The Sympathetic Division of the Autonomic Nervous System

To connect the spinal nerves to this sympathetic chain, the body utilizes specialized bridges called rami communicantes:

• White rami communicantes: These carry preganglionic axons from the spinal nerve into the sympathetic trunk. They are called "white" because the axons are myelinated.
• Gray rami communicantes: These carry postganglionic axons from the sympathetic trunk back out to the spinal nerve. They are "gray" because these axons lack myelin.

Exam Tip: Remember that "White" = Myelinated = Preganglionic, and "Gray" = Unmyelinated = Postganglionic.

Cervical Sympathetic Ganglia

The uppermost portion of the sympathetic trunk is divided into three distinct ganglia that provide innervation to the head, neck, and thoracic viscera:

1. Superior cervical ganglion: Distributes postganglionic axons to the head and neck.
2. Middle cervical ganglion: Extends to thoracic viscera.
3. Inferior cervical ganglion: Also extends to thoracic viscera.

The inferior cervical ganglion and the first thoracic ganglion fuse to form a single structure known as the cervicothoracic (stellate) ganglion.

Splanchnic Nerves and Prevertebral Ganglia

Not all preganglionic axons synapse within the sympathetic trunk. Some axons pass through the trunk without synapsing and continue traveling anteriorly toward the abdominal viscera. These pathways are known as sympathetic splanchnic nerves.

These splanchnic nerves typically terminate in prevertebral ganglia, which are clusters of ganglia located immediately anterior to the vertebral column. These ganglia are often located near major arteries and are named accordingly. Key prevertebral ganglia include:

• Celiac ganglion
• Superior mesenteric ganglion
• Inferior mesenteric ganglion

Specialized Pathways: The Adrenal Medulla

The sympathetic nervous system has a unique direct connection to the endocrine system via the adrenal medulla pathway. Instead of synapsing in a ganglion, certain preganglionic axons travel directly to the internal region of the adrenal gland.

When these preganglionic neurons are stimulated, they trigger the cells within the adrenal medulla to release large amounts of epinephrine and norepinephrine directly into the bloodstream. This allows the sympathetic response to become a systemic, body-wide event, rapidly increasing heart rate and mobilizing energy during stress.

Summary of Sympathetic Pathways

To visualize how these different routes function, we can categorize them into three main patterns: