Role in the Autonomic Reflex Arc
The autonomic nervous system functions via reflex arcs. Its efferent pathway has a characteristic structural feature—a two-neuron chain with synaptic relay in ganglia.
- Preganglionic neuron — located within the brain or spinal cord; its axon (preganglionic fiber) projects to an autonomic ganglion.
- Ganglionic neuron — whose cell body resides directly within the ganglion. Its axon is the postganglionic fiber, which travels to the target effector cells (muscles or glands).
The mechanism of signal transmission from the preganglionic fiber to the postganglionic neuron is identical in both the sympathetic and parasympathetic systems. Ganglionic synapses release acetylcholine, which binds to nicotinic acetylcholine receptors ($N_n$ receptors).
Morphological Features
In histological preparations of mixed nerves (e.g., H&E staining), axonal processes differ based on fiber type.
- Sympathetic postganglionic fibers — are unmyelinated. Multiple axons are enclosed within a single Schwann cell (Remak fiber). Furthermore, they are typically long, as sympathetic preganglionic fibers are short and terminate in the truncus sympathicus or prevertebral ganglia.
- Exception to the two-neuron rule — chromaffin cells of the adrenal medulla. Embryologically related to sympathetic ganglia, they functionally replace the postganglionic neuron and receive direct innervation from preganglionic fibers.
Sympathetic Innervation of Organs
Sympathetic postganglionic fibers originate from various ganglia and produce specific target organ effects:
- Eye — fibers from the superior cervical ganglion cause mydriasis (pupillary dilation).
- Heart — fibers from the cervical and upper thoracic ganglia increase heart rate and dilate coronary arteries.
- Abdominal organs — fibers from the celiac and mesenteric ganglia inhibit peristalsis and suppress gastric, intestinal, and pancreatic secretion.
- Urinary bladder — fibers originate from the inferior mesenteric ganglion (ganglion mesentericum inferius) and travel within the inferior hypogastric plexus (plexus hypogastricus inferior). They relax the detrusor muscle and contract the internal urethral sphincter (m. sphincter urethrae internus).
- Skin and limbs — fibers from cervical, thoracic, lumbar, and sacral ganglia mediate vasoconstriction, sudomotor function (sweating), and piloerection (contraction of arrector pili muscles).
Parasympathetic Innervation of Organs
- Eye — fibers from the ciliary ganglion cause miosis (pupillary constriction) and contraction of the ciliary muscle (accommodation).
- Head glands — fibers from the pterygopalatine and otic ganglia stimulate lacrimation, watery salivation, and vasodilation.
- Heart — fibers from the cardiac plexus cause bradycardia and constriction of coronary arteries.
- Intestines — fibers from the myenteric (Auerbach's) and submucosal (Meissner's) plexuses stimulate motility, secretion, and evacuation.
- Male genitalia — fibers from the pelvic splanchnic/hypogastric plexus mediate erection, vasodilation, and secretion.