The impulse that triggers a contraction travels deep into a muscle fiber by means of sarcoplasmic reticulum. a. transverse tubules. G mitochondria. D. motor end plates. 2. A sarcomere is best described as A a portion of the sarcoplasmic reticulum. the cytoplasm of a muscle fiber. a group of myofibrils. a unit within a myofibril. 3. Myofibrils are composed primarily of actin and myosin. ATP and ADP. troponin. tropomyosin. Neurotransmitters are stored in vesicles within A myofibrils. 6. motor units. motor end plates. motor neuron endings. 5 The enzyme cholinesterase causes acetylcholine to A bind to actin. B be secreted from the motor end plate. 6 decompose. D form cross bridges. Creatine phosphate serves to A cause the decomposition of ATP. B cause the decomposition of ADP. supply energy for the synthesis of ATP. supply energy for the change of ATP to ADP. 7. Rep molecules on the surface of actin serve as active sites for the formation of cross bridges with molecules of A АТР. 8 myosin. troponin. tropomyosin. The amount of oxygen needed to support the conversion of lactic acid to glucose is called the A refractory quantity. oxygen debt. anaerobic concentration. aerobic conversion. 9 A muscle cramp is likely due to a lack of A actin. myosin. АТР. ADP. 10. An example of a partial but sustained contraction would be A muscle tone. a. tetany. C twitch.