1. : the final chemical messenger and "trigger" for muscle contraction. it binds to troponin. 2. : a neurotransmitter released at motor end plates by the axon terminals. 3. : it diffuses across the cell membrane resulting in depolarization. 4. : activates synaptic vesicles in axon terminals to fuse with plasma membrane of axon terminal. 5. : where the axon of a motor neuron connects with the muscle fibers. 6. : breaks down ach into its building blocks, rendering it inactive.