Final answer:
Norepinephrine is the neurotransmitter and hormone involved in stress responses in the sympathetic nervous system, crucial for fight-or-flight responses.
Step-by-step explanation:
The neurotransmitter and hormone involved in the stress response in the sympathetic nervous system is norepinephrine. During stress, the sympathetic nervous system activates and norepinephrine is released by postganglionic sympathetic fibers as a neurotransmitter and by the adrenal medulla into the bloodstream as a hormone. This plays a crucial role in the body's fight-or-flight response, preparing the body to react quickly to stressful situations.