Sliding Filament Mechanism & Neuromuscular Junction flashcards

Structural Components of a Sarcomere

  • Thick filament
    • Built from myosin molecules
    • Each myosin has a head (cross-bridge) and a tail that anchors it to the M line
  • Thin filament
    • Built from actin (forms the helical backbone)
    • Regulatory proteins
    • Tropomyosin: long, rope-like protein that lies in the actin groove and hides myosin-binding sites when the fiber is relaxed
    • Troponin: globular complex attached to tropomyosin; contains the Ca²⁺-binding site
  • Supporting structures
    • Z disc: anchors thin filaments and marks sarcomere boundaries
    • M line: anchors thick filaments
    • Zone of overlap: region where thick and thin filaments interdigitate and cross-bridges can form

Sliding Filament Mechanism: Step-by-Step

  • Preconditions
    • Ca2+\text{Ca}^{2+} must be present in the cytosol
    • ATP must be available and hydrolyzable
  • Molecular sequence
    1. Ca²⁺ binds troponin ➜ troponin changes conformation ➜ tropomyosin moves off the actin binding sites
    2. ATP hydrolysis on myosin
    • ATP→ADP+Pi+Energy\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}
    • Energy “cocks” the myosin head (pivot toward the actin filament)
    1. Cross-bridge formation: energized myosin head attaches to exposed binding site on actin
    2. Power stroke: P_i then ADP are released ➜ head pivots, pulling thin filament toward the M line ➜ sarcomere shortens
    3. Cross-bridge detachment: a new ATP binds the myosin head ➜ head releases from actin
    4. Reset: ATP is hydrolyzed again → head re-cocks; as long as Ca²⁺+ATP remain, cycle repeats ("grab-pull-release-reset")

Requirements for Contraction

  • Calcium: opens the actin binding sites (regulatory)
  • ATP: two roles
    • Powers the cocking of the myosin head (chemical→mechanical energy)
    • Causes detachment of the myosin head (without ATP, rigor results)

Source & Regulation of Intracellular Ca²⁺

  • Sarcoplasmic reticulum (SR)
    • Terminal cisternae store Ca²⁺
    • Ca²⁺ release channels open in response to depolarization events transmitted via transverse (T) tubules
  • T tubules
    • Invaginations of the sarcolemma that carry the action potential deep into the fiber
  • Trigger for SR release
    • Depolarization along T tubules (carried by Na⁺ influx) mechanically/electrically opens SR Ca²⁺ channels

Sarcomere Length vs. Force Production

  • Force varies with initial sarcomere length
    • Under-stretched (already partially shortened): limited further shortening ➜ reduced force
    • Optimal length: maximal overlap of thick and thin filaments without interference ➜ greatest cross-bridge number ➜ maximal force
    • Over-stretched: minimal overlap ➜ few cross-bridges ➜ weak force

Neuromuscular Junction (NMJ): Anatomy

  • Axon terminal (synaptic end bulb) of a somatic motor neuron
  • Filled with synaptic vesicles containing the neurotransmitter acetylcholine (ACh)
  • Synaptic cleft: extracellular space between neuron and muscle
  • Motor end plate (specialized sarcolemma)
    • Contains ACh receptors (ligand-gated Na⁺ channels)
    • Highly folded to increase surface area

NMJ: Events Leading to Muscle Action Potential

  1. Nerve action potential arrives at the axon terminal
  2. Depolarization opens voltage-gated Ca²⁺ channels ➜ Ca²⁺ enters the synaptic bulb
  3. Ca²⁺ triggers vesicle fusion (exocytosis) ➜ ACh released into the cleft
  4. ACh diffuses across cleft ➜ binds to ACh receptors on motor end plate
  5. Ligand-gated Na⁺ channels open ➜ Na⁺ influx depolarizes sarcolemma (muscle action potential)
  6. Depolarization propagates along sarcolemma & down T tubules ➜ SR releases Ca²⁺ ➜ contraction sequence begins
  7. ACh removal
    • Enzyme acetylcholinesterase (AChE) hydrolyzes ACh into acetate + choline
    • Prevents continuous stimulation; allows muscle relaxation & readiness for next signal

Entire Signal Pathway in Order

  1. Motor neuron action potential → Ca²⁺ entry in terminal
  2. ACh release → crosses synaptic cleft
  3. ACh binds receptor → Na⁺ influx (muscle action potential)
  4. Depolarization via T tubules → SR Ca²⁺ release
  5. Ca²⁺ binds troponin → tropomyosin shift → actin sites exposed
  6. ATP-driven cross-bridge cycling → sarcomere shortens
  7. Ca²⁺ actively pumped back into SR by Ca²⁺-ATPase pumps (ATP required)
  8. Tropomyosin re-covers binding sites → muscle fiber relaxes

Key Chemical & Ionic Notations (LaTeX)

  • ATP hydrolysis: ATP→ADP+Pi+Energy\text{ATP} \rightarrow \text{ADP} + P_i + \text{Energy}
  • Principal ions: Na+,  Ca2+\text{Na}^+ ,\; \text{Ca}^{2+}

Practical / Clinical Connections

  • Rigor mortis: absence of ATP after death keeps myosin heads bound ➜ sustained contraction
  • Myasthenia gravis: autoimmune loss of ACh receptors ➜ weak muscle activation
  • Botulinum toxin: blocks ACh release ➜ flaccid paralysis
  • Cholinesterase inhibitors (e.g. nerve agents, organophosphates): prevent ACh breakdown ➜ spastic paralysis, respiratory failure

Study Aids/Next Steps

  • Instructor advised two Blackboard animations: (1) Sliding filament; (2) Ca²⁺ & ATP cycle; plus YouTube animation of NMJ events
  • Recommended: pause lecture, view animations in sequence to solidify spatial & temporal relationships