Definition and Overview
The sympathetic nervous system is one of the two major divisions that make up the autonomic nervous system. It is responsible for the fight-or-flight response, which quickly activates the body in crisis situations.
The concept of the sympathetic nervous system originates from the word sympatheia, which means ‘empathy’ in ancient Greek medicine. The term used to describe the phenomenon in which a disease in one organ affects another organ later became established as an anatomical term referring to a branch of the autonomic nervous system.
The sympathetic nervous system is distributed to almost all organs, including the heart, lungs, blood vessels, gastrointestinal tract, bladder, eyes, and sweat glands. It is continuously involved in maintaining everyday bodily functions, such as standing, exercising, and regulating body temperature, as well as in emergency situations.
anatomy
The neuron cell bodies of the sympathetic nerves are located in the thoracic and lumbar regions of the spinal cord, specifically in the intermediolateral cell column between the first thoracic vertebra (T1) and the second lumbar vertebra (L2). For this reason, the sympathetic nervous system is also called the thoracolumbar system.
Sympathetic nerve fibers from the spinal cord enter the sympathetic ganglia that line the spine. This ganglion is connected like a chain on both sides of the spine from the neck to the tailbone and is called the sympathetic trunk. There are approximately 22 to 23 ganglia on both sides of the sympathetic nerve trunk.
The sympathetic pathway consists of two stages. The first level nerve (preganglionic nerve) extends from the spinal cord to the ganglion and uses acetylcholine as a signaling substance. The second level nerve (postganglionic nerve) extends from the ganglion to the target organ and uses norepinephrine as its main signaling substance.
The exception is the adrenal medulla, which is directly connected to the preganglionic nerve without the postganglionic nerve. From the adrenal medulla, epinephrine (about 80%) and norepinephrine (about 20%) are secreted directly into the bloodstream and act throughout the body.
function
The core function of the sympathetic nervous system is to prepare the body to respond immediately in crisis situations. This is called the fight-or-flight response.
When the sympathetic nervous system is activated, the following changes occur:
- Heart: Heart rate and contractility increase, increasing blood supply
- Blood vessels: Blood vessels in the skin and digestive organs constrict, while blood vessels in skeletal muscles and the heart dilate.
- Breathing: The bronchi expand and oxygen intake increases.
- Eyes: Pupils are enlarged (mydriasis), widening the field of view.
- Metabolism: Increased release of glucose from the liver increases energy supply
- Digestion: Gastrointestinal motility and secretion of digestive juices are suppressed.
- Sweat glands: Increased sweating to regulate body temperature
This response allows the body to move quickly, focus, and use energy efficiently in crisis situations.
The sympathetic nervous system doesn't only work in emergency situations. It is continuously involved in maintaining daily homeostasis, such as maintaining blood pressure by constricting blood vessels in the lower extremities when standing, increasing heart rate during exercise, and narrowing skin blood vessels when cold.
Symptoms of sympathetic nerve abnormalities
When the activity of the sympathetic nervous system is too high or too low, various symptoms occur.
The following symptoms are common in sympathetic overactivity:
- Palpitations: Feeling that the heart is beating fast even when resting.
- Increased blood pressure: Chronic sympathetic hyperactivity is a risk factor for high blood pressure. According to studies, sympathetic nerve hyperactivity is observed in approximately 40-65% of patients with essential hypertension.
- Excessive sweating: Abnormally excessive sweating on the palms, soles, armpits, etc.
- Anxiety/Nervousness: Physical symptoms caused by hyperactivity of the sympathetic nervous system worsen anxiety.
- Insomnia: Initiating and maintaining sleep becomes difficult in a state of sympathetic dominance.
- Indigestion: When the sympathetic nervous system is hyperactive, gastrointestinal tract movement and secretion of digestive juices are suppressed.
When sympathetic nerve function declines, the following symptoms appear:
- Orthostatic hypotension: When standing up, blood vessels do not contract sufficiently, resulting in dizziness or fainting.
- Reduced sweating: Difficulty regulating body temperature, increasing the risk of heatstroke in hot environments
According to heart rate variability (HRV) studies, autonomic imbalance (sympathetic hyperactivity, parasympathetic inhibition) is significantly associated with increased cardiovascular disease mortality.
Inspection method
The main tests to evaluate sympathetic nerve function are as follows.
- Heart rate variability (HRV) analysis: Measures minute changes in heartbeat intervals to determine the balance of sympathetic and parasympathetic nerves. In HRV frequency analysis, the low frequency (LF, 0.04~0.15Hz) component is related to sympathetic nerve activity, and a high LF/HF ratio suggests sympathetic nerve dominance.
- Tilt table test: Tilt from a lying down position to a standing position and observe changes in blood pressure and heart rate. Normally, when you stand up, the sympathetic nerves should be activated, blood vessels should constrict, and your heart rate should increase appropriately.
- Valsalva maneuver: Analyze blood pressure and heart rate responses after closing the nose and mouth and exhaling forcefully. If sympathetic nerve function is normal, reflex vasoconstriction and increased heart rate are observed when blood pressure decreases.
- Blood and urine catecholamine test: Directly evaluates the level of sympathetic nerve activity by measuring norepinephrine and epinephrine concentrations.
- Sympathetic skin response (SSR): Evaluates the function of sympathetic nerve endings by measuring the sweating response to electrical stimulation.
life management
Lifestyle rules for alleviating sympathetic nerve overactivity and maintaining autonomic nervous system balance are as follows.
- Aerobic exercise: Walking, swimming, or cycling for more than 30 minutes, 3 to 5 times a week, reduces sympathetic nerve hyperactivity and improves parasympathetic nerve function. Research results have reported that 12 weeks of regular aerobic exercise significantly improves HRV.
- Abdominal breathing: Slow, deep abdominal breathing (6 times per minute) stimulates the vagus nerve and suppresses sympathetic nerve activity.
- Sleep hygiene: Regular sleep of 7 to 8 hours a day is essential for autonomic recovery. Lack of sleep is a major cause of sympathetic hyperactivity.
- Limit caffeine and alcohol: Excessive caffeine stimulates the sympathetic nervous system, and alcohol disturbs the balance of the autonomic nervous system.
- Stress management: Meditation, progressive muscle relaxation, and regular leisure activities can help prevent chronic sympathetic overactivity.
- Water intake: Sufficient water intake of 1.5 to 2 liters per day maintains blood volume and reduces the excessive compensation response of the sympathetic nervous system.
If symptoms persist or interfere with daily life, it is recommended to consult an autonomic neurologist.