Final answer:
Norepinephrine binds to α and β adrenergic receptors, leading to cellular excitation including vasoconstriction and increased heart muscle contraction associated with the fight-or-flight response.
Step-by-step explanation:
Norepinephrine (NT) binds to α (alpha) and β (beta) receptors; activation leads to a series of responses depending on the receptor type. When bound to α-adrenergic receptors on target cells such as those in the blood vessels, norepinephrine typically causes vasoconstriction. Conversely, when norepinephrine binds to β-adrenergic receptors, found on cells like those in the heart and in certain other tissues, it often leads to cellular excitation such as increased heart muscle contraction and smooth muscle relaxation, as well as other sympathetic nervous system responses associated with the fight-or-flight response. The binding of norepinephrine to these receptors activates a G-protein, which then activates adenylyl cyclase, converting ATP to cAMP. The cAMP acts as a second messenger initiating various cellular responses until it is broken down by phosphodiesterase, terminating the signal.