The human brain contains an estimated 100 billion nerve cells, called neurons. The neurons support a massive communications system that receives and sends information throughout the body. Although the brain has the largest number of neurons in the body, neurons are also present in the cardiovascular system, the gastrointestinal tract and in the muscles.

Neurons have two special structures: axons and dendrites. Axons are long, thin projections from nerve cells that transmit information via electrical signals. Dendrites are projections from nerve cells that receive electrical information from the axons of other cells. Small gaps, called synapses, separate every axon from the dendrite with which it communicates.

At every synapse, the neuron changes its electrical signal into a chemical signal. These chemical signals, released at the axon terminals, are called neurotransmitters. There are many different kinds of neurotransmitters, each having a distinct chemical structure and shape. The distinct shape of a neurotransmitter is important because it allows the neurotransmitter to act as a "key."

The "locks" that are opened by each of these neurotransmitter "keys" are called receptors, which are located on the dendrites. When a neurotransmitter binds to a receptor that is shaped to match, it sends a message—for example, causing a memory trace to be formed.

A particular type of receptors, called neuronal nicotinic receptors (NNRs), serve as key regulators of nervous system function. When the natural neurotransmitter acetylcholine, or a drug that mimics acetylcholine, binds to an NNR, the NNR normalizes chemical signaling, allowing neurons to communicate properly. This action, known as neuromodulation, results in increased signaling when the nervous system is understimulated and decreased signaling when the nervous system is overstimulated. As such, NNRs can be thought of as the nervous system's "volume knob."