Sensory (Afferent) Link
The receptor apparatus of the sympathetic division is located directly in the walls of blood vessels and internal organs. Impulses travel from them via dendrites that run within mixed nerves (including cranial nerves). The cell bodies of these primary sensory neurons are located in spinal ganglia or the ganglia of corresponding cranial nerves.
The parasympathetic arc utilizes pseudounipolars at this stage. Their cell bodies are also localized in sensory ganglia along nerve trunks. Information transfer from target organs occurs in two directions:
- To the brain: as part of cranial nerves, primarily via the vagus nerve (n. vagus).
- To the spinal cord: via pelvic nerves to the sacral-coccygeal segments.
Central (Associative) Link
The central apparatus is where the cell bodies of associative neurons reside and where autonomic functions are coordinated.
In the sympathetic arc, this center has a strict segmental localization. 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 $L_{III}$). Axons of these neurons form myelinated preganglionic fibers. They emerge from the spinal cord with the anterior roots and then branch off as white communicating rami. Their destination is the sympathetic trunk, where they either synapse in ganglia or pass right through.
Parasympathetic centers are divided into two distant poles:
- Cranial division: located in the brainstem (midbrain and medulla oblongata) and associated with the nuclei of cranial nerve pairs III, VII, IX, and X.
- 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 switching from one nerve to another along the way.
Effector Link and Return Pathways
Efferent neurons receive signals from the associative center and transmit them to the effector organ.
In the sympathetic system, the cell bodies of effector neurons can be located at three levels:
- In the ganglia of the paired sympathetic trunk (paravertebrally).
- In the ganglia of sympathetic plexuses (prevertebrally).
- In intraorgan ganglia (intramurally) — though least characteristic for the sympathetic system.
Unmyelinated postganglionic fibers emerge from these neurons. The bulk of them form nerves traveling to visceral organs. However, a return pathway to the "soma" also exists. Some fibers form gray communicating rami that return to somatic nerves. They are responsible for innervating blood vessels, sweat glands, and smooth muscle of the skin (arrector pili muscles).
In the parasympathetic system, effector ganglia lie differently: they are located either in specific ganglia close to the target organ or directly inside it (intramurally). It is within these intraorgan ganglia that the parasympathetic system anatomically intersects with the enteric nervous system. Due to this close proximity of ganglia to the target organ, parasympathetic effector neuron axons are always short.