Sympathetic Nervous System: A guide to understanding how it works

Our bodies rush with adrenaline and we get into fight-or-flight mode, which is the activation of our sympathetic nervous system. But what is it? How does it work? How does it affect our bodies? Our brains? Our organs? What disorders can derive from our sympathetic nervous system? This article will explain it all!

Sympathetic Nervous System
Sympathetic Nervous System

What is a sympathetic nervous system?

Our nervous system consists of three nerve types: sensory nerves, motor nerves, and autonomic nerves. Sensory nerves allow us to hear, see, smell, taste and feel. Motor nerves allow our brain to command and order our muscles to move (or not move). Autonomic nerves help regulate our internal organs and how they function. The autonomic nervous system (ANS) is divided into the sympathetic nervous system, shortened to SNS, and the parasympathetic nervous system, shortened to PNS.

Merriam-Webster defines it as, “the part of the autonomic nervous system that contains chiefly adrenergic fibers and tends to depress secretion, decrease the tone and contractility of smooth muscle, and increase heart rate”.

Essentially, the sympathetic nervous system activates what is known as the flight or fight response by going through a series of connected neurons.

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In the 18th century, a man named Jacob B. Winslow took the concept of sympathy and applied the word “sympathetic” to nerves and our nervous system. However, prior to that, the word was coined as a medical term to mean “the connection between parts” from a Greek philosopher known as Aelius Galenus or Claudius Galenus. It comes from the Greek words “sun” (with) and “pathos” (feeling).

What is the difference between the sympathetic nervous system and the parasympathetic?

Both the sympathetic and parasympathetic are complementary to each other. Both originate in the spinal cord and then they branch out. People aren’t really able to self-control the parasympathetic nervous system. However, they can have some sort of control over their sympathetic nervous system responses (like exercise and experience). While the sympathetic system is working, the parasympathetic system won’t and vice versa. That is to say, they don’t function at the same time.

The sympathetic nerves are nerves in our autonomic nervous system that are associated with the fight-or-flight response. They prepare our bodies to react to stress like threat or injury. Our muscles contract and our heart races.

The parasympathetic nerves are nerves in our autonomic nervous system that help us with functions like sleeping and digestion. It stimulates our body to “rest and digest”. It helps us maintain a homeostasis, and equilibrium, within the body. Our muscles relax and our heartbeat decreases- the opposite of the sympathetic nerves.

When faced with danger, our body will divert blood flow from our parasympathetic nerve functions, like digestion, to the sympathetic nerve functions, like heavy breathing and muscle contractions.

What is the function of the sympathetic nervous system?

The function of the sympathetic nervous system is simple:

The sympathetic neurons that are found in the spinal cord are called preganglionic neurons or presynaptic neurons while peripheral sympathetic neurons are called postganglionic neurons or postsynaptic neurons. The sympathetic neurons that are in the spinal cord (which make up part of the central nervous system) communicate with the peripheral sympathetic neurons through a series of sympathetic ganglia, the motor neurons, to be able to join sympathetic neurons together through chemical synapses.

The synapses that happen within the sympathetic ganglia work together with the preganglionic neurons that release acetylcholine, a chemical neurotransmitter that binds together and activates nicotinic acetylcholine receptors.

As a response to the stimulus, the postganglionic neurons mostly release noradrenaline, also called norepinephrine. Noradrenaline is an organic neurotransmitter. If it’s activated long enough, our brains release adrenaline in the adrenal medulla. When noradrenaline and adrenaline are released at the same time, they bind together with the adrenergic receptors in our peripheral tissues. This binding causes what we know as the fight-or-flight response.

sympathetic nervous system
Sympathetic Nervous System

How does a sympathetic nervous system affect the body?

Although stress works differently in every person, whenever everyone’s bodies get stressed, the hypothalamus sends signals to our autonomic nervous system and the pituitary gland begins to process epinephrine, also known as adrenaline, and cortisol, also known as the stress hormones. When our body receives these signals, it immediately puts forth all of its energy resources to fighting off a life threat or fleeing from the enemy. These hormones can cause an increased heartbeat, dilation of the blood vessels in the arms and legs, a changed digestive process, and an increase in glucose levels within the bloodstream.

Some overall effects on our bodies from our fight-or-flight response can be pupil dilation, increased heart rate, sweating, and increased blood pressure. However, it can be incredibly damaging to our bodies if our sympathetic nervous system is frequently activated. For example, men can have a decrease in testosterone levels and woman may have irregular menstrual cycles. A lowered and weaker immune system and fatigue are also possible.

How does the sympathetic nervous system affect the brain?

If you think of the brain as a computer that can control all of our bodily functions, the nervous system is the network that relays messages back and forth from the brain to the different parts of the body via the spinal cord. Our autonomic nerves travel to our organs and receive the parasympathetic signals from the vagus nerve and the sympathetic signals from the splanchnic nerves. From there, they reach the spinal cord which connects back to the brain. When our flight or fight mode is activated, our brain begins to send neurotransmitters throughout the rest of the body in order to notify it of incoming danger. These neurotransmitters act on the adrenergic receptors. Acetylcholine is released as a preganglionic neurotransmitter. At the adrenal medulla, epinephrine (adrenaline) is released into the bloodstream which acts on adrenoceptors. Adrenoceptors act as a mediator for the central and peripheral actions of the neurotransmitter.

How does the sympathetic nervous system affect our organs?

  • Our greater splanchnic nerves affect our stomach, duodenum, jejunum and the ileum, and the spleen.
  • Our celiac plexus affects out gallbladder and our liver.
  • Our lesser/least splanchnic nerves affect the colon.
  • Our thoracic splanchnic nerves affect the pancreas.
  • The nerves to the superior mesenteric plexus affect the appendix.
  • The thoracic and lumbar splanchnic nerves affect the kidneys and the ureters.

How does the sympathetic nervous system affect our hormones?

In our body, there are two major systems that control our hormones– the endocrine system and the nervous system. Hormones are essentially chemical messengers that are carried throughout the bloodstream to different cells within the body. There are three groups of hormones: the polypeptide hormones, an amino acid group, and a steroid group. Each of these hormones has their time, place, and effect within the body. Within the nervous system, the most essential region to look at in regard to hormones is the pituitary gland– found at the base of the brain and is attached to the hypothalamus.

How does the sympathetic nervous system affect stress?

In our adrenal glands, located near the kidneys, the hypothalamus produces cortisol and the adrenal medulla produces epinephrine. This begins the process that gives your body the energy to run from danger.

When cortisol and epinephrine are released, our liver begins to produce glucose, a blood sugar that gives us the energy for our “fight or flight” mode. The majority of us, if we don’t use this extra energy, our body is able to reabsorb the blood sugar. However, for some people, especially those who are prone to Type 2 diabetes, this extra blood sugar can provoke diabetes because they aren’t able to reabsorb the glucose. This is why many people with diabetes are told to keep their stress down and maintained.

sympathetic nervous system
Sympathetic Nervous System

Health issues and disorders that come from the sympathetic nervous system

Some disorders that come from the sympathetic nervous system are:

  • Reflex Sympathetic Dystrophy, known as RSD, is what happens when the nerves in our hands or arms are injured and thus, they become overactive. The cause is unclear, but it is thought by the American Academy of Orthopaedic Surgeons that it comes from injured nerves from an injury, surgical complications, or infections. It can be detected by a feeling of burning or warmth in the arm, swelling, or even discoloration.
  • Parkinson’s disease is a disorder that occurs in our nervous system, although the causes are unclear. What happens is that the brain produces inadequate amounts of dopamine. It’s mainly associated with tremors, slowed movement, or loss of balance/control. It was reported by the National Institute of Neurological Disorders and Stroke that Parkinson’s disease causes damage to the sympathetic nerves to the heart. Parkinson’s can lead to low blood pressure, also known as hypotension.
  • Diabetes can cause damage to the nerves that run throughout the body. Damage to these nerves due to diabetes is known as diabetic neuropathy. It commonly happens to people who have a long history with insufficiently untreated diabetes. It can disrupt the cardiovascular system’s ability to regulate our blood flow and the heart’s ability to contract.

Do you have any questions regarding the sympathetic nervous system? Let us know below!

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