Comprehensive Lecture: Autonomic Nervous System Pathways

By SWA

Introduction

The autonomic nervous system (ANS) controls involuntary physiological functions of the body, including heart rate, digestion, respiration, pupillary response, urination, and sexual arousal. It operates through two primary divisions:

  • Sympathetic nervous system (SNS) – mobilizes the body’s resources for “fight or flight”

  • Parasympathetic nervous system (PNS) – conserves energy and promotes “rest and digest”

Both divisions use efferent (motor) pathways to control organs, and both also include afferent (sensory) pathways that carry information from the organs back to the central nervous system (CNS).

Structure of Autonomic Pathways

Autonomic motor pathways always involve a two-neuron chain:

  1. Preganglionic neuron – cell body in the CNS (either brainstem or spinal cord), axon synapses in an autonomic ganglion

  2. Postganglionic neuron – cell body in the ganglion, axon extends to the target organ

The difference between sympathetic and parasympathetic pathways lies in where the preganglionic neurons originate and where the ganglia are located.

Parasympathetic Nervous System (PNS)

Origin

Parasympathetic fibers arise from two main areas:

  • Cranial region: Cranial nerves III, VII, IX, and especially X (vagus nerve)

  • Sacral region: Spinal cord levels S2–S4

This distribution is called craniosacral outflow.

Pathways

Cranial outflow:

  • The vagus nerve (cranial nerve X) is the most important parasympathetic nerve.

  • It carries both efferent (motor) fibers to thoracic and abdominal organs and afferent (sensory) fibers from those organs back to the brain.

  • The vagus nerve originates from the brainstem, travels down through the neck and thorax, and branches extensively in the abdomen.

At the level of the esophageal plexus:

  • The left vagus nerve becomes the anterior vagal trunk

  • The right vagus nerve becomes the posterior vagal trunk

This rearrangement is anatomical and does not imply any change in function or "switching" of sides. Parasympathetic control remains bilateral and is not confined to one side of the body.

Sacral outflow:

  • Preganglionic neurons from S2–S4 travel via the pelvic splanchnic nerves to pelvic organs (bladder, rectum, reproductive organs).

  • These synapse in ganglia close to or within the organ walls.

Characteristics

  • Preganglionic fibers are long

  • Postganglionic fibers are short

  • Ganglia are located near or within the target organs

  • Functions include:

    • Slowing heart rate

    • Stimulating digestion

    • Promoting glandular secretion

    • Enhancing urinary and sexual function

Parasympathetic Afferents

Approximately 80% of the vagus nerve is made up of afferent (sensory) fibers. These convey information such as:

  • Organ stretch (e.g., stomach fullness)

  • Blood pressure

  • Blood chemistry

These sensory signals travel from the organs to the brainstem, where they help regulate autonomic reflexes.

Sympathetic Nervous System (SNS)

Origin

Sympathetic fibers originate exclusively from the thoracolumbar spinal cord, specifically levels T1 to L2. This is called thoracolumbar outflow.

Pathways

Sympathetic efferent fibers follow one of three general routes after exiting the spinal cord:

1. To body wall and limbs (sweat glands, blood vessels, arrector pili muscles)

  • Preganglionic neuron exits the spinal cord via the ventral root

  • Enters the sympathetic chain (paravertebral) ganglia through the white ramus communicans

  • Synapses in the ganglion

  • Postganglionic fiber exits via the gray ramus communicans, rejoins spinal nerve, and travels to target structures in the skin

This pathway regulates thermoregulation and peripheral vasoconstriction.

2. To thoracic organs (heart, lungs)

  • Preganglionic fibers enter the sympathetic chain and synapse at levels T1–T4

  • Postganglionic fibers exit directly from the ganglia as cardiopulmonary splanchnic nerves

  • These go directly to the heart and lungs, increasing heart rate and bronchodilation

3. To abdominal and pelvic organs

  • Preganglionic fibers enter the sympathetic chain but do not synapse there

  • They exit the chain as thoracic splanchnic nerves (greater, lesser, and least)

  • These fibers synapse in prevertebral (collateral) ganglia, such as:

    • Celiac ganglion

    • Superior mesenteric ganglion

    • Inferior mesenteric ganglion

  • Postganglionic fibers then follow arteries to reach abdominal and pelvic organs

These fibers regulate:

  • Inhibition of gastrointestinal motility

  • Vasoconstriction in abdominal organs

  • Adrenal medulla stimulation (which releases epinephrine and norepinephrine into the blood)

Characteristics

  • Preganglionic fibers are short

  • Postganglionic fibers are long

  • Ganglia are located near the spinal cord (in sympathetic chain or prevertebral ganglia)

  • Functions include:

    • Increasing heart rate and blood pressure

    • Dilating pupils and airways

    • Inhibiting digestion and urinary activity

    • Mobilizing energy (e.g., glycogen breakdown)

Sympathetic Afferents

Sensory information from visceral organs (particularly pain) travels alongside sympathetic pathways. These include:

  • Ischemia (reduced blood flow)

  • Stretch (e.g., from obstruction or inflammation)

  • Chemical irritation

These afferents enter the spinal cord at the same levels (T1–L2) where sympathetic efferents originate.

This creates referred pain — where visceral pain is perceived as somatic pain:

  • Heart pain enters the spinal cord at T1–T4, which also receives sensory input from the left chest and arm — hence left arm pain during a heart attack

  • Gallbladder pain may be referred to the right shoulder or scapula


Conclusion

The autonomic nervous system is a vital part of human physiology, continuously balancing internal processes to adapt to changing external and internal environments. The parasympathetic system promotes rest, digestion, and conservation of resources, primarily through the vagus nerve and sacral outflow. The sympathetic system prepares the body for action, distributing its fibers widely through the body via complex routes involving sympathetic chains and prevertebral ganglia.

Both systems also contain afferent sensory fibers, which provide the CNS with critical feedback from internal organs. While parasympathetic afferents are more involved in non-painful sensations, sympathetic afferents often transmit visceral pain, giving rise to clinically important phenomena like referred pain.

Understanding these pathways is crucial for clinical diagnosis, surgical planning, and appreciating the integrative function of the nervous system in health and disease.

Reference: 

Path of the vagus nerve (anatomy)
https://www.youtube.com/watch?v=rFZcW-KrRuU

© 2000-2025 Sieglinde W. Alexander. All writings by Sieglinde W. Alexander have a fife year copy right. Library of Congress Card Number: LCN 00-192742

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