Sensory (afferent) link
The receptor apparatus of the sympathetic division is embedded directly in the walls of blood vessels and internal organs. From there, the impulse travels along dendrites that run as part of mixed nerves (including cranial). The cell bodies of these primary sensory neurons are located in spinal ganglia or the ganglia of corresponding cranial nerves.
The parasympathetic arc uses pseudounipolar neurons at this stage. Their cell bodies are also localized in sensory ganglia along nerve trunks. Information is transmitted from target organs in two directions:
- To the brain: as part of cranial nerves, primarily via the vagus nerve (n. vagus).
- To the spinal cord: via the pelvic nerves to the sacrococcygeal segments.
Central (associative) link
The central apparatus is where associative neuron cell bodies are located and where the control center for autonomic functions resides.
In the sympathetic arc, this center has a strict segmental localization. The neurons lie in the lateral horns of the spinal cord in the thoracolumbar region—encompassing all thoracic and the upper three lumbar segments (from $T_1$ to $L_{III}$). The axons of these neurons form myelinated preganglionic fibers. They emerge from the spinal cord together with the ventral roots and then branch off as white communicating rami. Their destination is the sympathetic trunk, where they either synapse in ganglia or pass through further.
Parasympathetic centers are divided into two distant poles:
- Cranial division: located in the brainstem (midbrain and medulla oblongata) and connected to the nuclei of the III, VII, IX, and X cranial nerve pairs.
- Sacral division: occupies the intermediate zone of the sacral and coccygeal segments of the spinal cord ($S_{II}—Co_{III}$).
Preganglionic parasympathetic fibers form very long pathways. They extend all the way to the effector ganglion, often transitioning from one nerve to another along the way.
Effector link and recurrent pathways
Efferent neurons receive signals from the associative center and transmit them to the effector organ.
In the sympathetic system, effector neuron cell bodies can be located at three levels:
- In the ganglia of the paired sympathetic trunk (paravertebrally).
- In the ganglia of sympathetic plexuses (prevertebrally).
- In intra-organ ganglia (intramurally) — although this is least characteristic of the sympathetic system.
Unmyelinated postganglionic fibers depart from these neurons. The bulk of them form nerves supplying visceral organs. However, a recurrent pathway to the "soma" also exists. Some fibers form gray communicating rami that return to somatic nerves. They are responsible for the innervation of blood vessels, sweat glands, and smooth muscles of the skin (arrector pili muscles).
In the parasympathetic system, effector ganglia are arranged differently: they lie either in specific ganglia near the target organ or directly within it (intramurally). It is within intra-organ ganglia that the parasympathetic system anatomically intersects with the enteric (metasympathetic) nervous system. Due to this close proximity of ganglia to the effector organ, axons of parasympathetic effector neurons are always short.
Briefly: answers to topical questions
What neurotransmitters mediate signal transmission in ganglia and at target organs?
Signal transmission in autonomic ganglia and at target organs is mediated primarily by acetylcholine and noradrenaline.
- Acetylcholine is the main neurotransmitter in ganglia (ensuring transmission from preganglionic to effector neurons in both sympathetic and parasympathetic systems) and also acts on the effector organ for most parasympathetic fibers.
- Noradrenaline serves as the neurotransmitter at the effector organ for sympathetic postganglionic fibers.
Other neurotransmitters can include serotonin, ATP, and amino acids.
White rami are the "entrance" to the sympathetic trunk (preganglionic myelinated), while gray rami are the "exit" back to the soma (postganglionic unmyelinated).
Class and exam questions
Where are the cell bodies of central sympathetic neurons located?
In the lateral horns of the spinal cord at the level of all thoracic and the upper three lumbar segments ($T_1$–$L_{III}$).
What do recurrent sympathetic fibers (gray rami) innervate?
They return to somatic nerves and innervate somatic structures: blood vessels, sweat glands, and arrector pili muscles.
What is the characteristic feature of parasympathetic postganglionic fibers?
They are very short because the effector neuron cell bodies lie in ganglia located close to or directly inside the innervated organ.
Where does the parasympathetic system intersect with the metasympathetic system?
Inside organs — in intramural ganglia, where parasympathetic effector neurons are located.
What else to study on this topic
- Topography of cranial nerve preganglionic fibers
- Classification of sympathetic ganglia (paravertebral and prevertebral)
- Innervation of skin smooth muscle
- Connection of intra-organ ganglia with the metasympathetic system
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