Acetylcholine (ACh) is a neurotransmitter. Acetylcholine in vertebrates is the major transmitter at neuromuscular junctions, autonomic ganglia, parasympathetic effector junctions, a subset of sympathetic effector junctions, and at many sites in the central nervous system. Its physiological and pharmacological effects, metabolism, release, and receptors have been well documented in several species. ACh has been considered an important excitatory neurotransmitter in the carotid body (CB). Various nicotinic and muscarinic ACh receptors are present in both afferent nerve endings and glomus cells. Therefore, ACh can depolarize or hyperpolarize the cell membrane depending on the available receptor type in the vicinity. Binding of ACh to its receptor can create a wide variety of cellular responses including opening cation channels (nicotinic ACh receptor activation), releasing Ca2+ from intracellular storage sites (via muscarinic ACh receptors), and modulating activities of K+ and Ca2+ channels. Interactions between ACh and other neurotransmitters (dopamine, adenosine, nitric oxide) have been known, and they may induce complicated responses. Cholinergic biology in the CB differs among species and even within the same species due to different genetic composition. Development and environment influence cholinergic biology. Pharmacological data clearly indicate that both muscarinic and nicotinic acetylcholine receptors have a role in the encoding of new memories. Localized lesions and antagonist infusions demonstrate the anatomical locus of these cholinergic effects, and computational modeling links the function of cholinergic modulation to specific cellular effects within these regions. Acetylcholine has been shown to increase the strength of afferent input relative to feedback, to contribute to theta rhythm oscillations, activate intrinsic mechanisms for persistent spiking, and increase the modification of synapses. These effects might enhance different types of encoding in different cortical structures. In particular, the effects in entorhinal and perirhinal cortex and hippocampus might be important for encoding new episodic memories. The role of ACh in attention has been repeatedly demonstrated in several tasks. Acetylcholine is linked to response accuracy in voluntary and reflexive attention and also to response speed in reflexive attention. It is well known that those with Attention-deficit/hyperactivity disorders tend to be inaccurate and slow to respond. Acetylcholine has been found to be a microbial product, urinary acetylcholine is produced by Lactobacillus.