During muscle contraction the myofilaments myosin and actin slide toward each other and overlap. This shortens the sarcomeres and the entire muscle. Muscle cells are "shocked" by nerve impulses from motor neurons. The point of attachment of the nerve to muscle is called a neuromuscular junction. A motor neuron and its muscle cells are referred to as a motor unit. The nerve impulse is carried from the neuron across the gap to the membrane or sarcolemma of the muscle cell by a chemical called acetylcholine. After the impulse is passed an enzyme called cholinesterase "de-activates" acetylcholine, readying the muscle for the next nerve impulse.