Final answer:
Norepinephrine is the signal molecule that elicits the strongest response from alpha receptors, as it has a higher affinity for these receptors compared to other catecholamines.
Step-by-step explanation:
The signal molecule that elicits the strongest response from alpha receptors is norepinephrine.
Norepinephrine is a signaling molecule released as a neurotransmitter by most postganglionic sympathetic fibers as part of the sympathetic response, or as a hormone into the bloodstream from the adrenal medulla.
Receptors for catecholamines, such as norepinephrine and epinephrine, are known as adrenergic receptors. Norepinephrine acts through alpha receptors predominantly. While both norepinephrine and epinephrine can bind to alpha receptors, norepinephrine has a stronger affinity for them and therefore typically elicits a stronger response than epinephrine.