Fasciculations originate at the very tips of the nerves, called axons, as they come close to being in contact with the muscle. The tips of the axons are thought to be overly sensitive to depolarizing (electrical firing), which is what triggers a muscle to contract.  When a nerve fires, zillions of times a day upon request, an electrical impulse starts in the nerve, moves out toward the muscle, triggers the release of a chemical (acetylcholine) that "swims" across the gap between the nerve axon and the muscle, and binds to a receptor on the muscle causing it to fire. The complicated process itself takes a small fraction of a second.  If any of this happens involuntarily, then the muscle fiber contracts without your permission and behold, a muscle twitch or fasciculation!