acetylcholine

n. (ACh) The first neurotransmitter to be scientifically identified. ACh is the primary neurotransmitter secreted by efferent (motor) axon terminals in the central nervous system. ACh is active in an ionic form at nicotinic receptors, including the neuromuscular junction, and in a metabolic form at muscarinic receptors in the postganglionic parasympathetic system, such as in the activity of the vagus nerve affecting the heart, as well as at sympathetic ganglia in the spinal cord.
In the brain itself, most cholinergic neurons (neurons using acetylcholine as a neurotransmitter) have excitatory muscarinic metabotropic sites, initiating actions such as rapid eye movement (REM) sleep (dream sleep). ACh in the forebrain facilitates learning, while ACh in the limbic system facilitates memory functions.   - VS