Final answer:
Norepinephrine is produced by neurons in the brain stem, specifically from the adrenal medulla, and serves as both a neurotransmitter and a hormone, playing a key role in the body's fight or flight response.
Step-by-step explanation:
The substance in question that is produced by neurons in the brain stem and serves dual roles as both a neurotransmitter and a hormone is norepinephrine. Within the brain stem, particularly from the adrenal medulla, norepinephrine is synthesized and released. As a neurotransmitter, it is released during the sympathetic nervous system response from the postganglionic sympathetic fibers. However, as a hormone, it is secreted into the bloodstream from the adrenal medulla, which is part of the adrenal glands located atop the kidneys. Norepinephrine, along with its closely related compound epinephrine, plays a crucial role in the body's fight or flight response, impacting systems such as heart rate, blood pressure, and muscle readiness.
Epinephrine and norepinephrine are both categorized as catecholamines, and they are derived from the amino acid tyrosine. Epinephrine, also known as adrenaline, is secreted in greater amounts than norepinephrine and is considered the more potent hormone. However, norepinephrine itself is significant for its role as a neurotransmitter in the brain, influencing attention, emotions, and the sleep-wake cycle, among other functions. In the adrenal medulla, chromaffin cells are responsible for the production of these neurotransmitters that double as hormones.