The sympathetic nervous system is a subset of the autonomic nervous system that stimulates the body's "fight or flight" response to stressful situations, though processes in this system are almost always active to maintain homeostasis. Due to its role as a direct mediator between the central nervous system and peripheral organs, researchers have hypothesized in its involvement in human pathophysiology and immunity. 
- Sensory nerves innervate the lungs to regulate various cardiopulmonary functions, lending to the hypothesis of its ability to modulate the severity of respiratory diseases.

The image below illustrates sympathetic nerve innervation of their target organs. 

Sympathetic Nervous System

The Enteric Nervous System is a subset of the Autonomic Nervous System. It controls the gastrointestinal functioning. It is also called the " second brain" as it can function independently of the brain and spinal cord, although it can be influenced by them. Being imbedded in the gastrointestinal tract from the oesophagus to the anus, it controls the motor functioning of the G.I tract along with controlling the secretions of the gastrointestinal enzymes. The neurons of ENS are embedded into two types of ganglia: Myenteric Plexus and Submucosal  Plexus. The myenteric plexus is embedded between the inner and outer layer of muscularis externa (layer of smooth muscles). The submucosal plexus is embedded in the submucosal layer. 

Enteric Nervous System

After a perceived threat has been resolved, the parasympathetic nervous system becomes active, initiating restorative processes that return the body to a relaxed state of equilibrium, often called the 'rest-and-digest' response. This system counteracts the fight-or-flight response by normalizing bodily functions. For example, it causes heart rate and blood pressure to decrease to normal levels, pupils to constrict, and bladder control to be restored. Additionally, it promotes energy conservation by stimulating the liver to store glucose in the form of glycogen for future use.

Parasympathetic Nervous System

The autonomic nervous system (ANS) is a subset of the peripheral nervous system. The word "autonomic" describes its role in regulating involuntary and unconscious bodily function, such as heart rate, respiratory rate, and sexual arousal. The system consists of motor neurons (ganglia) and its axonal connections, and is functionally divided into three components:
- sympathetic nervous system
- parasympathetic nervous system 
- enteric nervous system

Smith College

The peripheral nervous system are all nerve components (nerve, ganglia) outside of the brain and spinal cord. The word "peripheral" describes its function in connecting limbs and organs to the central nervous system. It is functionally divided into two components:
- somatic nervous system
- autonomic nervous system

Peripheral Nervous System

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Some COVID-19 patients have presented with peripheral neurological symptoms. Although it is not a common complication of COVID-19, it is worth investigating all possible causes of the neurological symptoms that arise.

Effects of COVID-19 on the Peripheral Nervous System

Autonomic Nervous System

The somatic nervous system is a part of Peripheral Nervous System. It controls the voluntary functions of the body through skeletal muscles. There are two types of nerves in the somatic nervous system: afferent (sensory) nerves and efferent (motor nerves). Afferent nerves relay information from the body to the central nervous system. It primarily relays sensory information like pain. smell, etc. Efferent nerves relay information from the central nervous system to the body. Somatic Nervous System consists of two parts: cranial nerves and spinal nerves. Cranial nerves are responsible for carrying information to and fro from the brain stem. Spinal nerves carry information to and fro from the spinal cord. 

Somatic Nervous System

Affective scientists can measure the peripheral nervous system to make inferences about one's emotional state. For example, they can measure eye movements, blood pressure and volume, heart rate, and electrodermal activities to infer one' emotional states (e.g. fear, anxiety, anger, etc.)

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