Final answer:
Norepinephrine has four types of receptors within the adrenergic system: α1, α2, β1, and β2. These are G protein-coupled receptors that mediate responses of the sympathetic nervous system such as heart rate increase and energy mobilization.
Step-by-step explanation:
Different Types of Norepinephrine Receptors
The norepinephrine receptors in the adrenergic system are classified into two main types: alpha (α)-adrenergic receptors and beta (β)-adrenergic receptors. There are three subtypes of alpha-adrenergic receptors, designated as α1, α2, and α3, and two subtypes of beta-adrenergic receptors, designated as β1 and β2. All these receptors are G protein-coupled receptors (GPCRs) which are significant in mediating physiological responses.
Norepinephrine and its closely related compound epinephrine, which has an extra methyl group, both interact with these receptors to elicit various sympathetic nervous system responses such as increasing heart rate and mobilizing energy. However, it's noteworthy that there's a common misconception about the existence of α3 and β3 adrenergic receptors; the correct classification includes α1 and α2 receptors, and β1 and β2 receptors primarily. The nature of the response depends on the type of receptor and the type of cell the receptor is on.