Choose the neurotransmitter that best fits each definition or description. 1. crosses the synapse between a motor neuron and a muscle fiber to produce a muscle contraction. 2. biogenic amine responsible for the "flight or fight" response of the sympathetic nervous system. 3. major inhibitory amino acid neurotransmitter that causes ligand-gated cl- channels to open. 4. neurotransmitter involved in control of some body movement; excess levels are linked to schizophrenia. 5. neurotransmitter involved in regulation of sleep and implicated in various emotional states. 6. major excitatory neurotransmitter in the vertebrate nervous system.