General structure
The sympathetic system has a strictly two-neuron efferent pathway structure. The central apparatus (first neurons) is located in the lateral horns of the spinal cord—specifically the intermediolateral cell column at levels from Th1 to L2 (thoracolumbar system).
The axons of the first neurons exit the spinal cord as part of the ventral root and then branch off to form the white rami communicantes. The white color is due to heavy myelination. These fibers enter the sympathetic trunk, where they follow one of three pathways:
- Synapse with the second neuron in the ganglion of the same segment.
- Travel up or down the sympathetic trunk and synapse in a different segment.
- Pass completely through the trunk (in transit) and terminate in remote prevertebral ganglia.
The cell bodies of the second neurons are always located peripherally—either in paravertebral ganglia (sympathetic trunk) or prevertebral ganglia. Their unmyelinated processes emerge as the gray ramus communicans and head toward the target organs.
Neurotransmitters of autonomic regulation
Neurotransmitter transmission in the sympathetic nervous system has specific features. Signal transmission from the first neuron to the second (within the ganglia) is always mediated by acetylcholine.
The second (postganglionic) neurons release noradrenaline at target organs, which is why the system itself is often called adrenergic. However, there is an important exception: the second sympathetic neurons that innervate the sweat glands use acetylcholine (are cholinergic).
Innervation of the head, neck, and thorax
There are no sympathetic centers in the cervical spinal cord. Innervation of the head and neck is provided by neurons from the upper thoracic segments (Th1–Th5). Their fibers ascend along the sympathetic trunk and synapse in three cervical ganglia: the superior, middle, and cervicothoracic (stellate) ganglia.
From here, postganglionic fibers travel directly along blood vessel walls:
- The external carotid plexus (from the superior cervical ganglion) regulates the facial blood vessels, sweat glands, and arrector pili muscles.
- The internal carotid plexus controls the eye (dilates the pupil), lacrimal glands, and salivary glands.
Cardiac and pulmonary nerves branch from the cervical and upper thoracic ganglia to supply the thoracic organs (heart, bronchi, lungs).
Innervation of the abdominal and pelvic cavities
Neurons from segments Th5–Th12 form the greater and lesser splanchnic nerves (nn. splanchnici major et minor). They pass through the sympathetic trunk without synapsing and terminate in the prevertebral ganglia around the abdominal aorta:
- Celiac ganglion (ganglion celiacum);
- Superior mesenteric ganglion;
- Inferior mesenteric ganglion (receives fibers from the lumbar segments).
These ganglia innervate the entire gastrointestinal tract, liver, spleen, kidneys, and pelvic organs.
Effects on the body
Master control of the sympathetic system is exerted by the caudal hypothalamus. Signals descend via the dorsal longitudinal fasciculus of Schütz and the reticular formation.
Stimulation triggers a classic fight-or-flight response:
- Heart and blood vessels: blood pressure and heart rate increase. Blood vessels in skeletal muscles dilate, while those in the gut constrict.
- Respiration: respiratory rate increases, bronchi dilate.
- Gastrointestinal tract and excretion: peristalsis is suppressed, digestive gland secretion is inhibited, urination is delayed.
- Metabolism: blood glucose rises, adrenal epinephrine release increases, pupils dilate.
Briefly: answers to topical questions
Which branches arise from the superior cervical sympathetic ganglion?
The superior cervical sympathetic ganglion gives rise to branches that form plexuses and innervate structures of the head, neck, and heart. These include:
- External carotid plexus — formed by unmyelinated fibers spread across the external carotid artery and its branches.
- Internal carotid nerve (n. caroticus internus) — ascends into the cranial cavity.
- Superior cervical cardiac nerve (n. cardiacus cervicalis superior) — descends along the sheath of the common carotid artery.
Which ganglia are classified as the prevertebral ganglia of the abdominal cavity?
The prevertebral ganglia of the abdominal cavity are unpaired autonomic ganglia located along the abdominal aorta near the origins of its corresponding branches. These ganglia are the sites where preganglionic sympathetic fibers synapse onto postganglionic neurons. They include:
- Celiac ganglion (ganglion celiacum).
- Superior mesenteric ganglion (ganglion mesentericum superius).
- Inferior mesenteric ganglion (ganglion mesentericum inferius).
Which organs are innervated by postganglionic fibers from the inferior mesenteric ganglion?
Postganglionic fibers from the inferior mesenteric ganglion innervate the pelvic organs and distal parts of the intestine. The main target organs include:
- Large intestine (including the descending colon and rectum).
- Urinary bladder (innervating the muscular wall and smooth muscle of the internal urethral sphincter).
- Genitals (internal genital organs).
Fibers to these organs travel as part of the hypogastric plexus.
The sympathetic system is the "fight or flight" (ergotropic) response. Imagine encountering a bear: your pupils dilate from fear, your heart races, blood rushes to your muscles for running, and digestion (peristalsis) completely shuts down.
Class and exam questions
Where are the cell bodies of the first neurons of the sympathetic nervous system located?
They lie in the intermediolateral cell columns (lateral horns) of the spinal cord from the first thoracic to the second lumbar segment (Th1–L2).
What is the difference between the white communicating ramus and the gray communicating ramus?
The white ramus contains myelinated processes of first neurons running from the spinal cord to the sympathetic trunk. The gray ramus consists of unmyelinated fibers of second neurons running from the ganglion to the organs.
What neurotransmitter is released by postganglionic sympathetic fibers?
Most organs release noradrenaline (adrenergic transmission). Sweat glands are the exception — they release acetylcholine.
How does the hypothalamus control the autonomic nervous system?
The hypothalamus regulates it via neural pathways (such as conduction tracts like the fasciculus of Schütz) and hormonal pathways (hypothalamic-pituitary system). The caudal part activates the sympathetic system, while the rostral part activates the parasympathetic system.
What else to study on this topic
- Anatomy of the parasympathetic nervous system (craniosacral division)
- Hypothalamic pathways (fasciculus of Schütz, medial forebrain bundle)
- Structure of intramural ganglia of the autonomic system
- Formation of the hypogastric plexus
- Hormonal regulation: hypothalamic-pituitary-adrenal axis
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