Receptors
The peripheral and autonomic nervous systems work through various types of receptors. Understanding these receptors is essential to understanding nervous system pharmacology.

There are two basic types of receptors associated with the PNS:
adrenergic and cholinergic receptors. Each of these receptors is divided into diff erent subtypes. Activation of each subtype of these receptors causes a characteristic set of physiological responses. Some drugs aff ect all receptor subtypes while others only aff ect one type of receptor. Different doses of a drug may activate one type of receptor, while increased doses may activate other receptor subtypes. It is important to memorize the various receptor types and their responses.

Cholinergic Receptors
Cholinergic receptors are defi ned as receptors that mediate responses to acetylcholine. Th ese receptors mediate responses at all junctions where acetylcholine is the transmitter. Th ere are three major subtypes of cholinergic receptors, which are referred to as nicotinicN, nicotinicM, and muscarinic.

Adrenergic Receptors 
Adrenergic receptors are defined as receptors that mediate responses to epinephrine (adrenaline) and norepinephrine. These receptors mediate responses at all junctions where norepinephrine or epinephrine is the transmitter.

Adrenergic receptors are divided into two types: alpha-receptors (a-receptors) and beta-receptors (b-receptors). Alpha-receptors are divided into alpha1 and alpha2 receptors, and beta-receptors are divided into beta1 and beta2 receptors. In addition to the four major subtypes of adrenergic receptors, there is another adrenergic receptor type, referred to as the dopamine receptor.

Dopamine receptors respond only to dopamine, a neurotransmitter found primarily in the CNS.