Division of the Autonomic Nervous System (ANS)

The autonomic nervous system (ANS) is a division of the peripheral nervous system of the human brain. This control system of the cerebral cortex is responsible for the regulation and control of bodily functions which are mostly unconscious, such as the heart rate, breathing, digestive processes, pupillary response, urination, etc.

 

All these functions that are not under voluntary control of the brain are the reason why the autonomic nervous system is also regarded as the self-regulating system. Above them, the ANS is involved in the most instinctive function of our body – the fight-or-flight response.

What Is a Fight-or-Flight Response?

The fight-or-flight response is a biological reaction that prepares our body to face danger or run away from the source of danger. This response is of vital importance to our survival.

 

When danger lurks and threatens to harm us, this vitally important instinctive reaction is triggered by the release of hormones. The hormones make way for a rapidly developing series of reactions in our body – our heartbeat picks up, we start to breathe faster, and the entire body becomes tense and ready to act.

 

All these processes in the body are activated with a single and unique goal – to give us a burst of energy in order to prepare us to either stay and fight the source of danger in the environment or run away to safety as a response to acute stress.

What Are the Two Main Divisions of the Autonomic Nervous System?

We can distinguish three divisions of the autonomic nervous system:

 

    • Sympathetic division (SNS) – The excitatory  fight-and-flight system
    • Parasympathetic division (PSNS) – The inhibitory rest-and-digest system
  • Enteric division (ENS)

 

However, the third (enteric) division is often considered both as a part of the autonomic nervous system, and as an individual entity of the brain.

 

Sympathetic Division of the Autonomic Nervous System

The sympathetic nervous system is the quick response mobilizing system of the brain. Namely, this division of the autonomic nervous system is responsible for controlling the fight or flight response.

Where Are the Neurons of the Sympathetic Division of the ANS Synapse Located?

This division of the sympathetic nervous system stems from the spinal cord in the thoracic and lumbar areas. It comprises of preganglionic neurons that create synapses on the following locations:

 

  • Paravertebral ganglia (Including the cervical ganglia – 3, thoracic ganglia – 12, and rostral lumbar ganglia – 2 or 3)
  • Prevertebral ganglia (Including the celiac ganglion, aortic renal ganglion, superior mesenteric ganglion, inferior mesenteric ganglion)
  • Chromaffin cells of the adrenal medulla.

 

These ganglia are the location of the postsynaptic preganglionic neurons that branch toward the other brain structures:

 

  • Cervical cardiac nerves and thoracic visceral nerves (Creating synapses in the sympathetic chain)
  • Thoracic splanchnic nerves (They synapse in the prevertebral ganglia)
  • Lumbar splanchnic nerves (Synapsing in the prevertebral ganglia)
  • Sacral splanchnic nerves (Making synapses in the inferior hypogastric plexus)

Parasympathetic Division of the Autonomic Nervous System

The rest-and-digest, i.e. the parasympathetic system of the brain branches its neurons from the cranial nerves (most notably from the oculomotor nerve, facial nerve, glossopharyngeal nerve, and vagus nerve).

 

This division of the autonomic nervous system regulates and controls the involuntary responses of the body under calm and normal conditions. Hence, the parasympathetic nervous system is active during periods of digestion and rest.

Where Are the Neurons of the Parasympathetic Division of the ANS Synapse Located?

The presynaptic preganglionic neurons of the parasympathetic nervous system are located in the brainstem and the sacral spinal cord. They create synapses with the postsynaptic neurons in several locations:

 

    • Parasympathetic ganglia of the head (Ciliary – Cranial nerve III, Submandibular – Cranial nerve VII, Pterygopalatine – Cranial nerve VII, and Otic – Cranial nerve IX).
    • Cranial nerve X or Sacral nerves (S2, S3, S4)
  • The postganglionic parasympathetic splanchnic (visceral) nerves
  • The vagus nerve (Innervating the heart, lungs, liver and stomach)

Enteric Division of the Autonomic Nervous System

The enteric nervous system is also labeled as ‘the second brain’ due to its ability to function independently from the central nervous system. Located in our gastrointestinal tract, it is stimulated by the parasympathetic nervous system.

 

This division of the ANS regulates motility, secretion, and growth. It reacts swiftly to the digestive stimuli.

What Are the Functions of the Autonomic Nervous System?

In order to enable the body to adapt more easily to the continual stresses to which it is exposed in our environment, the autonomic nervous system (ANS) is largely involved in the automatic and subconscious regulation of the cardiovascular, renal, gastrointestinal, thermoregulatory, and other systems. Hence, it controls functions like:

  • Blood pressure – The autonomic nervous system controls our blood pressure to the normal levels by toning the blood vessels and regulating the excretion of fluids and heart rate. It reduces the heart and respiratory rates, thus conserving energy through relaxation and rest.
  • Heart rate – By controlling the force of contraction of the cells comprising the heart, the autonomic nervous system also regulates our heart rate, as one of the unconscious functions of the body. The heart rate also depends on blood flow, exercising, temperature, etc.
  • Blood flow – The autonomic nervous system supports the function of the other organs in the human body by increasing or decreasing the blood supply to a particular organ. It also enhances blood flow to skeletal muscles and the lungs, dilates the bronchioles of the lung in order to allow greater oxygen exchange, and increases heart rate.
  • Digestion – The process of digestion is under the control of the autonomic nervous system, especially the enteric division. Namely, once we swallow a piece of food and it reaches the pharynx, from there all processes are regulated by the ANS.
  • Breathing – The ANS controls the rate of breathing and the secretions present in the airways. It also plays a vital role in the instinctive removal of a foreign particle when it enters the airways.
  • Urination – The smooth muscles of the urethra and the bladder contractions are under the rule of the autonomic nervous center.  The enteric division again plays a big role in the regulation of these processes.
  • Pupillary response – The control system of the ANS controls the reaction of the pupils in our eyes under light. It supports how our eyes accommodate to light, as well as how they adapt to both near and far away objects.
  • Body Temperature – The autonomic nervous system regulates the body’s loss of heat by regulating the blood flow to the skin. In addition, the secretion of sweat is involved in this process, and it’s also controlled by the ANS.
  • Metabolism – This control center of the brain is directly involved in the metabolism of lipids, playing a great role in the regulation of our metabolism.

Which Organs of the Body Are Supplied With Nerve Fibers from the Autonomic Nervous System?

The autonomic nervous system provides connections between the cerebral cortex and the parts of the body, including the internal organs.

 

This list includes:

 

  • Blood vessels
  • Stomach
  • Intestine
  • Liver
  • Kidneys
  • Bladder
  • Genitals
  • Lungs
  • Pupils
  • Heart
  • Sweat glands
  • Salivary glands
  • Digestive glands
  • Eyes

What Happens If the Autonomic Nervous System Is Dysfunctional?

It’s more than obvious that if the autonomic nervous system is dysfunctional, it would have a serious impact on our overall well being.

 

The autonomic dysfunction of this control system of the brain can affect either a small part of it or the entire ANS. Here are some of the conditions that influence the proper functioning of the autonomic nervous system:

 

    • Alcohol abuse
    • Diabetes
    • Drug abuse
    • Spinal cord injuries
    • Peripheral neuropathy
  • Cancer

 

When the nerves of the autonomic nervous system are dysfunctional, some severe diseases and medical conditions can occur. The affected individual may experience either chronic or temporary and reversible conditions, such as:

 

  • Autonomic neuropathy or dysautonomia
  • Diabetes
  • Autonomic Paralysis
  • Baroreceptor failure
  • Orthostatic hypotension

  • Postural orthostatic tachycardia syndrome (POTS)

  • Multiple system atrophy (MSA)

 

The symptoms announcing the aforementioned dysfunctionalities of the autonomic nervous system include:

  1. Dizziness and fainting upon standing up.
  2. An inability to alter heart rate with exercise.
  3. Sweating abnormalities, ranging from sweating too much to not sweating enough.
  4. Digestive problems, such as loss of appetite, bloating, diarrhea, constipation, or difficulty swallowing.
  5. Urinary problems, such as difficulty starting urination, incontinence, and incomplete emptying of the bladder.
  6. Vision problems, such as blurry vision or an inability of the pupils to react to light quickly

To Sum Up

Both the sympathetic and parasympathetic divisions of the autonomic nervous system have complementary roles. Namely, the sympathetic division of the ANS regulates and controls the actions that require a fast response (like the fight-or-flight response), while the parasympathetic division of the ANS regulates the functions that do not require speedy responses (rest and digest).

Functioning in a seemingly different manner, both the SNS and PSNS as divisions of the autonomic nervous system regulate some of the most vital functions respectively, in order to achieve homeostasis.

Fun Facts

 

Did you know?

 

  1. The American physiologist Walter Cannon was the first who described the fight-or-flight response in the 1920s.
  2. Hans Selye’s general adaptation syndrome is a theory that describes our reaction to stress. Consisting of three stages – the alarm phase, stage of resistance, and stage of exhaustion, it considers the fight-or-flight reaction as a part of the first phase of the syndrome.
  3. The pupillary response is a physiological reaction that regulates the width of the pupils via the optic and oculomotor cranial nerves.
  4. The peripheral nervous system (PNS) is one of two control centers that comprise the nervous system of humans and the bilateral animals. The other part of this control system of the brain consists of the central nervous system.
  5. The sympathetic preganglionic neurons are located within the central nervous system. They originate in the lateral horns of the 12 thoracic and the first 2 or 3 lumbar segments of the spinal cord.
  6. The sacral spinal cord is the lowest part of the spinal cord, involved with the control and regulation of the bladder contractions. It’s also known as the sacral reflex center.
  7. In the sympathetic nervous system, a single motor neuron connects the central nervous system in order to target skeletal muscles. However, in the autonomic nervous system, the connection between the CNS and the cell that acts as a response to the sensory stimuli consists of two neurons – the preganglionic neuron and the postganglionic neuron.
  8. Orthostatic hypotension occurs when your blood pressure drops significantly as you stand up. This usually causes lightheadedness, fainting, and heart palpitations.
  9. The Holmes-Adie syndrome (HAS) affects the nerves controlling the muscles of the eye, causing vision problems. It’s characterized by an enlargement of only one of the pupils, while the other remains the same size.
  10. Damage to the nerves of the ANS is often irreversible.