Acetylcholine (ACh), the first neurotransmitter ever to be identified, is a small- molecule excitatory neurotransmitter with a wide variety of known functions. In the sympathetic and parasympathetic nervous systems and at all neuromuscular junctions, ACh is used to signal muscle movement. Within the central nervous system, cholinergic cells (neurons that use ACh as a neurotransmitter) are found in several different locations of the brain, including the striatal complex, the basal forebrain, the diencephalon, pontomesencephalic cell groups, and the medulla. Depending on which area of the brain it is found in, ACh may be involved in any one of several different functions. Some of these functions include the conduction of pain, the regulation of neuroendocrine function, the regulation of REM sleep cycles, and the process of learning and memory formation.

Within the basal forebrain, it is the cholinergic cells of the septal nuclei that play a large role in learning and memory. These neurons send major projections to the hippocampus, a structure particularly important for the normal formation of declarative memories. Several noteworthy clinical cases have shown that lesions in this area of the brain alone can greatly impair an individual's ability to form new declarative memories. It is this cholinergic system, among others, that has been shown to suffer serious neurodegeneration in Alzheimer's disease, a condition characterized by a significant failure of memory and other cognitive functions.