Final answer:
Norepinephrine acts as a neurotransmitter in the CNS and as a hormone when released from the adrenal medulla into the bloodstream, playing a significant role in the fight-or-flight response by increasing the heart rate and blood pressure.
Step-by-step explanation:
Norepinephrine (NE) is a vital signaling molecule that operates differently depending on its location in the body. In the central nervous system (CNS), norepinephrine functions as a neurotransmitter released by most postganglionic sympathetic fibers as part of the sympathetic response. However, norepinephrine also plays a role as a hormone when it is released into the bloodstream from the adrenal medulla.
During stress or the so-called fight-or-flight response, the adrenal medulla secretes the catecholamines, epinephrine, and norepinephrine. These hormones have similar effects by binding to beta-1 receptors and opening sodium and calcium ion chemical- or ligand-gated channels, which contributes to increased heart rate and blood pressure. Nonetheless, excessively huge releases of these hormones and sympathetic nerve activity may result in arrhythmias.
The sympathetic nervous system (SNS) stimulation can lead to the release of norepinephrine and epinephrine by the adrenal medulla. The release process is managed by neuronal impulses that originate from the hypothalamus and travel through neurons from the thoracic spinal cord as part of the sympathomedullary (SAM) pathway. Therefore, while norepinephrine can be considered a neurotransmitter when released by nerve axons at synapses, it is regarded as a hormone when secreted by the adrenal medulla into the bloodstream.