Our nervous system mainly consists of the peripheral and central nervous systems. It is considered one of the most complex systems in our body. It collects information from the nerve, sends it through our spinal cord, and processes the data in the brain. 

The Autonomic Nervous System controls the operations of our internal organs such as the intestines, lungs, heart, and stomach without our conscious recognition. It operates involuntarily and impulsively and plays a big part during emergency situations as it triggers our “fight or flight” mode. 

The Autonomic Nervous System is composed of two antagonistic nerve sets: the sympathetic and parasympathetic nervous systems.

Sympathetic Nervous System

Pre-ganglionic and post-ganglionic are the neurons involved in signal transmission in the sympathetic system. These neurons are located in the lumbar and thoracic areas of the body, including the heart, lungs, eyes, liver, salivary gland, stomach, uterus, and kidney. It is responsible for the localized body response like sweating and faster heartbeats. 

When a person is under stress, the sympathetic nervous system triggers the “fight or flight” response that releases enormous amounts of adrenaline from the adrenal glands and causes bronchial dilation, faster heart rates, increased cardiac output, and pupillary dilation preparing our body for imminent emergencies.  

How Does the Sympathetic Nervous System Work?

When a person is placed under a lot of stress or pressure, the response starts from the brain. The primary human senses send information to the amygdala, the part of the brain that processes our emotion; the sound or the image will then be interpreted. When danger is perceived, a signal is sent to the hypothalamus, the brain’s command tower. The hypothalamus uses the nervous system to communicate the danger to other parts of the body.

The sympathetic nervous system initiates the fight or flight response that primes the body for action and bursts of energy for immediate response to perceived danger. It signals the adrenal glands to release adrenaline to the blood, glucose, and fats from our body’s temporary storages to help with the energy supply. 

Structure of the Sympathetic Nerves

• The sympathetic nerves are located at the sides of our vertebral column running laterally with our spinal cords. 
• The pre-ganglionic neurons are found in between T1-T12 and L1-L3 intermediolateral to the spinal cord. 
• The fibers of the pre-ganglionic neurons leave the spine through the rami of nerves and continue as white rami communicantes or connecting nerves.
• The fibers of the post-ganglionic neurons, gray rami communicantes, join the connecting nerves of the spinal cord. 
• C2-C8 areas of the spinal nerve carry the upper limbs, head, neck, and thorax’s sympathetic innervation.
• T1-L2 carries the trunk wall as well as the abdominopelvic viscera.
• L3-Co carries the lower limbs’ sympathetic innervation.

The Effects Caused by the Sympathetic Nerves

• Contraction of the radial muscles of the iris allows more light to enter, improving vision
• Increase of heart rate and cardiac output to supply other body organs with enough blood and oxygen
• Bronchodilation of the lungs for more airflow
• Decrease enzyme and insulin secretion by the endocrine
• Increases the renin secretion in the kidney for intravascular volume
• Sweating
• The kidney’s adrenal gland is releasing catecholamines, epinephrine, and norepinephrine for a continuous sympathetic response

Parasympathetic Nervous System

The parasympathetic nervous system assists with the body’s “rest and digestion” after the “flight and fight” response. It decreases the heart rate and respiration and prepares it for digestion. 

When the parasympathetic nerves are stimulated, it decreases the heartbeat and triggers secretions in the digestive system. Its nerve fibers are primarily the lumbar nerve of the spine and vagus nerve. 

The pre-ganglionic neurons of the parasympathetic system are found in the sacral areas of the spine’s lateral horns or the nuclei of the brain stem. These pre-ganglionic and post-ganglionic neurons secrete acetylcholine that acts as a neurotransmitter. The salivary, nasal mucous, and lacrimal glands are some of the glands controlled by the parasympathetic nerves. 

How Does the Parasympathetic Nervous System Work?

The parasympathetic nervous system, also known as the craniosacral division of the ANS, works in harmony with the sympathetic nervous system. Its function also starts with the brain and continues through long fibers that attach to the neurons near the organs they intend to act on. Thus, when the parasympathetic nerves send out signals, the neurons travel a shorter distance to their prospective organs. 

The “rest and digest” function of the parasympathetic nervous system involves the relaxation of the muscle sphincter of the urinary and gastrointestinal tracts, which increases intestinal activity. In contrast with sympathetic function, this response conserves energy and regulates normal bodily function. 

Structure of the Parasympathetic Nerves

• The parasympathetic system has presynaptic nerves found in the sacral area of the spine and the medulla oblongata. 
• These presynaptic nerves release fibers that leave the Central Nervous System and travel to the postsynaptic neurons. 
• The presynaptic fibers will then synapse or transmit electrical nerve impulses using acetylcholine as a neurotransmitter. This is also why the parasympathetic pathways are also known as cholinergic pathways. 
• Cholinergic pathways are located in the two major parts of the CNS.
• The medulla oblongata contains the presynaptic parasympathetic neurons that innervate the head and neck.
• The grey horn of the S2-S4 sacral segment of the spine contains the presynaptic parasympathetic neurons that innervate the pelvis, thorax, and abdominal viscera. 

The Effects Of The Parasympathetic Nervous System

• Increased production of saliva in the parotid glands
• Increased production of tears in the lacrimal gland
• Decreased heart rate and lower blood pressure
• Constriction of the pupils
• Increased bowel movement and gastric motility
• Contraction of the urinary bladder and urges to void

The autonomic nervous system is one of the branches of the peripheral nervous system, aside from the somatic nervous system, that is composed of the sympathetic and parasympathetic nervous systems. The autonomic nervous system regulates uncontrolled functions of the glands and internal organs. The parasympathetic nerves harmonize with our body after undergoing stress, and the sympathetic assists our body in dealing with stress.