Skeletal Muscle Fiber Contraction

  1. Activation of motor neuron

  2. Signal transmitted to muscle at the neuromuscular junction (NMJ)

  3. Excitation of muscle fiber

  4. Excitation-contraction coupling

  5. Contraction (sliding filament mechanism)

  6. Relaxation

  1. Activation of motor neuron:

Motor Control

  • Central nervous system = brain + spinal cord

  • Afferent division = somatic sensory, visceral sensory, special sensory

  • Efferent division = somatic motor (voluntary control) & autonomic motor (involuntary)

Motor Units

  • Each muscle fiber is innervated by one motor neuron

  • Each neuron innervates many fibers = motor unit

    • Activation of neuron causes all fibers in motor unit to contract

  • Large motor units = 100s-1000s of fibers

    • strength; arms, legs

  • Small motor units = 3-6 fibers

    • fine motor control; hands, eyes

  1. Neuromuscular Junction:

ACh is inside of the synaptic vesicles

  1. Excitation of Muscle Fiber:

Nerve signal arrives → ACh releases from synaptic vessels → ACh binds to receptor on the sarcolemma → Ligand-regulated ion gate open which creates an end-plate potential (Na+ flows in, K+ flows out) → Opening voltage-regulated ion gates causes action potentials (voltage spreading across entire membrane “excited”) →

  1. Excitation-Contraction Coupling:

→ Action potentials travel down T tubules via voltage-gated Na+ channels → Calcium is released from terminal cisternae which increases Ca2+ into the cytoplasm → Troponin binds to calcium → tropomyosin shifts which exposes active binding sites on actin →

Cross bridges form in presence of Ca2+

Cross bridge = connection between actin and myosin

  1. Contraction:

Cross-bridge cycling…

→ Myosin head @ rest is energized and wants to bind with actin → cross-bridge forms between myosin and actin → Bind triggers release of energy which causes movement “power stroke” (sliding of thick and thin filaments) → Binding of new ATP breaks the cross-bridge → REPEAT…

Sliding filament mechanism:

  • Relaxed → Shortened

    • Thick/thin filaments slide over each other

    • The muscle shortens but filament length stays the same

    • A band = unchanged

    • I band = reduced

    • H zone = reduced

  1. Relaxation:

Ending of nervous stimulation and ACh releases → AChE breaks down ACh that is being released → Reabsorption of calcium ions by the sarcoplasmic reticulum (pump uses ATP to move Ca2+ back into SR) → Loss of calcium ions from troponin → Return of tropomyosin to blocking position on binding sites of actin. (Elastic recoil and action of opposing muscles brings muscle back to resting length).