Sliding Filament Theory – 5-Phase Detailed Notes

Overview of the Sliding Filament Theory

• Total of five sequential phases (must be memorised):
  1. Resting Phase
  2. Excitation–Contraction Coupling Phase
  3. Contraction Phase
  4. Recharge Phase
  5. Relaxation Phase
• Purpose: Explains how skeletal muscle fibers generate tension and shorten via the interaction of the two primary myofilaments—actin (thin filament) and myosin (thick filament).
• Central requirements for continued cycling:
  • Sufficient Ca²⁺ to bind troponin.
  • Adequate ATP supply.
  • Functioning myosin ATPase to hydrolyse ATP.

Phase 1 – Resting Phase

• Motor neuron is silent → no action potentials arrive.
• Sarcolemma (muscle fiber cell membrane) unexcited; membrane potential at resting level.
• Actin–myosin interaction blocked:
  • Tropomyosin (rope-like regulatory protein) covers active binding sites on actin.
  • Troponin remains unbound to Ca²⁺, maintaining tropomyosin’s blocking position.
• No cross-bridge formation, no tension, muscle length unchanged.

Phase 2 – Excitation–Contraction Coupling

• Sequence of events linking neural stimulation to mechanical contraction.
• Steps and structures:
  • Nerve action potential travels along the α-motor neuron.
  • Reaches neuromuscular junction, depolarises the sarcolemma.
  • Depolarisation propagates internally via the T-tubules (continuations of sarcolemma).
  • T-tubules ensure that the interior of the fiber experiences the same electrical change almost simultaneously with the surface.
  • Electrical signal triggers the sarcoplasmic reticulum (SR) to release stored Ca²⁺ into the myofibril.
• Molecular consequences of Ca²⁺ release:
  • Ca²⁺ binds to troponin-C subunit of the troponin complex.
  • Binding causes a conformational shift → tropomyosin slides away from the active binding sites on actin.
  • Exposed sites now accessible to myosin globular heads (cross-bridges), permitting physical attachment.

Phase 3 – Contraction Phase

• Begins once myosin heads attach to actin.
• Cross-bridge cycling:
  1. Attachment – Energised myosin head (with ADP + P_i bound) binds to actin.
  2. Power stroke – Release of P_i → head pivots, pulling actin toward sarcomere centre; ADP then released.
  3. Detachment – New ATP molecule binds to myosin, causing it to detach from actin.
  4. Re-cocking – Myosin ATPase hydrolyses ATP → ADP + P_i + energy, re-energising the head for another cycle.
• Chemical equation for ATP hydrolysis:
  $(ATP \; + \; H<em>2O \; \xrightarrow{myosin\,ATPase} \; ADP \; + \; P</em>i \; + \; \text{energy})$
• Each single cross-bridge power stroke shortens the muscle fiber by ~1 % of resting length; therefore thousands of cycles across millions of sarcomeres are required for meaningful movement.

Phase 4 – Recharge Phase

• Not elaborated in depth in the transcript, but implied as the continuation/repetition of cross-bridge cycling as long as the three prerequisites persist:
  • Ca²⁺ remains bound to troponin (active sites still exposed).
  • ATP is continually resynthesised and available.
  • Myosin ATPase keeps catalysing ATP hydrolysis.
• Essentially a sustained looping of the contraction phase steps producing maintained or increasing tension.

Phase 5 – Relaxation Phase

• Triggered when the motor neuron stops firing (no more action potentials).
• Consequences:
  • Ca²⁺ re-uptake: Active pumps in the sarcoplasmic reticulum transport Ca²⁺ back into storage.
  • Ca²⁺ dissociates from troponin → tropomyosin re-covers the actin binding sites.
  • Cross-bridge formation ceases; without new attachments the elastic components restore the fiber to resting length.
• Net result: Muscle tension falls to zero → muscle relaxation.

Supporting Structures & Definitions

• Sarcolemma: Lipid-protein bilayer cell membrane of skeletal muscle; semi-permeable barrier regulating exchange of ions & nutrients.
• T-Tubules (Transverse tubules): Invaginations of sarcolemma that conduct action potentials deep into the fiber.
• Sarcoplasmic Reticulum (SR): Specialized smooth ER storing Ca²⁺; releases Ca²⁺ on stimulation and reabsorbs it during relaxation.
• Actin Filament: Thin filament containing binding sites; decorated with troponin and tropomyosin regulatory proteins.
• Myosin Filament: Thick filament; possesses ATPase activity in its heads and forms cross-bridges with actin.

Energy & Biochemistry Highlights

• ATP (adenosine triphosphate): Immediate energy currency for both cross-bridge power stroke and detachment.
• Myosin ATPase: Enzyme located on myosin head; catalyses ATP → ADP + P_i + energy.
• Repetition requirement: Continuous ATP turnover; if ATP or Ca²⁺ become limited the cycle halts.

Numerical / Statistical Reference

• Single power stroke → ≈ 1 % shortening of fiber’s resting length.

Practical / Clinical Relevance

• Understanding these phases underpins treatment strategies for muscular disorders, fatigue management, and athletic training protocols.
• Disruption in any component (e.g.
  • impaired Ca²⁺ handling in malignant hyperthermia,
  • ATP depletion in rigor mortis) directly maps to one of the phases above.

Key Take-Away Checklist (Exam Quick-Glance)

• Memorise the five phases in order.
• Know the role of Ca²⁺ and where it is stored.
• Recall the two distinct roles of ATP (power stroke energy & detachment).
• Understand the structural triad: Sarcolemma ↔ T-tubules ↔ SR.
• Recognise that measurable movement requires repetition of cross-bridge cycling across many sarcomeres.