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Autonomic Nervous System  
Overview 
The autonomic nervous system is a very complex multifaceted neural network that maintains 
internal physiologic homeostasis. This network includes cardiovascular, thermoregulatory, 
gastrointestinal (GI), genitourinary (GU), and ophthalmologic (pupillary) systems. Given the 
complex nature of this system, a stepwise approach to autonomic disorders is required for 
proper understanding. 
The goal for this chapter remains focused at step III on the anatomy of the autonomic nervous 
system.  
•  Step I: Understand the reason for testing 
•  Step II: Recognition and etiology (especially small fiber neuropathy) 
•  Step III: Understand basic anatomy and neurophysiology 
•  Step IV: Learn the methods for testing 
•  Step V: Diagnosis and management 
Almost 10% of the population (or over 30 million people in the United States) may acquire 
an autonomic disorder requiring medical attention. The autonomic nervous system is a 
versatile neural network maintaining internal physiologic homeostasis. Disorders of 
autonomic nervous system can be present with both central as well as peripheral nervous 
system localization. 
The image below depicts the autonomic nervous system. 
Autonomic nervous system.  
The etiology of autonomic dysfunction can be primary or idiopathic and secondary causes. 
Autonomic failure is seen in multiple system atrophy, pure or progressive autonomic failure, 
Parkinson and other neurodegenerative diseases, metabolic diseases such as Wernicke and 
cobalamin deficiency, diabetes mellitus, hyperlipidemia, trauma, vascular diseases, neoplastic 
diseases, and multiple sclerosis. In addition, autonomic dysfunction is associated with various 
medications. 
In addition to diabetes, autonomic dysfunction is associated with other neuropathies, 
including Guillain-Barr é syndrome, Lyme disease, HIV infection, leprosy, acute idiopathic 
dysautonomia, amyloidosis, porphyria, uremia, and alcoholism. Besides nerve localization in 

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the peripheral nervous system, it occurs in diseases of the presynaptic neuromuscular junction

such as botulism and myasthenic syndrome.

In addition to the acquired causes, inherited disorders like hereditary sensory-autonomic

neuropathy (HSAN), familial amyloid polyneuropathy (FAP), Tangier disease, and Fabry

disease also exist.

Clinically, postural lightheadedness, dry mouth, dry eyes, impotence, loss of sweating or

hyperthermia, nocturnal diarrhea, gastroparesis, impaired accommodation, urinary or bowel

incontinence, and small fiber neuropathy (SFN) are some of the presenting symptoms. Most

peripheral neuropathies affect all fiber sizes. Few peripheral neuropathies are associated with

pure or predominantly small-fiber involvement. A large proportion are associated with

diabetes. Painful burning feet is caused by a sensory neuropathy with small-fiber involvement

in more than 90% of cases. Patients with pure small-fiber involvement display normal large

fiber function. Muscle bulk, strength, muscle stretch reflexes, and large fiber sensory function

(ie, vibration, proprioception) are normal.

Small fibers are both myelinated and unmyelinated. Small myelinated fibers transmit

preganglionic autonomic efferents (B fibers) and somatic afferents (A delta fibers).

Unmyelinated (C) fibers transmit postganglionic autonomic efferents as well as somatic and

autonomic afferents. Both A delta and C fibers are widely distributed in skin and deep

tissues. The neurotransmitter for preganglionic sympathetic and parasympathetic nervous

system as well as postganglionic parasympathetic nervous system is acetylcholine. The

neurotransmitter for the postganglionic sympathetic nervous system (innervating sweat

glands) is acetylcholine, whereas remaining postganglionic sympathetic nervous system is

norepinephrine.

Electromyography plays a key role in the evaluation of most peripheral neuropathies and

helps in assessing only large myelinated fibers. Thus, pure SFNs may be associated with

normal findings on routine electro-physiologic studies. Elderly patients who lack sural

sensory responses can still be diagnosed with small-fiber neuropathy. Patients with symptoms

other than neuropathic certainly need autonomic function testing for appropriate diagnosis.

Gross Anatomy

Central integration

The central autonomic network is a complex network in the central nervous system

integrating and regulating autonomic function. The network involves cerebral cortex (the

insular and medial prefrontal regions), amygdala, stria terminalis, hypothalamus, and

brainstem centers (periaqueductal gray, parabrachial pons, nucleus of the tractus solitarius,

and intermediate reticular zone of the medulla).

[1]

Efferent pathways

The autonomic nervous system consists of the sympathetic and parasympathetic nervous

system. The sympathetic nervous system descends to the intermediolateral and

intermediomedial cells in the thoracolumbar regions of the spine, extending from TI to L2.

Preganglionic axons exiting the spinal cord enter the white rami communicantes to join a

network of prevertebral and paravertebral ganglia. They are relatively short, myelinated, and

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Autonomic Nervous System Overview The autonomic nervous system is a very complex multifaceted neural network that maintains internal physiologic homeostasis. This network includes cardiovascular, thermoregulatory, gastrointestinal (GI), genitourinary (GU), and ophthalmologic (pupillary) systems. Given the complex nature of this system, a stepwise approach to autonomic disorders is required for proper understanding. The goal for this chapter remains focused at step III on the anatomy of the autonomic nervous system. • • • • • Step I: Understand the reason for testing Step II: Recognition and etiology (especially small fiber neuropathy) Step III: Understand basic anatomy and neurophysiology Step IV: Learn the methods for testing Step V: Diagnosis and management Almost 10% of the population (or over 30 million people in the United States) may acquire an autonomic disorder requiring medical attention. The autonomic nervous system is a versatile neural network maintaining internal physiologic homeostasis. Disorders of autonomic nervous system can be present with both central as well as peripheral nervous system localization. The image below depicts the autonomic nervous system. Autonomic nervous system. The etiology of autonomic dysfunction can be primary or idiopathic and secondary causes. Autonomic failure is seen in multiple system atrophy, pure or progressive autonomic failure, Parkinson and other neurodegenerative diseases, metabolic diseases such as Wernicke and ...
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