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The contractile mechanism in skeletal muscle depends on proteins (myosin II, actin, tropomyosin, and troponin) as well as calcium ions.  The thick filaments in skeletal muscle are comprised of myosin molecules, with the heads of the myosin molecules forming cross-links with actin during muscular contraction.  Two actin chains comprise the thin filaments in skeletal muscle.  Tropomyosin molecules sit in the groove between the two actin chains, covering the myosin binding sites on actin when the muscle is at rest.  Troponin molecules are small globular proteins situated alongside the tropomyosin molecules.  Troponin is composed of three subunits: troponin T, troponin I, and troponin C.  Troponin T binds the other troponin components to tropomyosin, troponin I binds the troponin-tropomyosin complex to actin, and troponin C contains the binding sites for Ca²⁺.  During excitation-contraction coupling, Ca²⁺ is released from the sarcoplasmic reticulum.  When Ca²⁺ binds troponin C, tropomyosin shifts to expose the actin binding sites for myosin, allowing contraction to occur.

(Choice A)  Acetylcholine is the neurotransmitter that initiates muscle contraction in response to motor neuron stimulation.  Acetylcholine release from the motor neuron opens post-synaptic ligand-gated ion channels, resulting in depolarization of the muscle cell.  Depolarization then causes release of Ca²⁺ from the sarcoplasmic reticulum.

(Choice B)  Epinephrine is a catecholamine that is not directly involved in skeletal muscle contraction.  However, epinephrine stimulates β2-adrenergic receptors to increase skeletal muscle blood flow, glycogenolysis, and lipolysis.

(Choices C and D)  Calmodulin and myosin light-chain kinase are elements of the contractile mechanism in smooth muscle, not skeletal muscle.

Educational objective:
The contractile mechanism in skeletal muscle depends on proteins (myosin II, actin, tropomyosin, and troponin) as well as calcium ions.