Arrange the following physiological events during skeletal muscle contraction in their cor… — JAMB UTME

Arrange the following physiological events during skeletal muscle contraction in their correct sequence, starting from nerve excitation to the generation of the power stroke.

1An action potential arrives at the neuromuscular junction, causing acetylcholine release and depolarization of the sarcolemma.
2Action potentials travel down transverse tubules (T-tubules), triggering the release of calcium ions ( $Ca^{2+}$ ) from the sarcoplasmic reticulum into the sarcoplasm.
3Calcium ions ( $Ca^{2+}$ ) bind to troponin, inducing a conformational change that pulls tropomyosin away from the myosin-binding sites on actin filaments.
4Myosin heads bind to the exposed active sites on actin, forming cross-bridges.
5Bound myosin heads release inorganic phosphate ( $P_i$ ) and $ADP$ , executing the power stroke that slides actin filaments toward the center of the H-zone.

Cevap

The correct order of muscle contraction events is: 1. Action potential arrives at the neuromuscular junction releasing acetylcholine → 2. Calcium ions (

Ca^{2+}

) released from sarcoplasmic reticulum via T-tubules → 3. Calcium ions (

Ca^{2+}

) bind troponin, shifting tropomyosin → 4. Myosin heads bind actin forming cross-bridges → 5. Release of

ADP

and

P_i

drives the power stroke sliding actin filaments.

Excitation-contraction coupling progresses strictly from electrical activation at the neuromuscular junction to sarcoplasmic

Ca^{2+}

release, troponin binding, tropomyosin displacement, cross-bridge attachment, and finally the power stroke driven by release of

ADP

and

P_i

Adım Adım Çözüm

Identify the neuromuscular stimulus initiating contraction.

Depolarization of sarcolemma via acetylcholine release at the neuromuscular junction.

Electrical excitation precedes any intracellular chemical signaling in skeletal muscle.

Trace intracellular signal transduction.

Propagation down T-tubules induces sarcoplasmic reticulum release of

Ca^{2+}

Calcium acts as the key ionic messenger coupling membrane excitation to mechanical contraction.

Determine regulatory protein conformational changes.

Ca^{2+}

binds troponin, displacing tropomyosin to uncover myosin-binding sites on actin.

Tropomyosin sterically blocks cross-bridge formation until moved by

Ca^{2+}

-bound troponin.

Identify structural binding between contractile proteins.

Energized myosin heads attach to uncovered actin active sites, establishing cross-bridges.

Physical connection between thick and thin filaments is mandatory for force transmission.

Identify the mechanical force step.

Release of

ADP

and

P_i

causes the myosin head to pivot, sliding the actin filament toward the center of the sarcomere.

The power stroke produces microfilament sliding, resulting in muscle shortening.

Anahtar Kavram

Sliding Filament Mechanism and Excitation-Contraction Coupling

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