Noradrenaline or norepinephrine is a monoamine molecule (made of one amino acid, one base unit of protein) that functions as both a hormone and a neurotransmitter. Norepinephrine is incredibly important in cognitive awareness. It is released by a presynaptic sympathetic ganglion to act on the heart as a stress molecule that increases heart rate. Not only does norepinephrine play a role in the sympathetic branch of the autonomic nervous system (ANS), it also directly affects the amygdala, which is a brain structure important in controlling attention. Norepinephrine is synthesized mainly by the adrenal medulla (a structure in the adrenals that is associated with each kidney). However, it can also be synthesized and secreted by presynaptic noradrenergic neurons in the central nervous system. Treatment of hypotension (low blood pressure) is often done by administering noradrenaline to the patient.
Synthesis and Mechanism of Action
The adrenal medulla synthesizes a large amount of norepinephrine, which is then secreted by the adrenals directly into the blood without the use of a duct. It is also synthesized by postganglionic neurons in the sympathetic nervous system (SNS) of the ANS. Additionally, a region in the brainstem called the locus coeruleus is responsible for originating most of the pathways that utilize norepinephrine as their neurotransmitter in the brain. These signals are propagated through noradrenergic neurons that are bilateral (on both sides of the brain) and that run to various locations. Some of these locations in the brain include the cerebral cortex and the limbic system. These noradrenergic neurons also run to the spinal cord. All of these pathways combined are considered a neurotransmitter system that utilizes noradrenaline. Any noradrenergic neuron is capable of synthesizing and secreting norepinephrine when trying to propagate neural signals. Norepinephrine is fairly similar in structure to epinephrine, as norepinephrine is just an oxidized version of epinephrine. Dopamine is the major precursor to norepinephrine.
Norepinephrine is fairly important in the SNS response and “fight-or-flight.” The SNS is a part of the ANS that is important in responding to possible threats to the organism. When the SNS is stimulated, blood is funneled from the digestive and urinary organs to the heart, lungs, brain, and muscles. This results in the body’s ability to respond properly to a threat. The effects that are felt when there is an activation of noradrenergic neurons are an increase in alertness, arousal, and positive effects on the reward system. Norepinephrine has the physiologic response of constricting the blood vessels and serum glucose levels (by stimulating glycolysis in the muscles and liver and inhibiting the production and secretion of insulin by the pancreas). The constriction of blood vessels will eventually result in an increase in blood pressure. These physiologic responses help the body have the energy, nutrients, and oxygen to respond to the threat it is encountering.
Norepinephrine is extensively used to treat hypotension in patients. Hypotension is a condition in which systemic blood pressure is abnormally low. This can result in blood not circulating correctly and tissue not getting the correct amount of oxygen. Since one of the sympathetic effects of norepinephrine is the constriction of blood vessels, it can be used to treat this condition. The constriction of the blood vessels will result in higher blood pressure since the size of the vessels themselves will decrease. It is important to maintain proper levels of norepinephrine. If levels become too high, the patient might begin feeling highly stressed, anxious, and hyperactive. Low levels are linked to difficulties focusing and finding motivation along with a massive lack of focus. Additionally, there are some adverse effects of norepinephrine such as swelling of the face, lips, and tongue; uneven heart rate; alarmingly high blood pressure; blue lips and fingertips (a sign that tissues are not getting enough oxygen delivered to them); and difficulty breathing.
Riannon C. Atwater
See also: Adrenaline; Autonomic Nervous System; Carotid Body
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