Epinephrine and norepinephrine belong to a group of compounds known as catecholamines. These compounds act as both neurotransmitters, delivering signals between nerve cells, and hormones.
Epinephrine and norepinephrine have similar chemical structures. However, they produce different effects on the body. Both play a role in the regulation of the sympathetic nervous system, which is responsible for the body’s “fight or flight” response.
Another name for epinephrine is adrenaline, and some refer to norepinephrine as noradrenaline.
In this article, we discuss the similarities and differences between epinephrine and norepinephrine, along with their functions and medical uses. We also outline the health effects of having too much or too little of either compound in the body.
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Both epinephrine and norepinephrine are chemical messengers. They function as hormones, as well as neurotransmitters.
As hormones, epinephrine and norepinephrine travel through the bloodstream, along with other hormones that the endocrine and reproductive organs make. They tell organs and tissues to work in different ways.
Neurotransmitters have a similar function. However, they only occur in nerve cells and travel across synapses, which are junctions where two nerve fibers meet. Nerves cells produce neurotransmitters in response to electrical impulses.
The adrenal medulla is the inner portion of the adrenal gland. It regulates and secretes both epinephrine and norepinephrine in response to stress and other imbalances in the body, such as low blood pressure.
What does epinephrine do?
When the brain perceives danger, the amygdala triggers the hypothalamus to activate the autonomic nervous system (ANS).
Signals from the ANS stimulate the adrenal gland to start pumping epinephrine into the bloodstream. People often refer to this surge of epinephrine as an adrenaline rush or the fight or flight response.
Epinephrine activates alpha- and beta-adrenoreceptors in the cells of various body organs and tissues, including:
- the heart
- the lungs
- the muscles
- the blood vessels
The release of epinephrine into the bloodstream brings about several physiological changes, such as:
What does norepinephrine do?
The adrenal medulla produces norepinephrine in response to low blood pressure. Norepinephrine promotes vasoconstriction, which is a narrowing of the blood vessels. This, in turn, increases blood pressure.
Like epinephrine, norepinephrine also stimulates alpha-adrenoreceptors in the cells of the blood vessels. It increases heart rate and blood sugar levels.
Synthetic forms of epinephrine and norepinephrine have several medical uses.
Epinephrine
Doctors prescribe epinephrine to treat severe medical conditions that affect the heart and airways, such as anaphylaxis.
Anaphylaxis is a severe and life threatening allergic reaction that can interfere with a person’s ability to breathe. Epinephrine counters anaphylactic shock by:
- narrowing the blood vessels to increase blood pressure
- increasing heart rate to improve blood flow
- relaxing the muscles and airways, allowing a person to breathe
People at risk of anaphylaxis can carry an epinephrine autoinjector with them at all times.
Doctors may also use epinephrine to treat the following:
- severe asthma attacks
- cardiac arrest
- septic shock
During asthma attacks, doctors use epinephrine in a nebulized or inhaled form, rather than injecting it.
In cases of septic shock, doctors may use epinephrine and norepinephrine at the same time, delivering it via an IV line.
Norepinephrine
Norepinephrine can help raise systolic blood pressure (SBP) in people who have had a heart attack. Systolic blood pressure refers to the pressure that occurs when the heart is contracting and ejecting blood. A heart attack can damage and weaken the heart muscle, resulting in low SBP.
Doctors may also use norepinephrine to treat the following:
- Critical hypotension: This is the medical term for dangerously low blood pressure.
- Septic shock: This is a life threatening condition in which blood pressure drops dangerously low following an infection. Doctors may combine norepinephrine with IV fluids and antibiotics to treat septic shock.
- Pericardial tamponade: This is another life threatening condition in which the heart is unable to fully expand and fill with blood due to fluid in the pericardium, which is the membrane surrounding the heart. Doctors treat it via pericardiocentesis, which removes the excess fluid, as well as norepinephrine.
- Neurogenic shock: This occurs when damage to the nervous system causes difficulty maintaining a stable heart rate, blood pressure, and temperature.
Certain medical conditions can affect the adrenal glands, causing excess production of epinephrine and norepinephrine. Examples include:
- chronic stress
- obesity
- tumors
Symptoms of high levels of epinephrine or norepinephrine can include:
A 2018 review article states that having high levels of norepinephrine can increase a person’s risk of cardiovascular and kidney damage.
An epinephrine overdose can occur in people who use epinephrine injections to treat certain medical conditions. An overdose of injected epinephrine can lead to dangerously high blood pressure, stroke, or even death.
Epinephrine and norepinephrine are similar chemicals that act as both neurotransmitters and hormones in the body. Both substances play an important role in the body’s fight or flight response, and their release into the bloodstream causes increases in blood pressure, heart rate, and blood sugar levels.
Epinephrine acts on the alpha- and beta-adrenoreceptors in the muscles, lungs, heart, and blood vessels. Norepinephrine is a metabolite of dopamine that primarily acts on the alpha-adrenoreceptors in the blood vessels.
Doctors may prescribe epinephrine to treat potentially life threatening conditions, such as anaphylaxis, severe asthma attacks, and cardiac arrest. A doctor may prescribe norepinephrine to raise dangerously low blood pressure following a heart attack, critical hypotension, or septic shock.
Learning Outcomes
- Describe the organization and functions of the sympathetic nervous systems
- Describe the organization and functions of the parasympathetic nervous systems
In the autonomic nervous system, a preganglionic neuron of the CNS synapses with a postganglionic neuron of the PNS. The postganglionic neuron, in turn, acts on a target organ. Autonomic responses are mediated by the sympathetic and the parasympathetic systems, which are antagonistic to one another. The sympathetic system activates the “fight or flight” response, while the parasympathetic system activates the “rest and digest” response.
Figure 1. The sympathetic and parasympathetic systems
The autonomic nervous system serves as the relay between the CNS and the internal organs. It controls the lungs, the heart, smooth muscle, and exocrine and endocrine glands. The autonomic nervous system controls these organs largely without conscious control; it can continuously monitor the conditions of these different systems and implement changes as needed. Signaling to the target tissue usually involves two synapses: a preganglionic neuron (originating in the CNS) synapses to a neuron in a ganglion that, in turn, synapses on the target organ, as illustrated in Figure 1. There are two divisions of the autonomic nervous system that often have opposing effects: the sympathetic nervous system and the parasympathetic nervous system.
Which of the following statements is false?
- The parasympathetic pathway is responsible for resting the body, while the sympathetic pathway is responsible for preparing for an emergency.
- Most preganglionic neurons in the sympathetic pathway originate in the spinal cord.
- Slowing of the heartbeat is a parasympathetic response.
- Parasympathetic neurons are responsible for releasing norepinephrine on the target organ, while sympathetic neurons are responsible for releasing acetylcholine.
Sympathetic Nervous System
The sympathetic nervous system is responsible for the “fight or flight” response that occurs when an animal encounters a dangerous situation. One way to remember this is to think of the surprise a person feels when encountering a snake (“snake” and “sympathetic” both begin with “s”). Examples of functions controlled by the sympathetic nervous system include an accelerated heart rate and inhibited digestion. These functions help prepare an organism’s body for the physical strain required to escape a potentially dangerous situation or to fend off a predator.
Figure 2. The sympathetic and parasympathetic nervous systems often have opposing effects on target organs.
Most preganglionic neurons in the sympathetic nervous system originate in the spinal cord, as illustrated in Figure 2. The axons of these neurons release acetylcholine on postganglionic neurons within sympathetic ganglia (the sympathetic ganglia form a chain that extends alongside the spinal cord). The acetylcholine activates the postganglionic neurons. Postganglionic neurons then release norepinephrine onto target organs. As anyone who has ever felt a rush before a big test, speech, or athletic event can attest, the effects of the sympathetic nervous system are quite pervasive. This is both because one preganglionic neuron synapses on multiple postganglionic neurons, amplifying the effect of the original synapse, and because the adrenal gland also releases norepinephrine (and the closely related hormone epinephrine) into the blood stream. The physiological effects of this norepinephrine release include dilating the trachea and bronchi (making it easier for the animal to breathe), increasing heart rate, and moving blood from the skin to the heart, muscles, and brain (so the animal can think and run). The strength and speed of the sympathetic response helps an organism avoid danger, and scientists have found evidence that it may also increase LTP—allowing the animal to remember the dangerous situation and avoid it in the future.
Parasympathetic Nervous System
While the sympathetic nervous system is activated in stressful situations, the parasympathetic nervous system allows an animal to “rest and digest.” The parasympathetic system’s functions conserve energy: slowing down the heart rate, reducing contractile forces of both cardiac and gastrointestinal muscle, and reducing conduction velocity of the sinoatrial node and atrioventricular node.
One way to remember this is to think that during a restful situation like a picnic, the parasympathetic nervous system is in control (“picnic” and “parasympathetic” both start with “p”). Parasympathetic preganglionic neurons have cell bodies located in the brainstem and in the sacral (toward the bottom) spinal cord, as shown in Figure 2. The axons of the preganglionic neurons release acetylcholine on the postganglionic neurons, which are generally located very near the target organs. Most postganglionic neurons release acetylcholine onto target organs, although some release nitric oxide. Acetylcholine acts on two types of receptors, the muscarinic and nicotinic cholinergic receptors. Most transmissions occur in two stages: When stimulated, the preganglionic neuron releases acetylcholine at the ganglion, which acts on nicotinic receptors of postganglionic neurons. The postganglionic neuron then releases acetylcholine to stimulate the muscarinic receptors of the target organ.
The parasympathetic nervous system resets organ function after the sympathetic nervous system is activated (the common adrenaline dump you feel after a “fight-or-flight” event). Effects of acetylcholine release on target organs include slowing of heart rate, lowered blood pressure, and stimulation of digestion.
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