Skeletal Muscle Fibers – Key Vocabulary

Sarcomere Structure & Length–Tension Relationship

• Skeletal muscle fiber ≡ skeletal muscle cell; functional contractile unit is the sarcomere (distance between two Z discs).
• Diagram details (verbal):
  • Black zig-zag lines = Z discs.
  • Thick filament: parallel arrays of myosin molecules; four myosin heads drawn per side (eight total) for illustration.
  • Thin filament: actin; instructor drew one short strand of F-actin for simplicity, but physiologically there are two intertwined strands that span the entire half-sarcomere.
• H zone = region between the innermost ends of two adjacent thin filaments (i.e., bare region of thick filament that lacks actin overlap).
  • During contraction the H zone narrows or disappears as thin filaments slide toward the M line.
• Optimal length–tension relationship:
  • Achieved when all possible cross-bridges form (all myosin heads have an actin binding site available).
  • Characterized by a “goldilocks” H-zone width: not too wide (over-stretched) and not absent (over-shortened).
  • Produces maximum force because every myosin head contributes.
• Deviation scenarios:
  1. Over-stretched fiber
  • H zone very wide; only the terminal myosin heads reach actin → few cross-bridges.
  • Potential for large shortening exists but cannot be realized because most heads cannot attach.
  1. Over-shortened fiber
  • Thin filaments overlap each other; H zone already zero.
  • No further shortening possible → force production depressed.

ATP Sources for Skeletal Muscle Contraction

• Four sequential/parallel energy systems (listed in approximate order of utilization):
  1. Residual ATP in the cytosol
  • Pre-synthesised; supports < 1 s of activity.
  1. Substrate-level phosphorylation (phosphagen system)
  • Primary reaction: \text{Creatine~phosphate} + \text{ADP} \rightarrow \text{Creatine} + \text{ATP}
  • “Borrowing/lending” a phosphate; fast, anaerobic.
  1. Glycolysis
  • First stage of “cellular respiration”; can operate with or without O_2.
  • Rapid ATP delivery; produces pyruvate.
    • Without O_2 → pyruvate reduced to lactate (lactic-acid fermentation) → contributes to fatigue.
  • When combined with 1 & 2, can fuel ~1 min of maximal effort.
  1. Oxidative phosphorylation (mitochondrial ATP production)
  • Requires O_2, mitochondria, electron-transport chain, ATP synthase, proton motive force, chemiosmosis.
  • Slow to ramp up but yields orders of magnitude more ATP than systems 1–3.

Mechanical vs Electrical Events: Twitch, Summation & Tetanus

• Twitch = one contraction–relaxation mechanical cycle of a single muscle fiber.
  • Initiated by an action potential (AP) generated by a somatic motor neuron.
  • Electromechanical sequence:
  1. Motor neuron releases ACh → binds nicotinic receptors.
  2. End-plate potential depolarises sarcolemma → AP propagates.
  3. DHP receptor conformational change → opens RyR (ryanodine) channels in sarcoplasmic reticulum (SR).
  4. Ca^{2+} floods cytosol → binds troponin → tropomyosin shifts → cross-bridge cycling begins.
  • Timing:
    • AP duration ≈ 1\text{–}2\,\text{ms}.
    • Twitch duration varies 7\text{–}70\,\text{ms} (fast- vs slow-twitch fibers).
    • Latent period: brief delay between AP onset and force generation.
• Summation (temporal summation)
  • Multiple APs fired before complete relaxation → SR re-opens or remains open → [Ca^{2+}] stays elevated.
  • Result: larger, merged mechanical responses; decreases time available for relaxation.
• Tetanus
  • Mechanical state of sustained maximal tension.
  • Incomplete (unfused) tetanus: slight relaxation between stimuli; force oscillates.
  • Complete (fused) tetanus: no relaxation; plateau of maximal force.
  • Key distinction:
    • Summation = description of the rapid electrical stimuli.
    • Tetanus = resulting mechanical phenomenon.

Motor Unit Recruitment

• Motor unit = single somatic motor neuron + all skeletal muscle fibers it innervates (could be 3–>1000s fibers).
• Recruitment = activating additional motor units to increase whole-muscle force output.
  • Light task (e.g., lifting an Expo® marker) → few small motor units engaged.
  • Heavy task (e.g., 30-lb dumbbell curl) → progressively larger/more motor units activated.
  • Provides coarse-to-fine control of force.

Mechanisms to Increase Muscle Force

1. Optimal length–tension relationship (cross-bridge availability + H-zone width).
2. Recruitment of additional motor units (spatial summation).
3. Summation → tetanus (temporal summation; maintains cytosolic Ca^{2+} to keep cross-bridges cycling).

Skeletal Muscle Fiber Types: Slow-Twitch vs Fast-Twitch

• Type I (Slow-twitch)
  • Twitch time ≈ \sim 70\,\text{ms}.
  • Reliance on oxidative phosphorylation.
  • High mitochondrial density, rich capillary supply, abundant myoglobin → dark meat coloration.
  • Smaller fiber diameter; typical of endurance athletes (e.g., distance runners).
• Type II (Fast-twitch)
  • Twitch time ≈ \sim 7\,\text{ms}.
  • Predominantly glycolytic; ATP produced anaerobically → rapid availability.
  • Larger diameter; high concentration of glycolytic enzymes; fewer mitochondria & capillaries → white meat coloration.
  • Contain faster ATPases:
    • Myosin ATPase (cross-bridge cycling) & Ca²⁺-ATPase (SR calcium pump) work at higher rates, enabling rapid contraction/relaxation.
• (Intermediate/Type IIa fibers exist but were omitted for brevity.)

Integrative/Real-World Connections & Implications

• Stretching beyond optimal sarcomere length before heavy lifts can decrease force output despite feeling “loose.”
• Phosphagen & glycolytic systems dominate early in high-intensity sports (sprints, Olympic lifts), whereas oxidative phosphorylation powers long-duration events (marathons).
• Muscle tetanus in pathology (e.g., tetanus toxin) arises from spinal inhibition failure, not from voluntary summation, but the mechanical outcome—prolonged contraction—is analogous.
• Training adaptations:
  • Resistance training ↑ fiber diameter (especially Type II) and can shift Type IIx → IIa phenotype.
  • Endurance training ↑ mitochondrial biogenesis & capillarisation in Type I fibers.
• Creatine supplementation targets the phosphagen system, enhancing substrate-level phosphorylation capacity (useful for repeated short bursts).