Learning objective: The sympathetic nervous system (SNS) is one of the two primary divisions of the autonomic nervous system (ANS). It works in conjunction with the parasympathetic nervous system (PNS) to maintain homeostasis. The SNS dominates during periods of stress, preparing the body for the “fight or flight” response.

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Components of the Sympathetic Nervous System

 

Component	Description
Pre-ganglionic fibers	Short, myelinated fibers originating in the thoracic and lumbar spinal cord. Release acetylcholine.
Peripheral ganglia	Synapse sites for pre- and postganglionic fibers.
Post-ganglionic fibers	Long, unmyelinated fibers carry signals to target organs. Release noradrenaline or acetylcholine.

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Pre-Ganglionic Neurons

Pre-ganglionic neurons originate in the thoracic (T1-T12) and lumbar (L1-L2) spinal cord segments: Pathways within the sympathetic trunk:

1. Synapse at the same level
2. Ascend or descend to other levels
3. Pass without synapsing (to prevertebral ganglia or adrenal medulla as splanchnic nerves)

Key Features

1. Short and myelinated
2. Exit the spinal cord via the anterior spinal nerves
3. Enter the sympathetic trunk via the white ramus communicantes

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Peripheral Ganglia

Peripheral ganglia include the sympathetic trunks and the prevertebral ganglia:

Ganglia	Location	Function
Sympathetic trunks	Alongside the vertebral column	Relay signals between pre- and postganglionic fibers.
Prevertebral ganglia	Anterior to abdominal aorta	Control abdominal and pelvic organs.

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Regions of the Sympathetic Chain

Region	Number of Ganglia
Cervical	3 (superior, middle, inferior)
Thoracic	11–12
Lumbar	4
Sacral	4–5
Coccygeal	1 (ganglion impar)

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Postganglionic Neurons

Postganglionic neurons originate in the peripheral ganglia and transmit signals to target organs.

• Long and unmyelinated
• Release noradrenaline (except at sweat glands and erector pili muscles, where they release acetylcholine).

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Sympathetic Actions

The sympathetic nervous system affects multiple systems:

System	Effect
Eyes	Pupil dilation
Skin	Sweat secretion, erector pili contraction
Cardiovascular	Increased heart rate, vasodilation in muscles, vasoconstriction in the GI tract
Respiratory	Bronchodilation
Digestive	Decreased motility, glycogenolysis, and gluconeogenesis
Urinary	Renin secretion, bladder relaxation
Reproductive	Ejaculation in males, uterine effects in females, depending on pregnancy

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Clinical Correlation: Autonomic Dysreflexia

Definition: An exaggerated sympathetic response in spinal cord injury (SCI) patients above the T6 level.

Stimulus	Consequence
Bladder distension	Unchecked sympathetic reflex leads to widespread vasoconstriction and hypertension.
Faecal impaction	Afferent signals cannot pass the SCI level, causing unopposed sympathetic activation.

Key Features

• Sustained hypertension
• Risk of stroke
• Requires immediate medical attention

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Points to Remember

1. The fight-or-flight response is mediated by the sympathetic nervous system.
2. Pre-ganglionic fibers are short and release acetylcholine; post-ganglionic fibers are long and release noradrenaline.
3. Clinical emergencies like autonomic dysreflexia illustrate the significance of SNS regulation.
4. The sympathetic chain extends from the skull base to the coccyx, organized into distinct regions.

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