Acetylcholine (ACh) is a small-molecule neurotransmitter that is used by motor neurons of the nervous system (at the neuromuscular junction) to activate muscles. Neurons releasing ACh are known as cholinergic. ACh also plays a role in the autonomic nervous system and in the central nervous system, where it modulates plasticity, arousal, reward, enhancement of sensory perceptions, and sustained attention. ACh is involved in memory and learning: memory can be enhanced when drugs are used to increase the activity of ACh in the brain. Disruption of ACh neurotransmission can disrupt memory, as in the case of the neurodegenerative disease Alzheimer’s.