Put the steps of muscle contraction in order. Start with the arrival of the Action Potential (AP) arriving at the neuromuscular junction. Depolarization of the sarcolemma (plasma membrane) occurs. Repolarization (relaxation) occurs. Action potential from the nerve arrives at the Neuromuscular Junction. Myosin binds to exposed sites on actin, & pulls the actin towards the center of the sarcomere (contraction). Acetylcholine diffuses across the synaptic cleft and binds to receptors on the Sarcolemma. Action potential propagates down the t-tubules & causes the release of calcium ions (Ca2+), which binds to the troponin-tropomyosin complex which exposes the binding sites on actin. After contraction, ATP causes the myosin/actin connection to break.
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Sarcoplasmic reticulum releases calcium into the sarcoplasm of muscle fiber An action potential is triggered in the muscle fiber Thick filaments pivot at their hinges & pull thin filaments toward center of sarcomere An action potential reaches the axon terminal of a somatic motor neuron Myosin heads bind to actin forming cross bridges An action potential is conducted across the sarcolemma of muscle fiber and down T-tubules Neurotransmitter stops being released and calcium is pumped back into the sarcoplasmic reticulum Neurotransmitter binds the sarcolemma of muscle fiber Myosin-binding sites on actin become exposed
Sri K.
Complete the sentences describing the steps that occur during excitation-contraction coupling. Then put the steps in order. As the electrical impulse reaches the transverse tubules, the permeability of the sarcoplasmic reticulum to calcium increases. Calcium ions diffuse from the sarcoplasmic reticulum into the sarcoplasm. These ions bind to molecules of troponin. Interaction between troponin and tropomyosin results in exposure of binding sites on actin. Then, myosin heads bind to these sites. Thin (actin) filaments are pulled toward the center of the sarcomere by myosin cross-bridges. The muscle fiber shortens and contracts. After a muscle fiber is stimulated by a motor neuron, an electrical impulse travels along the sarcolemma and deep into the fiber via the transverse tubules.
\begin{equation} \begin{array}{l}{\text { During the contraction of a vertebrate skeletal muscle fiber, }} \\ {\text { calcium ions }} \\ {\text { (A) break cross-bridges as a cofactor in hydrolysis of ATP. }} \\ {\text { (B) bind with troponin, changing its shape so that the myosin- }}\end{array} \end{equation}\begin{equation} \begin{array}{l}{\text { binding sites on actin are exposed. }} \\ {\text { (C) transmit action potentials from the motor neuron to the }} \\ {\text { muscle fiber. }} \\ {\text { (D) spread action potentials through the T tubules. }}\end{array} \end{equation}
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