The sympathetic nervous system is one part of the autonomic nervous system, which controls involuntary body functions. When activated, the sympathetic nervous system initiates the “fight or flight” response, preparing the body to deal with perceived threats or stressors. It does this by increasing heart rate, dilating pupils, stimulating sweat glands, and diverting blood flow away from the gastrointestinal system and skin to the muscles needed for fight or flight. The sympathetic nervous system acts on many target organs throughout the body to bring about these widespread effects.
Heart
One of the major targets of the sympathetic nervous system is the heart. Sympathetic stimulation causes an increase in heart rate (positive chronotropic effect) and strength of heart muscle contraction (positive inotropic effect). This increases cardiac output to provide more oxygenated blood to the muscles and brain during times of stress. Specific effects on the heart include:
- Release of norepinephrine, which binds to beta-1 adrenergic receptors on the sinoatrial node, increasing heart rate
- Release of epinephrine from the adrenal medulla into the bloodstream, which also binds beta-1 adrenergic receptors to increase heart rate
- Enhanced conduction velocity through the AV node
- Increased strength of ventricular contraction
Together, these actions on the heart increase blood flow throughout the body to prepare for the fight-or-flight response.
Blood vessels
The sympathetic nervous system acts on blood vessels in different tissues in different ways to optimize blood flow during stress. Effects include:
- Constriction of blood vessels in the skin, gastrointestinal tract, and other non-essential tissues – This diverts blood flow away from areas not needed for fight or flight to priority areas like muscles.
- Dilation of blood vessels supplying skeletal muscle – More blood flows to the muscles to bring oxygen and energy sources.
- Dilation of blood vessels supplying the heart – Again, this provides more blood flow to the hard-working heart muscle.
- Constriction of vessels in the kidneys – This helps maintain blood pressure and volume.
In this way, the sympathetic nervous system uses precise control over the diameter of blood vessels throughout the body to optimize distribution of oxygenated blood.
Lungs
The sympathetic nervous system prepares the lungs for increased demand during fight-or-flight situations. Actions include:
- Dilation of bronchioles via beta-2 adrenergic receptors – This decreases airway resistance to allow more air flow.
- Relaxation of bronchial muscles – This also decreases airway resistance.
- Dilation of blood vessels in lungs – This meets the increased demand for gas exchange.
Together, these actions maximize air flow and gas exchange in the lungs to provide more oxygen to the working muscles.
Digestive system
During fight-or-flight situations, digestion becomes a low priority so the sympathetic nervous system inhibits activity of the digestive system. Specific effects include:
- Constriction of blood vessels supplying the GI tract – Reduces blood flow to the non-essential digestive organs.
- Reduction of peristalsis – Slows mechanical digestion.
- Reduction of saliva, gastric, pancreatic, and intestinal secretions – Slows chemical digestion.
- Constriction of the sphincters of the GI tract – Slows transit of food through.
- Relaxation of the internal anal sphincter – Causes defecation reflex during extreme stress.
These widespread effects basically shut down digestion temporarily so that blood and energy can be diverted elsewhere.
Eyes
The sympathetic nervous system acts on the eyes to optimize vision during the fight-or-flight response. Effects include:
- Dilation of the pupil – Allows more light to enter the eye to see better.
- Relaxation of the ciliary muscle – Adjusts the lens for distant vision to see threats from afar.
- Constriction of blood vessels in the retina – Enhances retinal activity.
Together, these actions improve visual perception and processing during stress.
Adrenal medulla
The adrenal medulla produces important hormones under sympathetic control:
- Epinephrine – Released into the bloodstream where it binds adrenergic receptors throughout the body, amplifying the fight-or-flight response initiated by direct sympathetic nerve stimulation.
- Norepinephrine – Also released into the bloodstream for additional adrenergic effects.
These catecholamine hormones are critical for the widespread effects of sympathetic activation.
Metabolic effects
The sympathetic nervous system initiates metabolic changes to provide energy for the fight-or-flight response. These include:
- Breakdown of glycogen into glucose – Provides readily available energy source.
- Breakdown of fat (lipolysis) into fatty acids – Provides fuel for muscles.
- Increased levels of blood glucose and fatty acids.
- Suppression of insulin release – So glucose remains available in the bloodstream rather than taken up by cells.
The combined result of these metabolic effects is to mobilize energy stores for use by muscles during sympathetic activation.
Other effects
Some other target organs and effects of the sympathetic nervous system include:
- Skin – Constriction of blood vessels causes pallor; stimulation of sweat glands.
- Kidneys – Constriction of blood vessels helps maintain blood pressure; decreased urine output.
- Spleen – Contraction causes release of stored red blood cells.
- Salivary glands – Decreased saliva production.
- Hairs – Erection of hairs causes “goose bumps.”
- Fat cells – Stimulation of lipolysis releases fatty acids into blood.
Synergistic hormones
Some hormones secreted from endocrine glands synergize with the sympathetic nervous system during fight-or-flight activation. These include:
- Thyroid hormone – Increases metabolic rate.
- Glucagon – Raises blood glucose levels by stimulating glycogen breakdown.
- Growth hormone – Mobilizes fat breakdown.
- Cortisol – Has widespread effects to provide energy and maintain blood pressure and volume.
Summary table
Here is a summary of some of the major target organs and effects of the sympathetic nervous system:
| Target Organ | Effects |
|---|---|
| Heart | Increases heart rate and contractility |
| Blood vessels | Constricts vessels to skin; dilates vessels to muscle |
| Lungs | Dilates bronchioles; dilates pulmonary blood vessels |
| Digestive tract | Constricts blood vessels; decreases motility |
| Eyes | Dilates pupils; relaxes ciliary muscle |
| Adrenal medulla | Releases epinephrine and norepinephrine |
| Liver | Stimulates glycogen breakdown |
| Fat cells | Stimulates lipolysis |
Conclusion
In summary, the widespread sympathetic outflow during the fight-or-flight response allows the body to react quickly and efficiently to danger or stress. This is accomplished by targeting multiple organs throughout the body, including the heart, blood vessels, lungs, liver, fat cells, digestive system, eyes, adrenal glands, and more. The net result is mobilization of energy stores, increased heart rate and breathing, dilation of the pupils, and constriction of blood vessels in non-essential organs – all physical changes that prepare the body for action. Understanding the diverse effects and target organs of the sympathetic nervous system provides insight into how the body adapts and responds to stressors.