein names, roles, and the sequence of events in the cross-bridge cycle.

Muscle Structure - Gross to Molecular

Understanding the sliding filament model requires knowing the hierarchical structure of muscle:

• A muscle is composed of bundles of muscle fibres (fascicles)
• A muscle fibre is a single, very large, multinucleated cell (formed by fusion of many myoblasts during development). It is surrounded by the sarcolemma (cell membrane) and contains specialised ER called the sarcoplasmic reticulum (which stores Ca²⁺)
• The cytoplasm is called sarcoplasm - rich in myofibrils, mitochondria, and glycogen
• A myofibril is a long cylindrical organelle running the length of the fibre, made of repeating units called sarcomeres
• The T-tubule system (transverse tubules) - invaginations of the sarcolemma that carry action potentials deep into the fibre, ensuring the signal reaches the sarcoplasmic reticulum

The Sarcomere

The sarcomere is the functional unit of muscle contraction. Under the electron microscope, the repeating pattern of sarcomeres gives skeletal muscle its striated (banded) appearance.

A sarcomere runs from one Z-line to the next. Within it:

• Thick filaments - made of myosin protein; extend across the A-band
• Thin filaments - made of actin (along with tropomyosin and troponin); extend from the Z-lines into the A-band

Banding Pattern

Key point: during contraction, the filaments do not shorten - actin and myosin slide past each other. Only the I-band and H-zone shorten; the A-band stays the same width (because myosin length doesn't change).

Proteins in Muscle

Myosin - thick filament protein. Each myosin molecule has a long tail and a globular head. The head:

• Has an actin-binding site - binds to actin during contraction
• Has an ATPase active site - hydrolyses ATP to ADP + Pᵢ, providing energy for the power stroke and for detachment from actin

Actin - thin filament made of two actin chains wound in a double helix. Tropomyosin is a fibrous protein that winds around the actin helix and, at rest, blocks the myosin-binding sites on actin. Troponin is a globular protein complex attached to tropomyosin and actin; it binds Ca²⁺.

The Sliding Filament Model - Cross-Bridge Cycle

Contraction occurs when myosin heads repeatedly attach to, pull, and release actin filaments - a cycle of cross-bridge formation and detachment powered by ATP.

Step-by-Step

1. Action potential arrives at the neuromuscular junction (see Neurons and Synapses). The motor neurone releases acetylcholine (ACh), which depolarises the sarcolemma.

1. Depolarisation spreads along the sarcolemma and down T-tubules to the sarcoplasmic reticulum.

1. Ca²⁺ released from the sarcoplasmic reticulum into the sarcoplasm. [Ca²⁺] rises.

1. Ca²⁺ binds to troponin on the thin filament. This causes a conformational change in troponin → tropomyosin shifts away from its blocking position, exposing the myosin-binding sites on actin.

1. Cross-bridge formation: myosin head (already "cocked" - in a high-energy position with ADP + Pᵢ bound from previous ATP hydrolysis) binds to the exposed actin site.

1. Power stroke: ADP + Pᵢ are released from the myosin head → the myosin head pivots, pulling the actin filament toward the centre of the sarcomere. The sarcomere shortens.

1. ATP binds to the myosin head → this causes the myosin head to detach from actin. (Without ATP, the head remains locked to actin - this is why muscles become rigid after death: rigor mortis.)

1. ATP is hydrolysed by ATPase in the myosin head → ADP + Pᵢ remain bound → the myosin head is re-cocked (returns to its high-energy position, pointing back toward the Z-line).

1. If Ca²⁺ is still present (nerve signal continues), the cycle repeats from step 5.

1. Relaxation: when the action potential stops, Ca²⁺ is actively pumped back into the sarcoplasmic reticulum (using ATP). [Ca²⁺] falls → Ca²⁺ dissociates from troponin → tropomyosin moves back to block myosin-binding sites → no new cross-bridges form → actin slides back (due to elastic recoil) → sarcomere lengthens.

Energy Supply to Muscles

The cross-bridge cycle requires ATP at two points:

• To drive the power stroke (ATP hydrolysis → ADP + Pᵢ)
• To detach the myosin head after the power stroke (ATP binding)
• To pump Ca²⁺ back into the sarcoplasmic reticulum during relaxation

ATP is replenished by:

1. Phosphocreatine (creatine phosphate) - stored in muscle; donates phosphate directly to ADP → ATP. Very fast, but limited supply (~10 seconds of maximal effort).

1. Anaerobic glycolysis - glucose → pyruvate → lactate. No O₂ needed; fast, but limited by lactate accumulation (which lowers pH and inhibits enzymes).

1. Aerobic respiration - glucose and fatty acids oxidised via the Krebs cycle and oxidative phosphorylation. Slow to ramp up but sustains prolonged activity. See Respiration.

Slow and Fast Twitch Muscle Fibres

Summary

• Sarcomere = functional unit of muscle; runs Z-line to Z-line
• Thick filaments = myosin; thin filaments = actin + tropomyosin + troponin
• On contraction: A-band unchanged; I-band and H-zone shorten; Z-lines move closer
• Cross-bridge cycle: Ca²⁺ → troponin → tropomyosin shifts → myosin binds actin → power stroke (ADP+Pᵢ released) → ATP binds → myosin detaches → ATP hydrolysed → head recocked
• ATP needed for: power stroke, detachment, Ca²⁺ pumping
• Relaxation: Ca²⁺ pumped back → tropomyosin re-blocks actin → sarcomere lengthens

AQA Exam Tips

• A-band width does NOT change - this is the most commonly tested fact about the sliding filament model. The A-band = length of myosin, which does not change. Only the I-band (actin only) and H-zone (myosin only) shorten.
• Rigor mortis: no ATP → myosin heads cannot detach from actin → muscles lock. AQA uses this to test understanding of ATP's role in detachment.
• Full cross-bridge cycle sequence: AQA often asks you to describe the cycle. The key sequence to remember is: Ca²⁺ binds troponin → tropomyosin moves → myosin binds actin → power stroke (ADP+Pᵢ released) → ATP binds → head detaches → ATP hydrolysed → head recocked.
• Tropomyosin blocks, troponin detects Ca²⁺: get these the right way round. Troponin is the Ca²⁺ receptor; tropomyosin is the physical blocker.
• Ca²⁺ source: sarcoplasmic reticulum (not the mitochondria, not the nucleus). Released when depolarisation arrives via T-tubules.
• Slow vs fast twitch: AQA may give data (mitochondria count, myoglobin level, fatigue rate) and ask you to classify the fibre type. High myoglobin + many mitochondria = slow twitch.