Central and peripheral structures
Anatomically, the sympathetic division (Pars sympathica) is strictly divided into the central apparatus and the peripheral part.
The bodies of the first (preganglionic) neurons are localized exclusively in the lateral horns of the spinal cord. This innervation zone spans segments from the first thoracic (T1) to the second or third lumbar (L2/L3). Due to this nuclear localization, the sympathetic nervous system has a thoracolumbar outflow.
The peripheral part of the system is represented by two main groups of nerve ganglia:
- Sympathetic trunk (Truncus sympathicus). It is a long chain of paravertebral (along the vertebral column) ganglia. These ganglia are located symmetrically on both sides of the spinal column along its entire length.
- Prevertebral ganglia. These are ganglia located further away from the spine, predominantly in the abdominal cavity. The most significant ones include the celiac plexus ganglia and mesenteric ganglia.
Fiber morphology and exceptions
The most important morphological criterion of the sympathetic division is the difference in length of nerve pathways:
- Preganglionic fibers are short. Their task is to cover only a short distance from the spinal cord to the nearest sympathetic trunk ganglion or prevertebral ganglion.
- Postganglionic fibers, by contrast, are long. They must travel a considerable distance from the peripheral ganglion directly to the target organ to transmit the signal.
There is one critical exception in this system—the innervation of the adrenal medulla. The preganglionic sympathetic fiber heads directly to the adrenal gland. It neither interrupts nor synapses on a second neuron in either the sympathetic trunk or the prevertebral ganglia. In this case, the adrenal medulla itself functionally replaces the postganglionic neuron, releasing its secretion in response to an impulse.
Topography of segmental innervation
Sympathetic influences are distributed to target organs depending on the level of fiber origin:
- Head and neck: Innervation is provided by fibers from the upper thoracic segments that synapse in the superior cervical ganglion. From there, signals travel to the eye (causingmydriasis), as well as to the lacrimal and salivary glands (inhibiting secretion and altering saliva composition).
- Thoracic cavity: Sympathetic fibers reach the heart, where their activation increases heart rate and contractility. In the lungs, sympathetic stimulation causes bronchodilation.
- Abdominal cavity: Fibers pass through in transit and synapse in the celiac and mesenteric ganglia. They innervate the kidneys and gastrointestinal organs—the stomach, liver, pancreas, and intestines. The result is inhibition of peristalsis and a decrease in digestive secretion.
- Pelvic region: Impulses originate from the lumbar segments. They control the reproductive organs and urinary bladder. In the bladder, the sympathetic system relaxes the detrusor muscle while contracting the sphincter, thereby preventing urination.
Briefly: answers to topical questions
From which specific segments of the spinal cord is the sympathetic innervation of the head and neck derived?
According to sources, for the head and neck in general: sympathetic innervation originates from the upper thoracic segments via the superior cervical ganglion.
Specific segments are given for the sympathetic pathway to the eye: the lateral horns of C8 and Th1–Th2 form the "ciliospinal center", after which preganglionic fibers proceed to the superior cervical sympathetic ganglion.
What neurotransmitters are released in sympathetic ganglia and at target organs?
Acetylcholine is released in sympathetic ganglia. At target organs, the main neurotransmitter is norepinephrine (accounting for 90% of released catecholamines, along with epinephrine at 7%, dopamine at 3%, and co-transmitters).
Exceptions include:
- Sweat glands and skeletal muscle blood vessels — innervated by fibers releasing acetylcholine.
- Gastrointestinal tract — contains serotonergic fibers.
What types of connecting branches join and leave the sympathetic trunk?
Two types of communicating branches (rami communicantes) join and leave the sympathetic trunk:
- White communicating branches (rr. communicantes albi) — contain preganglionic myelinated fibers, originate from spinal nerves Th1–L2, and run to the sympathetic trunk ganglia.
- Gray communicating branches (rr. communicantes grisei) — contain postganglionic unmyelinated fibers, originate from all sympathetic trunk ganglia, and return to the spinal nerves.
To easily remember fiber lengths, visualize the anatomical picture: the sympathetic trunk lies directly adjacent to the spine. Therefore, a preganglionic fiber from the spinal cord only needs to travel a "short" path to the ganglion, whereas a postganglionic fiber faces a "long" journey to the organ itself.
Class and exam questions
Where are the cell bodies of the first neurons of the sympathetic system localized?
They are located in the lateral horns of the spinal cord strictly at the level of the T1 to L2/L3 segments.
How is the adrenal medulla innervated?
Preganglionic fibers approach it directly without synapsing in ganglia. The medullary tissue itself functionally acts as a postganglionic neuron.
What type of outflow is characteristic of the sympathetic division?
The sympathetic system features a thoracolumbar outflow.
What else to study on this topic
- Structure and topography of the sympathetic trunk (Truncus sympathicus)
- Abdominal prevertebral ganglia and the celiac plexus
- Sympathetic regulation of visual organs and salivary glands
- Effect of the sympathetic division on the smooth muscle of the urinary bladder
- Functional connection of the adrenal medulla with the nervous system
Master the entire topic in the "Physiology" course
This is a summary. The course covers everything tested in classes and exams.
- Comprehensive study guide on "Morphology of the Sympathetic Nervous System" and 4,128 more pages on the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 114 video lessons on the subject
Other topics in the «Autonomic Nervous System» section
Free video lessons: PhysiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.