Complete the steps of the contraction of a skeletal muscle cell: A motor neuron releases neurotransmitters onto ion channels in the sarcolemma. The channels open and sodium ions move into the cell (primarily) creating a depolarizing potential. If enough ions diffuse (reach threshold level), voltage-gated sodium channels in the sarcolemma will open in a wavelike fashion all over the sarcolemma and down the T-tubules (action potential). The voltage wave in the T-tubules activates voltage-sensing Ca++ channels (dihydropyridine receptor -DHP). The DHP receptors are mechanically linked to Ca++ release channels in the sarcoplasmic reticulum called ryanodine channels. These channels open and stored calcium ions rush out of the sarcoplasmic reticulum into the sarcoplasm. Ca++ binds to troponin, and this protein will "roll over" moving the tropomyosin off the active sites on the actin filaments (exposure of active sites). The myosin heads will attach to the active sites in the actin filaments and inorganic phosphate is released. This binding is called cross-bridge formation. ATP is released and myosin "pulls" the actin filaments toward the center of the sarcomere (pivoting of myosin heads). This step is called power stroke. ADP binds to the myosin head and myosin detaches from actin (detachment). Myosin head splits ATP into ADP and Pi through hydrolysis (myosin reactivation). More cross bridges can be formed... The sarcolemma repolarizes and hyperpolarizes due to the opening of potassium channels. Potassium diffuses out of the cell.

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